Abstract:
A battery protection apparatus is disclosed. In one aspect, the apparatus includes a temperature-dependent resistor electrically connected to at least one battery cell, wherein the temperature-dependent resistor is configured to change internal resistance in a substantially inversely proportional relationship to temperature of one or more of the at least one battery cell. The apparatus further includes a battery protection unit connected between the temperature-dependent resistor and the at least one battery cell, wherein the battery protection unit is configured to block the current flowing through one or more of the at least one battery cell when the current exceeds a first reference value.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Provisional Patent Application No. 61/702,575 filed on Sep. 18, 2012 in the U.S. Patent and Trademark Office, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The described technology generally relates to a battery pack, a controlling method of the same, and an energy storage system including the battery pack. 
         [0004]    2. Description of the Related Technology 
         [0005]    As problems related to environmental destruction and natural resource depletion have arisen, systems for storing power and efficiently utilizing stored power are undergoing active research. Furthermore, renewable energies that are generated without producing significant environmental pollution are the subject of active commercial development. An energy storage system that interconnects such renewable energies, batteries storing power, and existing grid power to one another and is also the subject of research and development efforts. 
         [0006]    In such an energy storage system, efficient management of a battery is one of the most important factors. Secondary (rechargeable) batteries are generally managed by taking into consideration various factors such as charging, discharging and prevention of overheating. Efficient management of a battery generally increases the lifespan of the battery and enables the battery to supply stable power to a load. 
       SUMMARY 
       [0007]    One inventive aspect is an energy storage system including a battery pack, in which a fuse is cut when a battery is overheated. 
         [0008]    Another aspect is a battery protection apparatus, comprising: a temperature-dependent resistor electrically connected to at least one battery cell, wherein the temperature-dependent resistor is configured to change internal resistance in a substantially inversely proportional relationship to temperature of one or more of the at least one battery cell; and a battery protection unit connected between the temperature-dependent resistor and the at least one battery cell, wherein the battery protection unit is configured to block the current flowing through one or more of the at least one battery cell when the current exceeds a first reference value. 
         [0009]    In the above apparatus, the temperature-dependent resistor is a negative temperature coefficient (NTC) thermistor configured to decrease internal resistance when the battery temperature increases. In the above apparatus, the battery protection unit comprises a fuse. 
         [0010]    In the above apparatus, the battery protection unit is a single battery protection unit, wherein the temperature-dependent resistor is a single temperature-dependent resistor, and wherein the at least one battery cell comprises a plurality of battery cells connected to the single battery protection unit and the single temperature-dependent resistor. 
         [0011]    In the above apparatus, the battery cells comprise n battery cells connected in series, wherein n is a positive integer and greater than 1, wherein the battery protection unit is connected to the first of the n battery cells, wherein the temperature-dependent resistor is connected to the nth battery cell, and wherein the battery cells are configured to form a closed-loop with the battery protection unit and the temperature-dependent resistor. In the above apparatus, the battery protection unit comprises a plurality of battery protection units, wherein the temperature-dependent resistor comprises a plurality of temperature-dependent resistors, and wherein the at least one battery cell comprises a plurality of battery cells electrically connected to the battery protection units and the temperature-dependent resistors. 
         [0012]    In the above apparatus, each of the battery cells comprises first and second terminals, wherein each of the battery protection units is connected to the first terminal of at least one of the battery cells, wherein each of the temperature-dependent resistors is connected to the second terminal of at least one of the battery cells, and wherein at least one of the battery cells is configured to form a closed-loop with at least one of the battery protection units and at least one of the temperature-dependent resistors. 
         [0013]    The above apparatus further comprises at least one resistor connected in series with the temperature-dependent resistor. The above apparatus further comprises a battery management system (BMS) electrically connected to the at least one battery cell. The above apparatus further comprises a current fuse electrically connected to the battery protection unit and the BMS, wherein the BMS is configured to detect the temperature of the at least one battery cell and blow the current fuse when the detected temperature exceeds a second reference value. In the above apparatus, the battery protection unit is configured to block the current regardless of whether the BMS operates normally or not. In the above apparatus, the battery protection unit is configured to block the current without separately monitoring temperature of the at least one battery cell. 
