Patent Publication Number: US-2015070024-A1

Title: Battery pack, apparatus including battery pack, and method of managing battery pack

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2013-0108056, filed on Sep. 9, 2013, in the Korean Intellectual Property Of the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments of the present invention relate to a battery pack, an apparatus including the battery pack, and a method of managing the battery pack, and more particularly, to a battery pack including a battery cell, an apparatus including the battery pack, and a method of managing the battery pack by estimating a state of health (SOH) of the battery cell. 
     2. Description of the Related Art 
     Unlike a primary battery that is not designed to be recharged, a secondary battery can be repeatedly charged and discharged, and is widely used not only in high-tech small electronic devices including smartphones, notebook computers, person digital assistants (PDAs), or the like, but also used in electric vehicles and energy storage systems. The battery capacity is decreased according to a use environment, a use period, the number of charging and discharging, and/or the like. A state of health (SOH) of a battery is an index for indicating how much the battery capacity is decreased compared to its initial battery capacity stated in its specification, and is one of the key parameters for evaluating the battery. 
     In order to estimate the SOH of the battery, a current calculation method may be used. The current calculation method involves estimating a battery capacity by fully charging and discharging the battery, and estimating the SOH of the battery by comparing the battery capacity with the initial battery capacity. If temperature variation or charging speed variation can be appropriately compensated, the current calculation method may accurately estimate the SOH of the battery. However, because the battery has to be fully charged and then fully discharged, the current calculation method is not efficient (e.g., is time consuming). Alternatively, the SOH of the battery may be estimated by measuring impedance of the battery. In order to measure the impedance of the battery, an alternating voltage response of the battery has to be measured. However, the impedance measurement method requires an additional circuit, and is not efficient (e.g., may not be desirable) due to sensor errors, durability, costs, and/or the like. 
     SUMMARY 
     One or more aspects according to embodiments of the present invention include a battery pack having a battery cell whose state of health (SOH) may be estimated in real-time, and an apparatus including the battery pack. 
     One or more aspects according to embodiments of the present invention include a method of managing a battery pack by easily estimating an SOH of a battery cell in real-time. 
     Additional aspects will be set forth in part in the description that follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     According to an aspect of embodiments according to the present invention, there is provided a battery pack including: a battery coupled to a load and a charging device, and including at least one battery cell; and a battery management unit configured to control charging of the battery from the charging device and discharging of the battery to the load, wherein the battery management unit includes: a measuring unit configured to generate cell voltage data and current data by measuring a cell voltage and a current of the at least one battery cell; a capacity estimating unit configured to generate current capacity data by estimating a current capacity of the at least one battery cell based on the cell voltage data and the current data; an internal resistance estimating unit configured to generate current internal resistance data by estimating a current internal resistance of the at least one battery cell based on the cell voltage data and the current data; and a state of health (SOH) estimating unit configured to estimate an SOH of the at least one battery cell based on the current capacity data and the current internal resistance data. 
     The battery pack may further include a first storage unit for storing first data on a correlation between an open circuit voltage and a state of charge (SOC) of the at least one battery cell. 
     The capacity estimating unit may be configured to: store a first open circuit voltage that is a cell voltage of the at least one battery cell at a first time when the current of the at least one battery cell is 0; store a second open circuit voltage that is a cell voltage of the at least one battery cell at a second time when the current of the at least one battery cell is 0; calculate a varied capacity between the first time and the second time by integrating the current of the at least one battery cell from the first time to the second time; obtain a first SOC corresponding to the first open circuit voltage and a second SOC corresponding to the second open circuit voltage, based on the first data; and measure the current capacity of the at least one battery cell by dividing the varied capacity by a difference between the first SOC and the second SOC, and updates the current capacity data. 
     The first time and the second time may be set such that the difference between the first SOC and the second SOC may be equal to or greater than 36%. 
     The internal resistance estimating unit may be configured to: store a charging-discharging voltage that is a cell voltage of the at least one battery cell at a third time when the current of the at least one battery cell is not 0; store charging-discharging current that is the current of the at least one battery cell at the third time; obtain an open circuit voltage of the at least one battery cell corresponding to an SOC of the at least one battery cell at the third time, based on the first data; and estimate the current internal resistance of the at least one battery cell by dividing a difference between the open circuit voltage and the charging-discharging voltage by the charging-discharging current; and update the current internal resistance data based on the estimated current internal resistance of the at least one battery cell. 
     The internal resistance estimating unit may be configured to: store a third open circuit voltage that is a cell voltage of the at least one battery cell at a fourth time when current of the at least one battery cell is 0; and estimate the current internal resistance of the at least one battery cell by dividing a difference between a cell voltage of the at least one battery cell at a fifth time, near the fourth time, when current of the at least one battery cell is not 0 and the third open circuit voltage by current of the at least one battery cell at the fifth time; and update the current internal resistance data based on the estimated current internal resistance of the at least one battery cell. 
     The battery pack may further include a second storage unit for storing initial capacity data on an initial capacity of the at least one battery cell and initial internal resistance data on an initial internal resistance of the at least one battery cell. 
     The SOH estimating unit may be configured to measure a first SOH of the at least one battery cell, based on the initial capacity data and the initial internal resistance data. 
     The SOH estimating unit may include deterioration capacity data on a deterioration capacity of the at least one battery cell when the at least one battery cell is in a deterioration state, and wherein the first SOH may be calculated by dividing a difference between the current capacity and the deterioration capacity of the at least one battery cell by a difference between the initial capacity and the deterioration capacity of the at least one battery cell. 
     The SOH estimating unit may be configured to estimate a second SOH of the at least one battery cell based on the initial internal resistance data and the current internal resistance data. 
     The SOH estimating unit may include deterioration internal resistance data on a deterioration internal resistance of the at least one battery cell when the at least one battery cell is in a deterioration state, and wherein the second SOH may be calculated by dividing a difference between the deterioration internal resistance and the current internal resistance of the at least one battery cell by a difference between the deterioration internal resistance and the initial internal resistance of the at least one battery cell. 
     The SOH estimating unit may be configured to: estimate a first SOH of the at least one battery cell, based on the initial capacity data and the current capacity data; estimate a second SOH of the at least one battery cell, based on the initial internal resistance data and the current internal resistance data; and estimate the SOH of the at least one battery cell, based on the first SOH and the second SOH. 
     The SOH may be estimated as an average of the first SOH and the second SOH. 
     According to another aspect of embodiments according to the present invention, there is provided a device including: a battery pack including a battery and a battery management unit configured to control charging and discharging of the battery, the battery including at least one battery cell; a measuring unit configured to generate cell voltage data and current data by measuring a cell voltage and a current of the at least one battery cell; a capacity estimating unit configured to generate current capacity data by estimating a current capacity of the at least one battery cell based on the cell voltage data and the current data; an internal resistance estimating unit configured to generate current internal resistance data by estimating a current internal resistance of the at least one battery cell based on the cell voltage data and the current data; and a state of health (SOH) estimating unit configured to estimate an SOH of the at least one battery cell based on the current capacity data and the current internal resistance data. 
     The device may be an energy storage device including a power conversion device coupled between the battery pack and at least one of a power generation system, a load, and a grid, wherein the power conversion device may be configured to convert electric energy between the battery pack and at least one of the power generation system, the load, and the grid. 
     The device may include an electric vehicle including the battery pack and a motor, wherein the motor may be driven by using electric energy stored in the battery pack. 
     According to another aspect of embodiments according to the present invention, there is provided a method of managing a battery pack including a battery and a battery management unit for controlling charging and discharging of the battery, the battery including at least one battery cell, the method including: generating cell voltage data and current data by measuring a cell voltage and a current of the at least one battery cell; generating current capacity data by estimating a current capacity of the at least one battery cell based on the cell voltage data and the current data; generating current internal resistance data by estimating a current internal resistance of the at least one battery cell based on the cell voltage data and the current data; and estimating a state of health (SOH) of the at least one battery cell based on the current capacity data and the current internal resistance data. 