         [0014]    Another aspect is a battery protection apparatus, comprising: a temperature-dependent resistor electrically connected to one end of a plurality of battery cells, wherein the temperature-dependent resistor is configured to change internal resistance based at least in part on temperature of at least one of the battery cells; a battery protection unit connected between the temperature-dependent resistor and another opposing end of the battery cells, wherein the battery protection unit is configured to block the current flowing through the battery cells when the current exceeds a reference value, and wherein the temperature-dependent resistor and the battery protection unit are electrically connected to each other without having a resistor connected in parallel. 
         [0015]    In the above apparatus, the temperature-dependent resistor is a negative temperature coefficient (NTC) thermistor configured to decrease internal resistance when the battery temperature increases. In the above apparatus, the battery protection unit is an electrical fuse configured to be blown when current applied thereto exceeds the reference value. The above apparatus further comprises a battery management system (BMS) electrically connected to the battery cells 
         [0016]    Another aspect is an energy storage system, comprising: a plurality of batteries a fuse located adjacent to the batteries; a plurality of external terminals configured to be connected to a load; an NTC thermistor electrically connected to at least one of the batteries; and a resistor connected in series with the NTC thermistor, wherein the batteries, the fuse and the external terminals form a main current path, and wherein at least one of the batteries, the fuse, the resistor and the NTC thermistor form a battery protection path. 
         [0017]    In the above system, the fuse comprises a plurality of fuses, wherein the NTC thermistor comprises a plurality of NTC thermistors, and wherein the batteries are electrically connected to the fuses and the NTC thermistors. In the above apparatus, each of the batteries comprises first and second terminals, wherein each of the fuses is connected to the first terminal of at least one of the batteries, wherein each of the NTC thermistors is connected to the second terminal of at least one of the batteries, and wherein at least one of the batteries is configured to form a closed-loop with at least one of the fuses and at least one of the NTC thermistors. The above apparatus further comprises a battery management system (BMS) electrically connected to the batteries, wherein the fuse, the NTC thermistor and the resistor are separated from the BMS. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  illustrates an energy storage system according to an embodiment. 
           [0019]      FIG. 2  illustrates a battery and a battery management system (BMS). 
           [0020]      FIG. 3  illustrates a battery pack including a battery and a protection circuit according to an embodiment. 
           [0021]      FIG. 4  illustrates a battery pack including a battery and a protection circuit according to another embodiment. 
           [0022]      FIG. 5  is a flowchart showing an operation of a battery pack including a battery and a protection circuit according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Embodiments will now be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like numbers refer to like elements throughout. 
         [0024]      FIG. 1  illustrates an energy storage system  1  according to an embodiment. Referring to  FIG. 1 , the power storage system  1  works with a power generating system  2  and a grid  3  and supplies power to a load  4 . 
         [0025]    The power generating system  2  generates power by using an energy source and supplies the generated power to the energy storage system  1 . The power generating system  2  may include any of various power generating systems for generating power by using renewable energy, e.g., a solar power generating system, a wind power generating system, a tidal power generating system, etc. 
         [0026]    The grid  3  includes a power plant, a substation, and a power line. The grid  3  applies power to the energy storage system  1 , such that power is supplied to the load  4  and/or a battery  30 . Alternatively, the grid  3  receives power from the energy storage system  1 . 
         [0027]    The load  4  consumes power generated by the power generating system  2 , power stored in the battery  30 , or power supplied from the grid  3  and may be a household or a factory, for example. 
         [0028]    The energy storage system  1  may store power generated by the power generating system  2  in the battery  30  and supply the generated power to the grid  3 . Furthermore, the energy storage system  1  may supply power stored in the battery  30  to the grid  3  or store power supplied by the grid  3  in the battery  30 . Furthermore, if power is interrupted at the grid  3 , the energy storage system  1  functions as an uninterruptible power supply (UPS). 
         [0029]    The energy storage system  1  includes a power conversion system (PCS)  10 , a battery management system (BMS)  20 , the battery  30 , and a manual switch  40 . Depending on the implementation, certain elements/blocks may be removed from or additional elements/blocks may be added to the energy storage system  1  illustrated in  FIG. 1 . Furthermore, two or more elements/blocks may be combined into a single element/block, or a single element/block may be realized as multiple elements/blocks. 