     The method may further include storing first data on a correlation between an open circuit voltage and a state of charge (SOC) of the at least one battery cell, wherein the generating of the current capacity data may include: obtaining a first open circuit voltage that is a cell voltage of the at least one battery cell at a first time when the current of the at least one battery cell is 0; obtaining a second open circuit voltage that is a cell voltage of the at least one battery cell at a second time when the current of the at least one battery cell is 0; calculating a varied capacity between the first time and the second time by integrating the current of the at least one battery cell from the first time to the second time; obtaining a first SOC corresponding to the first open circuit voltage and a second SOC corresponding to the second open circuit voltage, based on the first data; and estimating the current capacity of the at least one battery cell by dividing the varied capacity by a difference between the first SOC and the second SOC. 
     The method may further include storing first data on a correlation between an open circuit voltage and a state of charge (SOC) of the at least one battery cell, wherein the generating of the current internal resistance data may include: storing a charging-discharging voltage that is a cell voltage of the at least one battery cell at a third time when the current of the at least one battery cell is not 0; storing charging-discharging current that is the current of the at least one battery cell at the third time; obtaining an open circuit voltage of the at least one battery cell corresponding to an SOC of the at least one battery cell at the third time, based on the first data; and estimating the current internal resistance of the at least one battery cell by dividing a difference between the open circuit voltage and the charging-discharging voltage by the charging-discharging current. 
     The estimating of the SOH may include: estimating a first SOH of the at least one battery cell, based on initial capacity data on an initial capacity of the at least one battery cell and the current capacity data; estimating a second SOH of the at least one battery cell, based on initial internal resistance data on an initial internal resistance of the at least one battery cell and the current internal resistance data; and estimating the SOH of the at least one battery cell, based on the first SOH and the second SOH. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects of the present invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram of a battery pack, according to an example embodiment of the present invention; 
         FIG. 2  is a block diagram of a battery pack, according to another example embodiment of the present invention; 
         FIG. 3A  illustrates graphs indicating a voltage, current, and a remaining capacity of a battery cell, according to an example embodiment of the present invention; 
         FIG. 3B  illustrates a graph indicating a correlation between an open circuit voltage and a state of charge (SOC) of the battery cell according to an example embodiment of the present invention; 
         FIG. 4  illustrates a graph of an SOH estimated according to the one or more example embodiments and an SOH estimated according to a comparative example; 
         FIG. 5  is a block diagram of an energy storage device including a battery pack, according to an example embodiment of the present invention; and 
         FIG. 6  is a block diagram of an electric vehicle including a battery pack, according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Thus, the present embodiments may include all revisions, equivalents, or substitutions that are included in the concept and the technical scope related to the present embodiments. 
     Like reference numerals in the drawings denote like elements. In the drawings, the dimension of structures may be exaggerated for clarity. 
     Furthermore, all examples and conditional language recited herein are to be construed as being without limitation to such specifically recited examples and conditions. Throughout the specification, a singular form may include plural forms, unless there is a particular description contrary thereto. Also, terms such as “include,” “including,” “comprise,” or “comprising” are used to specify existence of a recited form, a number, a process, an operation, a component, and/or groups thereof, not excluding the existence of one or more other recited forms, one or more other numbers, one or more other processes, one or more other operations, one or more other components and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Throughout the specification, while terms “first” and “second” are used to describe various components, it is intended that the components not be limited to the terms “first” and “second”. The terms “first” and “second” are used only to distinguish between each component. Throughout the specification, it will also be understood that when an element is referred to as being “on” another element, it can be directly on the other element, or intervening elements may also be present. 
     Unless expressly described otherwise, all terms including descriptive or technical terms, which are used herein, should be construed as having meanings that are obvious to one of ordinary skill in the art. Also, terms that are defined in a general dictionary and that are used in the following description should be construed as having meanings that are equivalent to meanings used in the related description, and unless expressly described otherwise herein, the terms should not be construed as being ideal or excessively formal. 
     Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     Herein, the use of the term “may,” when describing embodiments of the present invention, refers to “one or more embodiments of the present invention.” In addition, the use of alternative language, such as “or,” when describing embodiments of the present invention, refers to “one or more embodiments of the present invention” for each corresponding item listed. 
     The term “time” refers to a brief moment of time, and throughout the specification, each of the “first time” and the “second time” may refer to a brief moment of time or may refer to a time period. For example, the first time may refer to a first time period having a temporal length (e.g., a predetermined temporal length) and the second time may refer to a second time period that does not overlap with the first time period. 
       FIG. 1  is a block diagram of a battery pack  100 , according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the battery pack  100  includes a battery  110  and a battery management unit  120 . The battery management unit  120  includes a measuring unit  130 , a capacity estimating unit  140 , an internal resistance estimating unit  150 , and a state of health (SOH) estimating unit  160 . 
     The battery  110  stores power and includes at least one battery cell  111 . The battery  110  may include a plurality of the battery cells  111  that are connected in series, in parallel, or in combination of serial and parallel connections. The number of the battery cells  111  included in the battery  110  may be determined according to a desired output voltage. 
     The battery  110  may be connected to a load and a charging device via terminals  101 . When the battery  110  is discharged, the battery  110  outputs electric energy to the load via the terminals  101 , and when the battery  110  is charged, the battery  110  stores electric energy input from the charging device via the terminals  101 . In an example where the battery pack  100  is mounted in a pure electric vehicle that is driven by only electric energy or in a hybrid electric vehicle that is driven by electric energy or fossil fuel, the load may be a driving motor of the electric vehicle, and the charging device may be an electric vehicle charger and a regenerative generator that generates power by regenerating energy that occurs in braking. 
     When the battery pack  100  and a power conversion device make up an energy storage device, wherein the power conversion device is electrically coupled to (e.g., connected among) a generating system, the battery pack  100 , a load, and/or a grid, and converts electric energy therebetween, the load may be the load and/or the grid, and the charging device may be the generating system and/or the grid. 
     The battery cell  111  may include a chargeable secondary battery. For example, in one embodiment, the battery cell  111  includes a nickel-cadmium battery, a lead storage battery, a nickel metal hydride battery (NiMH), a lithium ion battery, a lithium polymer battery, and/or the like. 
     The battery  110  may be formed of a plurality of battery modules that may include the battery cells  111  connected in series, in parallel, or in any suitable combination of serial and parallel connections. 
     The battery management unit  120  monitors a state of the battery  110  and controls all operations including charging and discharging operations by the battery  110 . The battery management unit  120  may be referred to as a battery management system (BMS). 
     The battery management unit  120  may measure parameters, such as cell voltage, temperature, charging and discharging currents, and/or the like, that are related to the battery  110 , and may control charging and discharging of the battery  110 , based on data of the measured parameters. The battery management unit  120  may calculate a remaining amount of power, a lifetime, a state of charge (SOC) (measured, e.g., as a percentage of the fully charged state), and/or the like from the data or may determine whether or not an error has occurred in the battery  110 . For example, the battery management unit  120  may determine whether or not an error such as over-charging, over-discharging, over-current, an over-voltage, overheating, a battery cell imbalance, battery cell deterioration, and/or the like has occurred. If an error has occurred, the battery management unit  120  may perform a suitable operation (e.g., a preset operation) according to an internal algorithm. For example, the battery management unit  120  may control a charging switch and/or a discharging switch, or may cut a fuse. The battery management unit  120  may control a cell balancing operation by battery cells of the battery  110  according to the data and a suitable algorithm (e.g., a preset algorithm). 
     The battery management unit  120  includes the measuring unit  130  for generating cell voltage data VD and current data CD by measuring a cell voltage and current of the battery cell  111 ; the capacity estimating unit  140  for generating current capacity data CCD by estimating a current capacity of the battery cell  111  based on the cell voltage data VD and the current data CD; the internal resistance estimating unit  150  for generating current internal resistance data CIRD by estimating a current internal resistance of the battery cell  111  based on the cell voltage data VD and the current data CD; and the SOH estimating unit  160  for estimating an SOH of the battery cell  111  based on the current capacity data CCD and the current internal resistance data CIRD. Although the measuring unit  130 , the capacity estimating unit  140 , the internal resistance estimating unit  150 , and the SOH estimating unit  160  are illustrated as separate elements, the measuring unit  130 , the capacity estimating unit  140 , the internal resistance estimating unit  150 , and the SOH estimating unit  160  may be included (e.g., embodied) in one chip. In another embodiment, the measuring unit  130  may be included in a device called an analog front end (AFE), and the capacity estimating unit  140 , the internal resistance estimating unit  150 , and the SOH estimating unit  160  may be included in a microcontroller called a battery monitoring unit (BMU). 
     The measuring unit  130  generates the cell voltage data VD by measuring the cell voltage of the battery cell  111 . The measuring unit  130  is coupled to (e.g., connected to) both terminals of the battery cell  111  via lines and thus may directly measure the cell voltage of the battery cell  111 . The measuring unit  130  may include an analog to digital converter (ADC) so as to convert the measured cell voltage into the cell voltage data VD. In an embodiment, the measuring unit  130  may be coupled to nodes between the battery cells  111  via lines, and may generate a plurality of pieces of cell voltage data VD that correspond to cell voltages of the battery cells  111 . In consideration of cell voltage variation, a noise, and a measurement tolerance, the cell voltage data VD may correspond to an average of cell voltage values of the battery cell  111  during a time (e.g., a predetermined time). The time may be 1 second, 10 seconds, or 1 minute long. 
     The measuring unit  130  generates the current data CD by measuring the charging current and the discharging current of the battery cell  111 . The measuring unit  130  may measure the charging current and the discharging current of the battery cell  111  by using a current sensor. In an example in which the battery cells  111  are connected in series, the same amplitude of current flows in the battery cells  111  that are connected in series, thus, the measuring unit  130  may measure only one current with respect to the battery cells  111 . In an example in which the battery cells  111  are connected in parallel or are connected in series and parallel, the measuring unit  130  may measure cell current of each of the battery cells  111  or may measure cell current of each of the battery cells  111  that are connected in parallel. The measuring unit  130  may include an ADC so as to convert the measured current into the current data CD. In consideration of cell voltage variations, noise, and measurement tolerances, the current data CD may correspond to an average of current values of the battery cell  111  during a time (e.g., a predetermined time). The time may be 1 second, 10 seconds, or 1 minute long. 
     The measuring unit  130  may further measure parameters such as a temperature of the battery  110 , a terminal voltage, the cell voltage, the charging current, the discharging current of the battery cell  111 , and/or the like. 
     The capacity estimating unit  140  generates the current capacity data CCD by estimating the current capacity of the battery cell  111  based on the cell voltage data VD and the current data CD. The capacity estimating unit  140  may receive the cell voltage data VD and the current data CD from the measuring unit  130 , and may estimate the current capacity of the battery cell  111  by using the cell voltage data VD and the current data CD. 
     In an embodiment, the capacity estimating unit  140  determines a fully-charged state and a fully-discharged state by using the cell voltage data VD, and integrates current that flows into or flows out of the battery cell  111  between the fully-charged state and the fully-discharged state by using the current data CD, so that the capacity estimating unit  140  may calculate a full charging capacity or a full discharging capacity. 
     In another embodiment, the capacity estimating unit  140  estimates the current capacity of the battery cell  111  by using data OSD on a correlation between the open circuit voltage and the SOC of the battery cell  111 . The battery management unit  120  may further include a first storage unit  170  that stores the data OSD. The open circuit voltage of the battery cell  111  is a cell voltage a which a load or a charging device is not coupled to (e.g., connected to) the battery cell  111 , and is equal to a cell voltage at which current of the battery cell  111  is 0. The SOC of the battery cell  111  indicates a ratio of a capacity of the battery cell  111  to a remaining capacity of the battery cell  111 . 
     The capacity estimating unit  140  may determine a first time at which current of the battery cell  111  is 0, based on the cell voltage data VD and the current data CD, and may determine a first open circuit voltage that is a cell voltage of the battery cell  111  at the first time. 
     The capacity estimating unit  140  may determine a second time at which current of the battery cell  111  is 0, based on the cell voltage data VD and the current data CD, and may determine a second open circuit voltage that is a cell voltage of the battery cell  111  at the second time. The second time may be different from the first time. Additionally, the second open circuit voltage may be different from the first open circuit voltage. 
     The capacity estimating unit  140  may integrate current between the first time and the second time, based on the cell voltage data VD and the current data CD, and then may calculate a varied capacity of the battery cell  111  between the first time and the second time, i.e., a varied amount of the remaining capacity of the battery cell  111 . The varied capacity indicates a difference between the remaining capacity of the battery cell  111  at the first time and the remaining capacity of the battery cell  111  at the second time. 
     The capacity estimating unit  140  may determine a first SOC corresponding to the first open circuit voltage and a second SOC corresponding to the second open circuit voltage, based on the data OSD. For example, the first SOC indicates an SOC of the battery cell  111  at the first time, and the second SOC indicates an SOC of the battery cell  111  at the second time. In general, the first and second SOCs may be expressed as actual numbers between 0 and 1 or may be expressed in a percentage. 
     The capacity estimating unit  140  may measure the current capacity of the battery cell  111  by dividing the varied capacity by a difference between the first SOC and the second SOC. For example, when the first SOC is 90% and the second SOC is 40%, if the remaining capacity of the battery cell  111  is decreased by 1000 mAh between the first time and the second time, the current capacity of the battery cell  111  may be estimated as 2000 mAh. As another example, when the first SOC is 30% and the second SOC is 90%, if the remaining capacity of the battery cell  111  is increased by 1500 mAh between the first time and the second time, the current capacity of the battery cell  111  may be estimated as 2500 mAh. In the aforementioned examples, each of 1000 mAh and 1500 mAh may correspond to the varied capacity of the battery cell  111 , and may be calculated by integrating current of the battery cell  111  between the first time and the second time, based on the current data CD. 
     The capacity estimating unit  140  may update the current capacity data CCD by referring to the estimated current capacity. For example, when the estimated current capacity is 2000 mAh, the current capacity data CCD may be updated as 2000. That is, the current capacity data CCD may indicate the most-recently estimated current capacity of the battery cell  111 . 
     In an embodiment, noise may be included in the most-recently estimated current capacity of the battery cell  111 , so that the capacity estimating unit  140  may update the current capacity data CCD by referring to both a value of the current capacity data CCD before the update and the most-recently estimated current capacity of the battery cell  111 . For example, in a case where the value of the current capacity data CCD before the update is 2000 and the most-recently estimated current capacity is 1900 mAh, the current capacity data CCD may be updated as 1950, which is a numerical average of the two values. Instead of the numerical average, a weighted average may be applied thereto. 
     In another embodiment, the capacity estimating unit  140  updates the current capacity data CCD by using current capacities of the battery cell  111  that is recently estimated at a plurality of times. For example, when the most-recently estimated current capacity is 1910 mAh, an estimated current capacity just previous to the most recent estimate is 1870 mAh, and a previously estimated current capacity is 1920 mAh, the current capacity data CCD may be updated as 1900 mAh that is a numerical average of the three values. In an embodiment, instead of the numerical average, a weighted average may be used so that a greater weight may be applied to the most-recently estimated current capacity. 
     In other embodiments, the first time and the second time may be set so that the difference between the first SOC and the second SOC may be equal to or greater than 36%. When the difference between the first SOC and the second SOC is small, the estimated current capacity of the battery cell  111  may be inaccurate. Thus, when the difference between the first SOC and the second SOC is less than 36%, the second time may be re-determined. 
     In another embodiment, the capacity estimating unit  140  estimates the current capacity of the battery cell  111  by analyzing a pattern of the cell voltage data VD and the current data CD. The first storage unit  170  may store pattern data on patterns of cell voltage and cell current at different capacities of the battery cell  111 . The capacity estimating unit  140  may scan a pattern that is most similar to the pattern of the cell voltage data VD and the current data CD by using the pattern data, and may estimate the current capacity of the battery cell  111  based on a scanning result. 
     The internal resistance estimating unit  150  generates the current internal resistance data CIRD by estimating the current internal resistance of the battery cell  111  based on the cell voltage data VD and the current data CD. The internal resistance estimating unit  150  may receive the cell voltage data VD and the current data CD from the measuring unit  130 , and may estimate the current internal resistance of the battery cell  111  by using the cell voltage data VD and the current data CD. It is known that an internal resistance of the battery cell  111  increases as the battery cell  111  ages. 