         [0030]    The PCS  10  converts power from the power generating system  2 , the grid  3 , or the battery  30  to appropriate power and supplies the converted power to a power-demanding load. The PCS  10  includes a power converting unit  11 , a direct current (DC) linking unit  12 , a two-way inverter  13 , a two-way converter  14 , a first switch  15 , a second switch  16 , and an integrated controller  17 . 
         [0031]    The power converting unit  11  is connected between the power generating system  2  and the DC linking unit  12 . The power converting unit  11  transmits power generated by the power generating system  2  to the DC linking unit  12 , where the output voltage is converted to a DC link voltage. 
         [0032]    The power converting unit  11  may include a converter or a rectifying circuit, according to the type of the power generating system  2 . If the power generating system  2  generates DC power, the power converting unit  11  may be a converter for converting DC power to AC power. If the power generating system  2  generates AC power, the power converting unit  11  may be a converter circuit such as a rectifying circuit for converting AC power to DC power. If the power generating system  2  generates power from solar energy, the power converting unit  11  may include a maximum power point tracking (MPPT) converter which performs MPPT controls for the power generating system  2  to generate the maximum power based on changes of solar radiation and temperature. 
         [0033]    The DC linking unit  12  is connected between the power converting unit  11  and the two-way inverter  13 . The DC linking unit  12  prevents momentary voltage drops of the power generating system  2  or the grid  3  and peak load at the load  4 , thereby maintaining DC link voltage stable. 
         [0034]    The two-way inverter  13  is a power inverter connected between the DC linking unit  12  and the first switch  15 . The two-way inverter  13  inverts a DC link voltage output by the power generating system  2  and/or the battery  30  to AC voltage for the grid  3  and outputs the AC voltage in discharging mode. Furthermore, to store power from the grid  3  in the battery  30  in charging mode, the two-way inverter  13  rectifies AC voltage of the grid  3 , converts the rectified AC voltage to DC link voltage, and outputs the converted DC link voltage. 
         [0035]    The two-way inverter  13  may include a filter for removing harmonics from AC voltage output to the grid  3  and a phase-locked loop (PLL) circuit for synchronizing phase of output AC voltage to phase of AC voltage of the grid  3 . Furthermore, the two-way inverter  13  may perform functions including restriction of a range of voltage changes, phase compensation, DC ingredient removal, transient phenomena protection, etc. 
         [0036]    The two-way converter  14  DC-DC converts power stored in the battery  30  to a voltage level demanded by the two-way inverter  13 , that is, to DC link voltage and outputs the converted DC link voltage in discharging mode. Furthermore, in charging mode, the two-way converter  14  DC-DC converts power output by the power converting unit  11  or power output by the two-way inverter  13  to a voltage level demanded by the battery  30 , that is, charging voltage. 
         [0037]    The first switch  15  and the second switch  16  are connected in series between the two-way inverter  13  and the grid  3  and controls flow of a current between the power generating system  2  and the grid  3  by being turned on and off under the control of the integrated controller  17 . The first and second switches  15  and  16  may be turned on or off based on states of at least one of the power generating system  2 , the grid  3 , and the battery  30 . For example, if the load  4  demands a large amount of power, both of the switches  15  and  16  are turned on, such that power from both the power generating system  2  and the grid  3  may be used. In some embodiments, if power from the power generating system  2  and the grid  3  is insufficient to satisfy power demanded by the load  4 , power stored in the battery  30  may be provided to the load  4 . In another embodiment, if power is interrupted at the grid  3 , the second switch  16  is turned off and the first switch  15  is turned on. Therefore, power from the power generating system  2  or the battery  30  may be supplied to the load  4  and prevent power supplied to the load  4  from flowing to the grid  3 . In other words, islanding may be prevented, and thus accidents including electrification of a worker working on a power line of the grid  3  may be prevented. 
         [0038]    The integrated controller  17  monitors states of the power generating system  2 , the grid  3 , the battery  30 , and the load  4  and controls the power converting unit  11 , the two-way inverter  13 , the two-way converter  14 , the first switch  15 , the second switch  16 , and the BMS  20  based on a result of the monitoring. The integrated controller  17  may monitor various facts, such as whether power is interrupted at the grid  3  and whether power is generated by the power generating system  2 . Furthermore, the integrated controller  17  may monitor the amount of power generated by the power generating system  2 , charging state of the battery  30 , the amount of power consumed by the load  4 , time, etc. 
         [0039]    In some embodiments, the BMS  20  is connected to the battery  30  and controls charging and discharging of the battery  30  under the control of the integrated controller  17 . The BMS  20  may perform functions including, but not limited to, overcharging protection, over-discharging protection, overcurrent protection, overvoltage protection, and overheat protection to protect the battery  30 . To this end, the BMS  20  may monitor at least one of voltage, current, temperature, remaining power, lifespan, and charging state of the battery  30  and transmit a result of the monitoring to the integrated controller  17 . 
         [0040]    The battery  30  receives and stores power generated by the power generating system  2  or power from the grid  3  and supplies stored power to the load  4  or the grid  3 . The battery  30  may include at least one battery racks connected in series and/or in parallel. Here, the battery rack is a sub-component constituting the battery  30 . Furthermore, each of the battery rack may include at least one battery trays connected in series and/or in parallel. 
         [0041]    Here, the battery tray is a sub-component constituting the battery rack. Furthermore, each of the battery trays may include a plurality of battery cells. The battery  30  may be embodied of any of various types of battery cells. For example, the battery  30  may be a nickel-cadmium battery, a lead storage battery, a nickel metal hydride (NiMH) battery, a lithium ion battery, a lithium polymer battery, etc. 
         [0042]      FIG. 2  illustrates a battery  300  and a BMS  200 . 
         [0043]      FIG. 2  shows a circuit including the BMS  200 , the battery  300 , a terminal unit  410 , charging/discharging control switches  420  and  430 , and a high current fuse  450 . 
         [0044]    The battery  300  may include one or more battery cells  300 - 1  through  300 - n  . As described above, the battery  300  receives and stores power from external sources or supplies power to an external system or a load. As described above, the BMS  200  controls states of the battery  300 . The terminal unit  410  includes at least a positive electrode terminal  410   a  and a negative electrode terminal  410   b.  Power stored in the battery  300  may be supplied to an external system or a load via the terminal unit  410 . Furthermore, external power may be supplied to the battery  300  via the terminal unit  410  and charge the battery  300 . When the battery  300  is used in a mobile device, the terminal unit  410  may be connected to the mobile device or a charger. Alternatively, if the battery  300  is used in the energy storage system  1 , the terminal unit  410  may be connected to the two-way converter  14   
         [0045]    Furthermore, the charging control switch  420  or the discharging control switch  430  receives a charging control signal or a discharging control signal from the BMS  200  and blocks or connects charging/discharging path of the battery  300 . 
         [0046]    In some embodiments, to protect the battery  300 , the high current fuse  440  receives a signal from the BMS and blocks a high current path. For example, the BMS  200  may cut the high current fuse  440  if the battery  300  may be overheated. The high current fuse  450  may be replaced with the charging control switch  420  and the discharging control switch  430 . 
         [0047]    When the battery  300  is overheated in the  FIG. 2  battery pack, the BMS  200  detects temperature of the battery  300  and cuts the high current fuse  450  to protect the battery  300 . In other words, the BMS  200  continuously monitors temperature of the battery  300 . As shown in  FIG. 2 , the BMS  200  may receive values of temperature of the battery  300  via terminals V 1 , V 2 , . . . , and Vn. To this end, the BMS  200  may include a thermistor therein. 
         [0048]    A thermistor is a type of resistor and is an electric device using the principle that resistance of a material is changed according to temperature. The thermistor is also referred to as a thermally-varying resistor and is used for preventing a current of a circuit from exceeding a predetermined level or as a sensor for detecting temperature of a circuit. 