     In an embodiment, the internal resistance estimating unit  150  estimates the current internal resistance of the battery cell  111  based on the cell voltage data VD and the current data CD, by using a fact that a difference between an open circuit voltage and a cell voltage of the battery cell  111  is increased as current output from the battery cell  111  is increased. The current internal resistance of the battery cell  111  may be estimated based on a variation level of the cell voltage at a time at which the current output from the battery cell  111  sharply changes. 
     In another embodiment, the internal resistance estimating unit  150  determines a fourth time at which current of the battery cell  111  is 0, and then determines a third open circuit voltage that is a cell voltage of the battery cell  111  at the fourth time. 
     The internal resistance estimating unit  150  may determine a fifth time at which the battery  110  is turned to an SOC or a state of discharge. The fifth time may be as close to (e.g., adjacent to) the fourth time as possible. The fifth time may be set as a time after a period of time (e.g., a predetermined time) elapses from a time at which the battery  110  is turned to the SOC or the state of discharge. When charging or discharging of the battery  110  starts, the cell voltage of the battery cell  111  fluctuates. The period of time (e.g., a predetermined time) may indicate a time period in which the fluctuation of the cell voltage subsides (e.g., disappears) and then the cell voltage is stabilized. The fifth time may be set as a time at which the cell voltage is stabilized after the current of the battery cell  111  becomes greater or less than 0. 
     The internal resistance estimating unit  150  may estimate the current internal resistance of the battery cell  111  by dividing a difference between a cell voltage of the battery cell  111  in the fifth time and the third open circuit voltage by current of the battery cell  111  in the fifth time. 
     The internal resistance estimating unit  150  may update the current internal resistance data CIRD by referring to the estimated current internal resistance. The current internal resistance data CIRD may indicate a most-recently estimated current internal resistance of the battery cell  111 . In an embodiment, the current internal resistance data CIRD may be determined based on a value of the current internal resistance data CIRD before the update and the most-recently estimated current internal resistance of the battery cell  111  or may be determined based on current internal resistances of the battery cell  111  that is recently estimated at a plurality of times. 
     In another embodiment, the internal resistance estimating unit  150  estimates the current internal resistance of the battery cell  111  by using the data OSD. As described above, the battery management unit  120  may further include the first storage unit  170  that stores the data OSD. 
     The internal resistance estimating unit  150  may determine a third time at which current of the battery cell  111  is not 0. At the third time, the battery  110  may be in an SOC or a state of discharge. The internal resistance estimating unit  150  may determine a charging-discharging voltage that is a cell voltage of the battery cell  111  at the third time, and may determine charging-discharging current that is the current of the battery cell  111  at the third time. 
     The internal resistance estimating unit  150  may determine the SOC of the battery cell  111  at the third time. The internal resistance estimating unit  150  may have information about an SOC at a particular time before the third time. For example, the internal resistance estimating unit  150  may receive information about an SOC at the second time from the capacity estimating unit  140 . The internal resistance estimating unit  150  may calculate a varied capacity between the second time and the third time by integrating current of the battery cell  111  between the second time and the third time, and then may determine the SOC at the third time, based on the varied capacity and the current capacity of the battery cell  111  that is estimated by the capacity estimating unit  140 . 
     In an embodiment, because the SOC is 0% at a time at which the battery cell  111  is fully discharged and the SOC is 100% at a time in which the battery cell  111  is fully charged, the internal resistance estimating unit  150  may determine the SOC at the third time by integrating current of the battery cell  111  from after the fully-discharged time or the fully-charged time to the third time. 
     In another embodiment, the internal resistance estimating unit  150  determines an open circuit voltage that is a cell voltage of the battery cell  111  in a particular time at which current of the battery cell  111  is 0 before the third time, and determines an SOC corresponding to the open circuit voltage, based on the data OSD stored in the first storage unit  170 . By doing so, the internal resistance estimating unit  150  may determine the SOC at the particular time. The internal resistance estimating unit  150  may calculate a varied capacity between the particular time and the third time by integrating current of the battery cell  111  between the particular time and the third time, and then may determine the SOC at the third time, based on the varied capacity and the current capacity of the battery cell  111  that is estimated by the capacity estimating unit  140 . 
     The internal resistance estimating unit  150  may determine an open circuit voltage of the battery cell  111  at the third time, which corresponds to the determined SOC at the third time, based on the data OSD stored in the first storage unit  170 . The internal resistance estimating unit  150  may estimate the current internal resistance of the battery cell  111  by dividing a difference between the open circuit voltage and the charging-discharging voltage by the charging-discharging current. The internal resistance estimating unit  150  may update the current internal resistance data GIRD by referring to the estimated current internal resistance. 
     In another embodiment, the internal resistance estimating unit  150  estimates the current internal resistance of the battery cell  111  by analyzing a pattern of the cell voltage data VD and the current data CD. The first storage unit  170  may store pattern data on the patterns of cell voltage and the current according to internal resistances (e.g., at different internal resistances) of the battery cell  111 . The internal resistance estimating unit  150  may scan a pattern that is the most similar to the pattern of the cell voltage data VD and the current data CD by using the pattern data, and may estimate the current internal resistance of the battery cell  111 , based on a scanning result. 
     The SOH estimating unit  160  estimates the SOH of the battery cell  111 , based on the current capacity data CCD and the current internal resistance data CIRD. The SOH estimating unit  160  receives the current capacity data CCD from the capacity estimating unit  140 , receives the current internal resistance data CIRD from the internal resistance estimating unit  150 , and then estimates the SOH of the battery cell  111 , based on the current capacity data CCD and the current internal resistance data CIRD. 
     According to an embodiment, the SOH estimating unit  160  includes a fuzzy logic block that receives an input of the current capacity data CCD and the current internal resistance data CIRD and outputs the SOH of the battery cell  111 . The fuzzy logic block determines whether the current capacity of the battery cell  111  is good, normal, or bad, based on the current capacity data CCD. The fuzzy logic block also determines whether the current internal resistance of the battery cell  111  is good, normal, or bad, based on the current internal resistance data CIRD. The fuzzy logic block estimates the SOH of the battery cell  111  by applying an If-then rule to a result of determining the current capacity and a result of determining the current internal resistance. Examples of the If-then rule are as below. 
     If the current capacity is good and the current internal resistance is good, then the SOH of the battery cell  111  is good. Here, the SOH is determined between about 0.9 and about 1. 
     If the current capacity is good and the current internal resistance is normal, then the SOH of the battery cell  111  is slightly good. Here, the SOH is determined between about 0.7 and about 0.9. 
     If the current capacity is good and the current internal resistance is bad, then the SOH of the battery cell  111  is normal. Here, the SOH is determined between about 0.5 and about 0.7. 
     If the current capacity is normal and the current internal resistance is good, then the SOH of the battery cell  111  is slightly good. Here, the SOH is determined between about 0.7 and about 0.9. 
     If the current capacity is normal and the current internal resistance is normal, then the SOH of the battery cell  111  is normal. Here, the SOH is determined between about 0.5 and about 0.7. 
     If the current capacity is normal and the current internal resistance is bad, then the SOH of the battery cell  111  is slightly bad. Here, the SOH is determined between about 0.3 and about 0.5. 
     If the current capacity is bad and the current internal resistance is good, then the SOH of the battery cell  111  is normal. Here, the SOH is determined between about 0.5 and about 0.7. 
     If the current capacity is bad and the current internal resistance is normal, then the SOH of the battery cell  111  is slightly bad. Here, the SOH is determined between about 0.3 and about 0.5. 
     If the current capacity is bad and the current internal resistance is bad, then the SOH of the battery cell  111  is slightly bad. Here, the SOH is determined between about 0 and about 0.3. 
     In another embodiment, the SOH estimating unit  160  estimates the SOH of the battery cell  111  based on the current capacity data CCD and the current internal resistance data CIRD, by using initial capacity data ICD about an initial capacity of the battery cell  111  and initial internal resistance data IIRD about an initial internal resistance of the battery cell  111 . The battery management unit  120  may further include a second storage unit  180  that stores the initial capacity data ICD about the initial capacity of the battery cell  111  and the initial internal resistance data IIRD about the initial internal resistance of the battery cell  111 . The initial capacity is a capacity of the battery cell  111  according to product specification, which is attributed to (e.g., allocated to) the battery cell  111  at the time of its manufacture. The initial internal resistance indicates an internal resistance that is attributed to (e.g., allocated to) the battery cell  111  at the time of its manufacture. 