         [0049]    A thermistor is generally formed of a polymer or a ceramic material and is capable of detecting a temperature between about −90° C. and about 130° C. at high precision. It is the difference between a thermistor and a resistance thermometer which detects high temperature by using a pure metal. 
         [0050]    Thermistors may be categorized into two types according to degrees of temperature changes with respect to changes of temperatures. If resistance of a thermistor increases according to temperature, the thermistor is referred to as a positive temperature coefficient (PTC) thermistor. If resistance of a thermistor decreases when temperature increases, the thermistor is referred to as a negative temperature coefficient (NTC) thermistor. A general resistor, which is not a thermistor, is adjusted to exhibit little resistance changes according to temperatures. 
         [0051]    The BMS  200  detects temperature of the battery  300  based on a PTC thermistor or an NTC thermistor and blocks power supplied to the battery  300  or power supplied by the battery  300  by transmitting a signal to the high current fuse  440  if the battery  300  is being overheated. 
         [0052]    However, when the BMS  200  detects temperature of the battery  300  and cuts the high current fuse  440 , if the BMS  200  stops operation, the battery  300  cannot be protected even if the battery  300  is overheated. The BMS  200  may not operate normally due to deterioration or malfunction. 
         [0053]      FIG. 3  illustrates a battery pack including a battery and a protection circuit according to an embodiment. Hereinafter, a combination of the battery  30   a  and the protection circuit  40   a  controlling the same will be referred to as a battery pack. Referring to  FIG. 3 , the battery pack according to the present embodiment includes the battery  30   a  and the protection circuit  40   a  including BMS  20 . The battery  30   a  may include one or more battery cells  31 - 1  through  31 - n.  The battery  30   a  is connected to the protection circuit  40   a,  so that the battery  30   a  may supply power to an external system or a load or receive external power. 
         [0054]    Meanwhile, if the battery  30   a  is used in the energy storage system  1 , the reference numerals  31 - 1  through  31 - n  shown in  FIG. 3  may denote individual battery racks or battery trays constituting the battery  30   a.  Description will be given below under an assumption that the reference numerals  31 - 1  through  31 - n  denote a plurality of battery cells. However, the descriptions below may also be applied even when the reference numerals  31 - 1  through  31 - n  denote a plurality of battery trays or a plurality of battery racks. 
         [0055]    The protection circuit  40   a  controls charging and discharging of the battery  30   a  and controls components in a battery pack for stable operation. The protection circuit  40   a  may include a terminal unit  41 , a BMS  20 , a charging control switch  42 , a discharging control switch  43 , a resistor unit  44   a,  and a high current fuse  45 . 
         [0056]    The terminal unit  41  includes at least a positive electrode terminal  41   a  and a negative electrode terminal  41   b.  Power stored in the battery  30   a  may be supplied to an external system or a load via the terminal unit  41 . Furthermore, external power may be supplied to the battery  30   a  via the terminal unit  41  to charge the battery  30   a.  If the battery  30   a  is used in a mobile device, the terminal unit  41  may be connected to the mobile device or a charger. Alternatively, if the battery  30   a  is used in the energy storage system  1 , the terminal unit  41  may be electrically connected to the two-way converter  14  for power conversion. 
         [0057]    The BMS  20  monitors charging or discharge status of the battery  30   a,  current flow in a battery pack, etc., and performs charging control or discharging control. The BMS  20  may include a power terminal VDD, a ground terminal VSS, a charging control terminal CHG, a discharging control terminal DCG, at least one voltage detecting terminals V 1  through Vn, and a fuse control terminal FC. 
         [0058]    Power voltage and ground voltage are applied to the power terminal VDD and the ground terminal VSS, respectively. When there is a problem in the battery  30   a,  the charging control terminal CHG or the discharging control terminal DCG output a charging control signal for controlling operation of the charging control switch  42  or a discharging control signal for controlling operation of the discharging control switch  43 . Furthermore, if the battery  30   a  may be overheated, the fuse control terminal FC outputs a fuse control signal for controlling the high current fuse  45 . 
         [0059]    In some embodiments, at least one of the voltage detecting terminals V 1  through Vn measures intermediate voltage of the battery  30   a.  In other words, the voltage detecting terminals V 1  through Vn are electrically connected to a node between the battery cells  31 - 1  through  31 - n  and measure voltages of the battery cells  31 - 1  through  31 - n.    
         [0060]    Each of the charging control switch  42  and the discharging control switch  43  may include a field effect transistor FET and a parasitic diode. For example, the charging control switch  42  includes a field effect transistor FET 1  and a parasitic diode D 1 , whereas the discharging control switch  43  includes a field effect transistor FET 2  and a parasitic diode D 2 . A connecting direction between a source and a drain of the field effect transistor FET 1  of the charging control switch  42  is set to be opposite as compared to the field effect transistor FET 2  of the discharging control switch  43 . Here, the field effect transistors FET 1  and FET 2  of the charging control switch  42  and the discharging control switch  43  are switching devices. However, the present invention is not limited thereto, and any of various other types of electric switching devices may be used. For example, if the battery  30   a  is used in the energy storage system  1 , since a current flowing on a high current path is very large, a relay may be used. The switching devices may also include bipolar transistors, other digital or analog switches. 
         [0061]    The resistor unit  44   a  includes a fuse FUSE, a resistor R, and a thermistor resistor Rth. Instead of or in addition to the fuse FUSE, the resistor unit  44   a  may include other battery protection unit such as a circuit breaker. The thermistor resistor Rth may be located in adjacent or contact to the battery  30   a  to detect temperature. The thermistor resistor Rth included in the resistor unit  44   a  may be an NTC thermistor. Furthermore, if a current exceeding a reference value flows, the fuse FUSE may insulate two opposite ends thereof. 
         [0062]    Referring to  FIG. 3 , the resistor unit  44   a  may form a closed-loop with the battery  30   a.  In some embodiments, the battery  30   a,  the fuse FUSE and terminals  41   a  and  41   b  form a main current path whereas the battery  30   a  and the resistor unit  44   a  form a battery protection path. 
         [0063]    In one embodiment, if the battery  30   a  is overheated, since the thermistor resistor Rth is an NTC thermistor, the resistance of the thermistor resistor Rth decreases. When the resistor unit  44   a  forms a closed-loop with the battery  30   a,  current flowing in the fuse FUSE is inversely proportional to resistance of a resistor device. For example, the smaller the sum of resistances of the resistor R and the thermistor resistor Rth, the larger the current flowing in the fuse FUSE. 
         [0064]    Therefore, if the battery  30   a  is overheated, the resistance of the thermistor resistor Rth, which is an NTC thermistor, decreases, and thus a current exceeding a reference value flows in the fuse FUSE. The fuse FUSE may block a high current path via which the battery  30   a  exchanges power with an external device. 
         [0065]    In some embodiments, even if the BMS  20  does not monitor the state of the battery  30   a  and transmit a signal to the high current fuse  45 , overheating of the battery  30   a  may be automatically detected and the fuse FUSE may block a high current path via which the battery  30   a  exchanges power with an external device. That is, regardless of whether the BMS  20  operates normally or not, the battery  30   a  can be protected from a high current. Of course, while the BMS  20  is normally operating, the BMS  20  may transmit a signal to the high current fuse  45  to block the high current path. 
         [0066]      FIG. 4  illustrates a battery pack including a battery and a protection circuit according to another embodiment. Since the embodiment shown in  FIG. 4  is a modification of the  FIG. 3  embodiment, any repeated description will be omitted and only distinguishing features of the  FIG. 4  embodiment will be described below. 
         [0067]    Referring to  FIG. 4 , a battery pack according to the present embodiment includes the battery  30   b  and the protection circuit  40   b.  The battery  30   b  may include one or more battery cells  31 - 1  through  31 - n.  wherein the batteries, the fuse and the external terminals form a main current path, and wherein at least one of the batteries  31 - 1  to  31 - n,  at least one of the fuses F 1  to Fn, at least one of the resistors R 1  to Rn and at least one of the NTC thermistors Rth 1  to Rthn form a battery protection path. 
         [0068]    Same as in the previous embodiment, the battery  30   b  is connected to the protection circuit  40   b  and may either supply power to an external system or a load or receive external power. Furthermore, first through n th  fuses F 1  through Fn are respectively connected in series to the battery cells  31 - 1  through  31 - n  in the battery  30   b.    