     The SOH estimating unit  160  may estimate a capacity-based SOH of the battery cell  111 , according to the initial capacity data ICD and the current capacity data CCD. The capacity-based SOH is referred to as a first SOH. 
     The SOH estimating unit  160  may include deterioration capacity data DCD about a deterioration capacity that the battery cell  111  has when the battery cell  111  is in a deterioration state. The deterioration capacity may be determined according to a capacity of the battery cell  111 , which is guaranteed by a manufacturer of the battery pack  100 . When the current capacity of the battery cell  111  is less than the deterioration capacity, the SOH estimating unit  160  may determine that the battery cell  111  has deteriorated. For example, the deterioration capacity may be determined between about 60% and about 90% of the initial capacity. The deterioration capacity data DCD may indicate a ratio of the deterioration capacity to the initial capacity. The first SOH may be determined as a value obtained by dividing a difference between the current capacity and the deterioration capacity of the battery cell  111  by a difference between the initial capacity and the deterioration capacity of the battery cell  111 . 
     The SOH estimating unit  160  may estimate an internal resistance-based SOH of the battery cell  111 , according to the initial internal resistance data IIRD and the current internal resistance data CIRD. The internal resistance-based SOH is referred to as a second SOH. 
     The SOH estimating unit  160  may include deterioration internal resistance data DIRD about a deterioration internal resistance that the battery cell  111  has when the battery cell  111  is in a deterioration state. The battery cell  111  may have the deterioration internal resistance when the current capacity of the battery cell  111  is less than the deterioration capacity. For example, the deterioration internal resistance may be determined between about 130% and about 200% of the initial internal resistance. The deterioration internal resistance data DIRD may indicate a ratio of the deterioration internal resistance to the initial internal resistance. The second SOH may be determined as a value obtained by dividing a difference between the deterioration internal resistance and the current internal resistance of the battery cell  111  by a difference between the deterioration internal resistance and the initial internal resistance of the battery cell  111 . 
     The SOH estimating unit  160  may estimate the SOH of the battery cell  111 , based on the first SOH and the second SOH. For example, the SOH of the battery cell  111  may be determined as a numerical average of the first SOH and the second SOH. In another embodiment, the SOH of the battery cell  111  may be determined as a weighted average of the first SOH and the second SOH. According to a capacity of the battery cell  111 , a weight of the first SOH and a weight of the second SOH may be adjusted. For example, as the capacity of the battery cell  111  is increased, the weight of the first SOH may be greater than the weight of the second SOH. Conversely, as the capacity of the battery cell  111  is decreased, the weight of the second SOH may be greater than the weight of the first SOH. 
     As described above, the measuring unit  130  measures the cell voltage and current of the battery cell  111  of the battery pack  100  in operation, and generates the cell voltage data VD and the current data CD in real-time. The capacity estimating unit  140  and the internal resistance estimating unit  150  generate the current capacity data CCD in real-time by estimating the current capacity of the battery cell  111  based on only the cell voltage data VD and the current data CD without using separate additional circuit devices, and generate the current internal resistance data CIRD in real-time by estimating the current internal resistance of the battery cell  111 . The SOH estimating unit  160  measures the SOH of the battery cell  111  in real-time, based on the current capacity data CCD and the current internal resistance data CIRD. Thus, the battery pack  100  in operation may accurately estimate the SOH of the battery cell  111  in a relatively easy way. 
     In embodiments according to  FIG. 1 , the battery management unit  120  of the battery pack  100  estimates the SOH of the battery cell  111 , but in another embodiment, an upper controller or an external controller, which may communicate with the battery pack  100 , the measuring unit  130  of the battery pack  100 , or the battery management unit  120  of the battery pack  100 , estimates the SOH of the battery cell  111 . 
       FIG. 2  is a block diagram of a battery pack  100   a , according to another embodiment of the present invention. 
     Referring to  FIG. 2 , the battery pack  100   a  includes the battery  110 , an AFE  135 , and a micro controller unit (MCU)  125 . 
     The battery  110  includes the battery cells  111 . Referring to  FIG. 2 , the battery cells  111  are connected in series, but if desired, the battery cells  111  may be connected in series, in parallel, or in combination of serial and parallel connections. Also, the number of the battery cells may be selected based on (e.g., determined according to) a desired output voltage. The battery cells  111  that are positioned at both ends are connected to the terminals  101 . 
     The AFE  135  may correspond to the measuring unit  130  shown in  FIG. 1 . The AFE  135  includes a cell voltage measuring unit  131  for measuring a cell voltage of each of the battery cells  111 . The cell voltage measuring unit  131  may be coupled to lines that extend from nodes between the terminals  101  and the battery cells  111 . The cell voltage measuring unit  131  may measure a cell voltage and may convert the measured cell voltage into cell voltage data by using an ADC. The cell voltage data may be provided to the MCU  125 . 
     The AFE  135  includes a current measuring unit  132  that is coupled to a current sensor  133  for measuring charging-discharging current of the battery cells  111 . The current sensor  133  may be a shunt or a hall sensor, which is mounted in a high-current path between the battery  110  and the terminals  101 . The current measuring unit  132  may convert an analog current value into current data, wherein the analog current value corresponds to current that is measured by the current sensor  133 . The current data may be provided to the MCU  125 . 
     The MCU  125  may monitor a state of the battery  110  and may control all operations including charging and discharging operations by the battery  110 . The MCU  125  may receive measurement data from the AFE  135 . The measurement data may include the cell voltage data, the current data, temperature data indicating a temperature of the battery  110 , terminal voltage data indicating a terminal voltage between the terminals  101 , and/or the like. The terminal voltage data may be calculated by the MCU  125  using the cell voltage data. 
     The MCU  125  may control charging and discharging of the battery  110 , based on the measurement data. The MCU  125  may calculate a remaining amount of power, a lifetime, an SOC, and/or the like from a plurality of pieces of measured data, or may determine whether or not an error has occurred in the battery  110 . When the error has occurred in the battery  110 , the MCU  125  may control a charging switch  191  and/or a discharging switch  192 , or may cut a fuse. When the terminal voltage data is greater than a charging upper limit value (e.g., a preset charging upper limit value), the MCU  125  may open the charging switch  191  so as to discontinue charging, and when the terminal voltage data is less than a discharging lower limit value (e.g., preset discharging lower limit value), the MCU  125  may open the discharging switch  192  so as to discontinue discharging. When the current data is greater than an over-current reference value (e.g., a preset over-current reference value), the MCU  125  may cut the fuse so as to protect the battery pack  100 . 
     The MCU  125  measures an SOH of each of the battery cells  111 , based on the cell voltage data and the current data. The MCU  125  includes the capacity estimating unit  140 , the internal resistance estimating unit  150 , and the SOH estimating unit  160  shown in  FIG. 1 . 
     The MCU  125  may include the first storage unit  170  and the second storage unit  180  shown in  FIG. 1 . The first storage unit  170  and the second storage unit  180  may be non-volatile memory devices such as an electrically erasable programmable read-only memory (EEPROM), a flash memory, a ferroelectric RAM (FeRAM), a magnetoresistive random-access memory (MRAM), a phase-change memory (PRAM), and/or the like. Although the first storage unit  170  and the second storage unit  180  are illustrated as separate elements, the first storage unit  170  and the second storage unit  180  may be included in one memory device. 
     The MCU  125  may communicate with an external device and may transmit the SOH of each of the battery cells  111  to the external device. 
     Referring to  FIG. 2 , the battery pack  100   a  includes the AFE  135  and the MCU  125  as separate elements, but the AFE  135  and the MCU  125  may be integrated in one chip. Further, although the MCU  125  is illustrated as one micro-controller chip, functions of the MCU  125  may be embodied as at least two integrated circuit chips. Furthermore, in an embodiment where the battery pack  100   a  has a hierarchical structure in which a battery module includes a plurality of battery cells, a battery tray includes a plurality of the battery modules, a battery rack includes a plurality of the battery trays, and a battery system includes a plurality of the battery racks, a method of estimating an SOH of a battery cell may be performed by a tray management unit for managing and controlling the battery tray, a rack management unit for managing and controlling the battery rack, and/or a system management unit for managing and controlling the battery system. For example, the system management unit may estimate an SOH of each of the battery cells, based on cell voltage data and current data of each of the battery cells included in the battery system. If there is a monitoring system capable of communicating with the battery system, the monitoring system may estimate the SOH of each of the battery cells. 
       FIG. 3A  illustrates graphs indicating a voltage, current, and a remaining capacity of the battery cell  111 , according to an embodiment of the present invention.  FIG. 3B  illustrates a graph indicating a correlation between an open circuit voltage and an SOC of the battery cell  111 , according to an embodiment of the present invention. With reference to  FIGS. 1 ,  3 A, and  3 B, a method of estimating a current capacity and a current internal resistance of the battery cell  111  according to an embodiment will now be described. 
     The voltage graph of  FIG. 3A  indicates the cell voltage data VD of  FIG. 1  as a function of time, and the current graph of  FIG. 3A  indicates the current data CD of  FIG. 1  as a function of time. The current graph of  FIG. 3A  indicates discharging current of the battery cell  111 , and a negative current value indicates that the battery cell  111  is being charged. The voltage graph and the current graph of  FIG. 3A  are shown in relation to the method of estimating a current capacity and a current internal resistance of the battery cell according to an embodiment. The bottom graph of  FIG. 3A  indicates a remaining capacity of the battery cell  111  as a function of time, wherein the remaining capacity is calculated by integrating current of the battery cell  111 , based on the current data CD.  FIG. 3B  illustrates a graph indicating the correlation between the open circuit voltage and the SOC of the battery cell  111  as stored as the data OSD in the first storage unit  170  shown in  FIG. 1 . 
     First, the method of estimating the current capacity of the battery cell  111  is described below. 
     A time when current of the battery cell  111  is 0 includes a first time t1, a third time t3, and a fifth time t5. In order to estimate a current capacity between the first time t1 and the third time t3 in which the battery cell  111  is discharged, a cell voltage of the first time t1 is determined as a first open circuit voltage OCV1 and a cell voltage of the third time t3 is determined as a third open circuit voltage OCV3. A varied capacity between the first time t1 and the third time t3 may be calculated by integrating current between the first time t1 and the third time t3. Referring to the bottom graph of  FIG. 3A , which is a capacity graph, the varied capacity may be calculated as a difference (Q1−Q3) between a remaining capacity Q1 at the first time t1 and a remaining capacity Q3 at the third time t3. Referring to the graph of  FIG. 3B , an SOC at the first time t1 corresponding to the first open circuit voltage OCV1 is SOC1, and an SOC at the third time t3 corresponding to the third open circuit voltage OCV3 is SOC3. The current capacity of the battery cell  111 , which is calculated based on cell voltage data DV and current data CV between the first time t1 and the third time t3, may be determined as (Q1−Q3)/(SOC1−SOC3). 
     A current capacity of the battery cell  111  may be estimated between the third time t3 and the fifth time t5 in which the battery cell  111  is discharged. A cell voltage at the fifth time t5 is determined as a fifth open circuit voltage OCV5, and referring to the graph of  FIG. 3B , an SOC at the fifth time t5 corresponding to the fifth open circuit voltage OCV5 is SOC5. A varied capacity between the third time t3 and the fifth time t5 may be calculated by integrating current between the third time t3 and the fifth time t5. Referring to the capacity graph, the varied capacity may be calculated as a difference (Q5−Q3) between the remaining capacity Q3 at the third time t3 and a remaining capacity Q5 at the fifth time t5. The current capacity of the battery cell  111 , which is calculated based on cell voltage data DV and current data CV between the third time t3 and the fifth time t5, may be determined as (Q5−Q3)/(SOC5−SOC3). 
     A current capacity of the battery cell  111  may be estimated when the battery cell  111  is being charged, being discharged, or when the battery cell  111  is being sequentially charged and discharged. That is, the current capacity of the battery cell  111  may be estimated between the first time t1 and the fifth time t5. However, because a difference between the SOC at the first time t1 (i.e., SOC1) and the SOC at the fifth time t5 (i.e., SOC5) is small, the current capacity of the battery cell  111 , which is estimated between the first time t1 and the fifth time t5, may be inaccurate. However, when the difference between the SOC at the first time t1 (i.e., SOC1) and the SOC at the fifth time t5 (i.e., SOC5) is equal to or greater than a reference value (e.g., a predetermined value), e.g., 36%, the current capacity of the battery cell  111 , which is estimated between the first time t1 and the fifth time t5, may have sufficient reliability. 
     Hereinafter, the method of estimating the current internal resistance of the battery cell  111  is described below. As an example, it is assumed that a current internal resistance of the battery cell  111  is estimated at each of a second time t2 and a fourth time t4 that are times when current of the battery cell  111  is not 0. 
     At the second time t2, the current of the battery cell  111  is greater than 0, but a SOC of the battery cell  111  is constant between the first time t1 and the second time t2. Although a cell voltage of the battery cell  111  fluctuates at the second time t2 at which discharging starts before the cell voltage is stabilized,  FIG. 3A  does not show the fluctuation. The second time t2 may be selected as a time when the fluctuation disappears and the cell voltage is stabilized. A cell voltage at the second time t2 is determined as a charging-discharging voltage V2, and current at the second time t2 is determined as charging-discharging current I2. In another embodiment, the second time t2 may be defined as a time period, and the cell voltage and the current may be respectively defined as an average of cell voltages and an average of currents during the time period. Because a SOC at the second time t2 is equal to the SOC at the first time t1, an open circuit voltage at the second time t2 is equal to the first open circuit voltage OCV1 at the first time t1. A current internal resistance of the battery cell  111 , which is estimated at the second time t2, may be determined as (OCV1−V2)/I2. 
     Similarly, a current internal resistance of the battery cell  111  may be estimated at the fourth time t4. The current internal resistance of the battery cell  111 , which is estimated at the fourth time t4, may be determined as (V4−OCV5)/I4. Although I4 is a negative number, a value of a current internal resistance is always a positive number, thus, even when the value of the current internal resistance is a negative number, the value of the current internal resistance may be expressed as (e.g., recorded as) a positive number. Similarly, a current internal resistance of the battery cell  111  may be estimated when charging is started or discharging is ended. 
     Hereinafter, according to another embodiment, a method of estimating a current internal resistance of the battery cell  111  is described below. As an example, it is assumed that the current internal resistance of the battery cell  111  is estimated at a sixth time t6. 
     A cell voltage at the sixth time t6 is determined as a charging-discharging voltage V6, and current at the sixth time t6 is determined as charging-discharging current I6. As described above, the SOC of the battery cell  111  at the fifth time t5 is SOC5, and the internal resistance estimating unit  150  may obtain information indicating that the SOC of the battery cell  111  at the fifth time t5 is SOC5 in the same manner as the capacity estimating unit  140 , or may receive, from the capacity estimating unit  140 , the information indicating that the SOC of the battery cell  111  at the fifth time t5 is SOC5. A varied capacity between the fifth time t5 and the sixth time t6 may be calculated by integrating current between the fifth time t5 and the sixth time t6, and referring to the capacity graph, the varied capacity may be calculated as a difference (Q5−Q6) between the remaining capacity Q5 at the fifth time t5 and a remaining capacity Q6 at the sixth time t6. An SOC at the sixth time t6 may be determined based on the calculated varied capacity. SOC6, that is, the SOC of the battery cell  111  at the sixth time t6 may be calculated as SOC5−(Q5−Q6)/current capacity. Referring to the graph of  FIG. 3B , a sixth open circuit voltage OCV6 at the sixth time t6, which corresponds to the SOC (SOC6) at the sixth time t6, may be determined. The current internal resistance of the battery cell  111 , which is estimated at the sixth time t6, may be determined as (OCV6−V6)/I6. 
     As described with reference to the aforementioned methods, the current capacity and the current internal resistance of the battery cell  111  may be estimated based on the cell voltage data VD and the current data CD, and the data OSD on the correlation between the open circuit voltage and the SOC of the battery cell  111 . 