         [0069]    The protection circuit  40   b  controls charging and discharging of the battery  30   b  and controls components in a battery pack for stable operation. Same as in the previous embodiment, the protection circuit  40   b  may include the terminal unit  41 , the BMS  20 , the charging control switch  42 , the discharging control switch  43 , a resistor unit  44   b,  and the high current fuse  45 . 
         [0070]    In the present embodiment, the resistor unit  44   b  includes first through n th  resistors and first through n th  thermistor resistors Rth 1  through Rthn. The first through n th  thermistors Rth 1  through Rthn included in the resistor unit  44   b  may be NTC thermistors. 
         [0071]    In some embodiments, the resistances of the first through n th  thermistor resistors Rth 1  through Rthn respectively connected to the battery cells  31 - 1  through  31 - n  are changed according to temperatures of the battery cells  31 - 1  through  31 - n  and change currents flowing in the first through n th  fuses F 1  through Fn. In this embodiment, when currents exceeding a reference value flowing in the first through n th  fuses F 1  through Fn, the first through nth fuses are cut. 
         [0072]    For example, referring to  FIG. 4 , the first battery cell  31 - 1 , the first fuse F 1 , the first resistor R 1 , and the first thermistor resistor Rth 1  form a closed circuit. If the first battery cell  31 - 1  is overheated, the resistance of the first thermistor resistor Rth 1 , which is an NTC thermistor, decreases. Therefore, a current flowing in the closed circuit including the first battery cell  31 - 1  increases, and, when the current exceeds a reference value, the first fuse F 1  is cut to protect the first battery cell  31 - 1 . 
         [0073]    According to an embodiment, if the battery cells  31 - 1  through  31 - n  are normal, the sums of resistances of the first through n th  resistors R 1  through Rn and resistances of the first through n th  thermistors Rth 1  through Rthn are relatively large values, and thus a closed loop including the battery cells  31 - 1  through  31 - n  may operate as an open circuit. For example, if the first battery cell  31 - 1  is normal, the sum of resistances of the first resistor R 1  and the first thermistor Rth 1  may be large enough for a closed loop including the first battery cell  31 - 1  and the first resistor R 1  to operate as an open circuit. Of course, as described above, if the first battery cell  31 - 1  is overheated, the resistance of the first thermistor Rth 1  decreases, and thus a current flows in the closed loop, where the first fuse F 1  may be blown when the current exceeds a reference value. 
         [0074]    Accordingly, when the battery cells  31 - 1  through  31 - n  included in the battery  30   b  according to the present embodiment are overheated, the overheating may be detected and the first through n th  fuses F 1  through Fn respectively connected to the battery cells  31 - 1  through  31 - n  may be cut so that the battery  30   b  is protected against receiving an overly high current. 
         [0075]      FIG. 5  is a flowchart showing an operation of a battery pack including a battery and a protection circuit according to an embodiment. Depending on the embodiment, additional states may be added, others removed, or the order of the states changes in  FIG. 5 . In  FIG. 5 , the term ‘battery cell’ may be replaced with the term ‘battery.’ 
         [0076]    First, at least one of a plurality of battery cells of the battery is overheated (operation S 1 ). When the at least one battery cell is overheated, the resistance of an NTC thermistor electrically connected to the corresponding battery cell(s) decreases (operation S 2 ). Alternatively, the resistance of an NTC thermistor electrically connected to the entire battery may decrease. 
         [0077]    When the resistance of the NTC thermistor decreases, a current exceeding a reference value flows in a closed loop including the corresponding battery cell (operation S 3 ). A fuse connected to the corresponding battery cell is cut to protect the batter (operation S 4 ). According to at least one of the disclosed embodiments, in an energy storage system, when a battery overheats, the resistance of a negative temperature coefficient (NTC) thermistor is changed, and thus amount of a current flowing in a circuit including the battery and a fuse increases. As the amount of the current increases, the fuse blows to protect the battery. 
         [0078]    While the above embodiments have been described with reference to the accompanying drawings, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The disclosed embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description but by the appended claims.