       FIG. 4  illustrates a graph of an SOH estimated according to the one or more embodiments and an SOH estimated according to a comparative example. 
     The SOH according to the comparative example is an SOH of a battery cell that was calculated based on a capacity of the battery cell, which was measured by integrating current flowing into the battery cell while the fully discharged battery cell was being charged. As shown in the graph, the SOH of the battery cell is decreased over time. 
     The SOH according to the one or more embodiments is an SOH of a battery cell, which is estimated in a manner such that a current capacity and a current internal resistance of the battery cell is estimated based on the cell voltage data VD and the current data CD, and then the SOH of the battery cell is estimated based on the estimated current capacity and current internal resistance. As shown in  FIG. 4 , the SOH of the battery cell according to the one or more embodiments also decreases over time, and has a result similar to a result of the SOH according to the comparative example. 
     The SOH estimation according to the comparative example requires full discharging and full charging processes, such that it is difficult to apply the SOH estimation according to the comparative example to a battery pack in operation. However, the SOH estimation according to the one or more embodiments may be applied to a battery pack in operation, does not require an additional circuit device, and may be easily performed. Further, as shown in  FIG. 4 , the SOH result by the SOH estimation according to the one or more embodiments is similar to the SOH result by the SOH estimation according to the comparative example, thus, the SOH estimation according to the one or more embodiments may have sufficient reliability. 
       FIG. 5  is a block diagram of an energy storage device  500  including a battery pack  510 , according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the energy storage device  500  is in conjunction with a power generation system  501  and a grid  502 , thereby supplying power to a load  503 . The energy storage device  500  may be referred to as an energy storage system. 
     The power generation system  501  generates power from an energy source. The power generation system  501  may supply the generated power to the energy storage device  500 . The power generation system  501  may include, but is not limited to, at least one of a photovoltaic generation system, a wind power generation system, or a tidal power generation system. All types of power generation systems that generate power by using renewable energy such as solar heat or terrestrial heat may be included in the power generation system  501 . In particular, a solar cell that generates power by using solar light may be easily mounted in a house or a factory, so that the solar cell may be used with the energy storage device  500  in the house or the factory. The power generation system  501  may configure a large capacity energy system in which a plurality of power generation modules capable of generating power is arrayed in parallel. 
     The grid  502  may include a power generation plant, a substation, a power transmission line, and/or the like. When the grid  502  is in a normal state, the grid  502  may supply power to the energy storage device  500 , i.e., at least one of the load  503  and the battery pack  510 , or may receive power from the energy storage device  500 , in particular, the battery pack  510  or the power generation system  501 . When the grid  502  is in an abnormal state, power transmission between the grid  502  and the energy storage device  500  is discontinued. 
     The load  503  may consume power generated by the power generation system  501 , power stored in the battery pack  510 , and/or power received from the grid  502 . Electric devices in the house or the factory may be examples of the load  503 . 
     The energy storage device  500  may store power, which is generated by the power generation system  501 , in the battery pack  510  or may supply the power to the grid  502 . The energy storage device  500  may supply power stored in the battery pack  510  to the grid  502 , or may store power, which is received from the grid  502 , in the battery pack  510 . Further, when the grid  502  is in the abnormal state, e.g., when a power supply failure occurs, the energy storage device  500  may perform an uninterruptible power supply (UPS) function so that the energy storage device  500  may supply power generated by the power generation system  501  or power stored in the battery pack  510  to the load  503 . 
     In one embodiment, the energy storage device  500  includes a power conversion system (PCS)  520 , the battery pack  510 , a first switch  530 , and a second switch  540 . The PCS  520  may be referred to as a power conversion device. 
     The PCS  520  may convert power, which is supplied by the power generation system  501 , the grid  502 , and/or the battery pack  510 , into power in an appropriate form and may supply the power to the load  503 , the battery pack  510 , and/or the grid  502 . The PCS  520  may include a power conversion unit  521 , a direct current (DC) link unit  522 , an inverter  523 , a converter  524 , and an integrated controller  525 . 
     The power conversion unit  521  may be coupled between (e.g., connected between) the power generation system  501  and the DC link unit  522 . The power conversion unit  521  may convert power, which is generated by the power generation system  501 , into a DC link voltage and may transmit it to the DC link unit  522 . The power conversion unit  521  may include a power conversion circuit such as a converter circuit, a rectifying circuit, and/or the like, depending on types of the power generation system  501 . For example, in an embodiment in which the power generation system  501  generates DC power, the power conversion unit  521  includes a DC-DC converter circuit for converting the DC power generated by the power generation system  501  into another DC power. In an embodiment in which the power generation system  501  generates alternating current (AC) power, the power conversion unit  521  includes the rectifying circuit for converting the AC power into DC power. 
     In an embodiment in which the power generation system  501  is a photovoltaic generation system, the power conversion unit  521  includes a maximum power point tracking (MPPT) converter for performing an MPPT control so as to maximally obtain power generated by the power generation system  501 , according to variation of solar radiation, temperature, and/or the like. When the power generation system  501  does not generate power, the power conversion unit  521  discontinues its operation, so that power consumed by a power conversion device such as the converter or the rectifying circuit may be minimized or decreased. 
     Due to problems such as an instantaneous voltage-drop in the power generation system  501  or the grid  502 , and occurrence of a peak load in the load  503 , a level of a DC link voltage may be unstable. However, it is desired to stabilize the DC link voltage so as to normally operate the converter  524  and the inverter  523 . The DC link unit  522  may be coupled between the power conversion unit  521  and the inverter  523  and may constantly or substantially constantly maintain the DC link voltage. An example of the DC link unit  522  may include a large-capacity capacitor. 
     The inverter  523  may be a power conversion device coupled between the DC link unit  522  and the first switch  530 . The inverter  523  may include an inverter that converts a DC link voltage, which is output from at least one of the power generation system  501  and the battery pack  510 , into an AC voltage of the grid  502  and outputs the AC voltage. Additionally, in order to store power of the grid  502  in the battery pack  510  during a charging mode, the inverter  523  may include a rectifying circuit that converts an AC voltage from the grid  502  into a DC voltage and then outputs a DC link voltage. The inverter  523  may be a bidirectional inverter in which a direction of an input and an output may be changed. 
     The inverter  523  may include a filter to remove harmonics from an AC voltage that is output to the grid  502 . Further, in order to suppress or limit occurrence of reactive power, the inverter  523  may include a phase lock loop (PLL) to synchronize a phase of an AC voltage output from the inverter  523  with a phase of an AC voltage of the grid  502 . Furthermore, the inverter  523  may function to limit a voltage variation range, to improve a power factor, to remove a DC component, and to protect or decrease a transient phenomenon. 
     The converter  524  may be a power conversion device coupled between the DC link unit  522  and the battery pack  510 . The converter  524  may include a DC-DC converter that converts (e.g., DC-DC converts) a DC voltage of power stored in the battery pack  510  into a DC link voltage with an appropriate level and outputs it to the inverter  523  during a discharging mode. The converter  524  may include a DC-DC converter that converts (e.g., DC-DC converts) a voltage of power output from the power conversion unit  521  or from the inverter  523  into a voltage with an appropriate level, i.e., a charging voltage level that is requested by the battery pack  510 , and then outputs the voltage to the battery pack  510  during a charging mode. The converter  524  may be a bidirectional converter in which a direction of an input and an output may be changed. When charging or discharging with respect to the battery pack  510  is not performed, the converter  524  discontinues its operation, so that power consumption may be minimized or decreased. 
     The integrated controller  525  may monitor states of the power generation system  501 , the grid  502 , the battery pack  510 , and the load  503 . For example, the integrated controller  525  may monitor a number of operational parameters, such as, whether or not a power supply failure occurs in the grid  502 , whether or not power is generated by the power generation system  501 , an amount of generated power when the power generation system  501  generates power, an SOC of the battery pack  510 , an amount of power consumption by the load  503 , a time, and/or the like. 
     According to monitoring results and a suitable algorithm (e.g., a preset algorithm), the integrated controller  525  may control operations of the power conversion unit  521 , the inverter  523 , the converter  524 , the first switch  530 , and the second switch  540 . For example, when a power supply failure occurs in the grid  502 , the integrated controller  525  may control power stored in the battery pack  510  or generated by the power generation system  501  to be supplied to the load  503 . In a case where sufficient power cannot be supplied to the load  503 , the integrated controller  525  may assign priority order to (or set priority orders of) the electric devices of the load  503  and may control the load  503  to supply power to the electric devices having high priority orders. The integrated controller  525  may further control charging and discharging of the battery pack  510 . 
     The first switch  530  and the second switch  540  are connected in series between the inverter  523  and the grid  502 , and control a flow of current between the power generation system  501  and the grid  502  by performing an ON or OFF operation in response to a control by the integrated controller  525 . According to states of the power generation system  501 , the grid  502 , and the battery pack  510 , the ON or OFF state of the first switch  530  and the second switch  540  may be determined. In more detail, when power from at least one of the power generation system  501  and the battery pack  510  is supplied to the load  503 , or power from the grid  502  is supplied to the battery pack  510 , the first switch  530  is turned ON. When power from at least one of the power generation system  501  and the battery pack  510  is supplied to the grid  502 , or power from the grid  502  is supplied to at least one of the load  503  and the battery pack  510 , the second switch  540  is turned ON. 
     When a power supply failure occurs in the grid  502 , the second switch  540  is turned OFF and the first switch  530  is turned ON. By doing so, it is possible to supply power from at least one of the power generation system  501  and the battery pack  510  to the load  503  and simultaneously to prevent power, which is supplied to the load  503 , from flowing toward the grid  502 . As described above, the energy storage device  500  operates as a stand-alone system, so that it is possible to prevent an accident in which a person working near a power cable of the grid  502  receives an electric shock from the power generation system  501  or the battery pack  510 . 
     The first switch  530  and the second switch  540  may include a switching device such as a relay capable of enduring or processing high current. 
     The battery pack  510  may store power after receiving the power from at least one of the power generation system  501  and the grid  502 , and may supply stored power to at least one of the load  503  and the grid  502 . The battery pack  510  may include a part for storing power and another part for controlling and protecting the part. Charging and discharging of the battery pack  510  may be controlled by the integrated controller  525 . 
     The battery pack  510  may correspond to the battery packs  100  and  100   a  described with reference to  FIGS. 1 and 2 . The battery pack  510  includes a battery  511  including at least one battery cell, and a battery management unit  512  for controlling charging and discharging of the battery  511 . The battery management unit  512  may include a measuring unit for generating cell voltage data and current data by measuring a cell voltage and current of the battery cell; a capacity estimating unit for generating current capacity data by estimating a current capacity of the battery cell based on the cell voltage data and the current data; an internal resistance estimating unit for generating current internal resistance data by estimating a current internal resistance of the battery cell based on the cell voltage data and the current data; and an SOH estimating unit for estimating an SOH of the battery cell based on the current capacity data and the current internal resistance data. The battery management unit  512  may provide the estimated SOH of the battery cell to the integrated controller  525 . 
     In another embodiment, the battery management unit  512  generates cell voltage data and current data by measuring a cell voltage and current of the battery cell and transmits the cell voltage data and the current data to the integrated controller  525 . The integrated controller  525  may receive the cell voltage data and the current data from the battery management unit  512 . The integrated controller  525  may include a capacity estimating unit for generating current capacity data by estimating a current capacity of the battery cell based on the cell voltage data and the current data; an internal resistance estimating unit for generating current internal resistance data by estimating a current internal resistance of the battery cell based on the cell voltage data and the current data; and an SOH estimating unit for estimating an SOH of the battery cell based on the current capacity data and the current internal resistance data. 
       FIG. 6  is a block diagram of an electric vehicle  600  including a battery pack  610 , according to an embodiment of the present invention. 
     Referring to  FIG. 6 , the electric vehicle  600  may include an electronic control unit (ECU)  621 , an inverter controller  622 , an inverter  623 , a motor  624 , and the battery pack  610 . The battery pack  610  includes a battery  611  including at least one battery cell, and a battery management unit  612  for controlling charging and discharging of the battery  611 . The battery pack  610  may correspond to the battery packs  100  and  100   a  described with reference to  FIGS. 1 and 2 . 
     The battery  611  may include at least one battery cell, may support an output power of the motor  624  by supplying a voltage to the motor  624  in driving the electric vehicle  600 , and may recover and store regenerative braking energy of the motor  624  that operates as a power generator in braking the electric vehicle  600 . The battery  611  may be charged with DC power supplied from a DC charging unit  625  such as a PCS or an energy storage system of a power supply station. The battery  611  may be charged with AC power supplied from an AC charging unit  627  such as a commercial power source. To do so, the electric vehicle  600  may include a power conversion unit  626 . The battery  611  may be coupled to the power conversion unit  626 , and the power conversion unit  626  may convert the AC power supplied from the AC charging unit  627  into DC power. 
     The battery management unit  612  may detect information including a voltage, current, temperature, and/or the like of the battery  611 , may diagnose and manage an SOC of the battery  611 , and may control all operations such as charging and discharging of the battery  611 . The battery management unit  612  may provide information such as a voltage, current, temperature, an SOC, diagnosis information, and/or the like of the battery  611  to the ECU  621  via a communication line, for example, a Controller Area Network (CAN) communication line of the electric vehicle  600 . 
     The battery management unit  612  may include a measuring unit for generating cell voltage data and current data by measuring a cell voltage and current of the battery cell; a capacity estimating unit for generating current capacity data by estimating a current capacity of the battery cell based on the cell voltage data and the current data; an internal resistance estimating unit for generating current internal resistance data by estimating a current internal resistance of the battery cell based on the cell voltage data and the current data; and an SOH estimating unit for estimating an SOH of the battery cell based on the current capacity data and the current internal resistance data. The battery management unit  612  may provide the estimated SOH of the battery cell to the ECU  621 . 
     In another embodiment, the battery management unit  612  generates cell voltage data and current data by measuring a cell voltage and current of the battery cell and transmits the cell voltage data and the current data to the ECU  621 . The ECU  621  may receive the cell voltage data and the current data from the battery management unit  612 . The ECU  621  may include a capacity estimating unit for generating current capacity data by estimating a current capacity of the battery cell based on the cell voltage data and the current data; an internal resistance estimating unit for generating current internal resistance data by estimating a current internal resistance of the battery cell based on the cell voltage data and the current data; and an SOH estimating unit for estimating an SOH of the battery cell based on the current capacity data and the current internal resistance data. 
     The ECU  621  generally controls a vehicle state, a driving mode, and/or the like of the electric vehicle  600 , and helps a driver to stably drive the electric vehicle  600 , in consideration of information about the battery  611  that is provided from the battery management unit  612 . When an SOH of one of the battery cells of the battery  611  are equal to or less than a reference value (e.g., a preset reference value), the ECU  621  may identify (e.g., display) the deteriorating battery cell and/or display the SOH thereof to a manager of the electric vehicle  600 . The manager may take action such as replacement of the deteriorating battery cell and/or the like, so that the electric vehicle  600  may be driven safely. The ECU  621  may control the inverter  623  via the inverter controller  622 . The inverter  623  may provide AC power for driving the motor  624 , by converting DC power supplied from the battery  611  into the AC power. Further, when the electric vehicle  600  brakes, the inverter  623  may convert AC power supplied from the motor  624  into DC power and may provide the DC power to the battery  611 . 
     While  FIG. 6  illustrates the electric vehicle  600  including the battery pack according to the one or more embodiments, the battery pack may be applied to various other vehicles such as hybrid vehicles, electric bicycles, electric motorbikes, and/or the like. 
     As described above, according to the one or more embodiments of the present invention, an SOH of a battery cell may be measured by using a cell voltage and current of the battery cell in a simple and cost-effective manner without using an additional circuit or a complicated algorithm. According to the test results, the measurement result of the SOH of the battery cell may be reliable. In addition, a cell voltage and current of a battery cell that is being discharged to a load or that is being charged by a charging device may be measured, so that an SOH of the battery cell in operation may be measured in real-time without separating the battery cell from the load or the charging device. 
     It should be understood that the example embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
     While one or more embodiments of the present invention have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and equivalents thereof.