Abstract:
A battery rack includes: a plurality of battery packs; a plurality of slave battery management systems, each of the slave battery management systems being coupled to a corresponding one of the battery packs, being powered by an operating power, and being configured to use the corresponding one of the battery packs for the operating power in response to receiving a corresponding one of a plurality of operating power changing signals; and a master battery management system coupled to the slave battery management systems and configured to transmit the operating power changing signals to the slave battery management systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of U.S. Provisional Application No. 61/826,939, filed on May 23, 2013 in the U.S. Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to a battery management system and a method of driving the same. 
     2. Description of the Related Art 
     As environmental destruction and resource depletion become more significant, an interest in a system for storing power and efficiently using the stored power has increased. 
     A power storage system may store generated power of new and renewable energy in a battery or may store power of a commercial system in a battery with relation to the commercial system. The power storage system may supply power stored in the battery to the commercial system or to a load. 
     A rechargeable secondary battery may be used for the power storage system in order to store power. However, due to limitations on a capacity of the secondary battery, a number of secondary batteries may be coupled in parallel or may be serially coupled to form a battery pack. There exists a variation in capacities of battery cells that form the battery pack due to various causes in manufacturing processes. 
     Therefore, in the battery pack, a variation is generated in charge and discharge voltages of the battery cells in a charge and discharge cycle. Therefore, in the battery pack, a specific battery cell may be overcharged during charge and a specific battery cell may be over-discharged during discharge. As described above, when a specific battery cell is overcharged or over-discharged in the battery pack, a capacity of the battery pack is reduced, the battery pack is deteriorated, and a lifespan of the battery pack is reduced. 
     Therefore, a cell balancing operation that keeps uniformity of voltages of the battery cells may be performed. However, the cell balancing operation is performed based on a voltage of a battery cell having a lowest cell voltage so that energy of battery cells having relatively high voltages may be lost. 
     Hereinafter, a battery management system for reducing (or preventing) energy from being lost by the cell balancing operation is suggested. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of embodiments of the present invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. 
     SUMMARY 
     Embodiments of the present invention provide a battery management system for more efficiently using power stored in a battery pack and a method of driving the same. 
     In addition, embodiments of the present invention may provide a battery management system for reducing a time for which a cell balancing operation is performed and a method of driving the same. 
     Aspects of the present invention are not limited to the above, but other aspects that are not described may be clearly understood by those skilled in the art from the following description. 
     According to one embodiment of the present invention, there is provided a battery rack including: a plurality of battery packs; a plurality of slave battery management systems, each of the slave battery management systems being coupled to a corresponding one of the battery packs, being powered by an operating power, and being configured to use the corresponding one of the battery packs for the operating power in response to receiving a corresponding one of a plurality of operating power changing signals; and a master battery management system coupled to the slave battery management systems and configured to transmit the operating power changing signals to the slave battery management systems. 
     The slave battery management systems may be configured to measure information related to charging parameters of the battery packs and to transmit the measured information to the master battery management system. 
     The master battery management system may be configured to selectively transmit the operating power changing signals according to comparison results of states of charge or of voltages of the battery packs. 
     The master battery management system may be configured to transmit the corresponding one of the operating power changing signals to a slave battery management system of the slave battery management systems coupled to a battery pack of the battery packs having a relatively high state of charge or a relatively high voltage as determined by the comparison results. 
     The battery pack having the relatively high state of charge or the relatively high voltage may have a state of charge or a voltage higher than a state of charge or a voltage of another battery pack of the plurality of battery packs by more than a threshold state of charge or a threshold voltage. 
     Each of the slave battery management systems may be configured to selectively change a power source for the operating power between an external power source and the corresponding one of the battery packs in response to receiving the corresponding one of the operating power changing signals. 
     Each of the slave battery management systems may include a switch unit configured to select one of the external power source or the corresponding one of the battery packs as the power source for the operating power. 
     The switch unit may include a first relay and a second relay, and the switch unit may be configured to select one of the corresponding one of the battery packs or the external power source using the first relay or the second relay, respectively. 
     Each of the plurality of slave battery management systems may include a voltage converter coupled between the corresponding one of the battery packs and the second relay, and the voltage converter may be configured to convert a voltage received from the corresponding one of the plurality of battery packs to correspond to the operating power of the slave battery management system. 
     The corresponding one of the battery packs may include a plurality of battery cells, and each of the slave battery management systems may be configured to receive one of a plurality of cell balancing signals from the master battery management system and to perform a cell balancing operation on the battery cells in response to the cell balancing signal. 
     According to another embodiment of the present invention, there is provided a method of operating a battery rack comprising: selectively transmitting a plurality of operating power changing signals from a master battery management system; receiving each of the transmitted operating power changing signals at a corresponding one of a plurality of slave battery management systems, each of the slave battery management systems being powered by an operating power source and being coupled to the master battery management system; and using, as the operating power source, a corresponding one of a plurality of battery packs by the corresponding one of the slave battery management systems that receives the transmitted operating power changing signal. 
     The method of operating the battery rack may further include: measuring battery pack information related to charging parameters of the battery packs by the plurality of slave battery management systems; and transmitting the measured battery pack information to the master battery management system. 
     The selectively transmitting may occur according to comparison results of states of charge or of voltages of the battery packs. 
     The selectively transmitting may include transmitting the operating power changing signals to slave battery management systems that are coupled to battery packs having relatively high states of charge or relatively high voltages as determined by the comparison results. 
     The battery pack having the relatively high state of charge or the relatively high voltage may have a state of charge or a voltage higher than a state of charge or a voltage of another battery pack of the plurality of battery packs by more than a threshold state of charge or a threshold voltage. 
     The using the corresponding one of the battery packs as the operating power source may include selectively changing the operating power source between an external power source and the corresponding one of the battery packs in response to the corresponding one of the slave battery management systems receiving the transmitted operating power changing signal. 
     The selectively changing the operating power source may include operating a corresponding switch unit configured to select one of the external power source or the corresponding one of the plurality of battery packs as the operating power source. 
     The operating the corresponding switch unit may include driving a first relay or a second relay to select the corresponding one of the battery packs or the external power source, respectively. 
     The method of operating the battery rack may further include converting a voltage received from the external power source or the corresponding one of the battery packs to an operating voltage of the slave battery management systems. 
     The method of operating the battery rack may further include: selectively sending a plurality of cell balancing signals from the master battery management system; receiving a corresponding one of the sent cell balancing signals at a cell balancing slave battery management system of the plurality of slave battery management systems coupled to a corresponding cell balancing battery pack of the plurality of battery packs; and selectively performing a cell balancing operation on a plurality of battery cells in the cell balancing battery pack in response to receiving the cell balancing signal at the cell balancing slave battery management system. 
     Aspects of the battery management system according to embodiments of the present invention will be described as follows. 
     According to at least one example embodiment of the present invention, power stored in a battery pack may be more efficiently used. 
     In addition, according to at least one example embodiment of the present invention, a time for which a cell balancing operation is performed may be reduced. Aspects of the present invention are not limited to the above, but other aspects that are not described may be clearly understood by those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a power storage system according to an example embodiment of the present invention. 
         FIG. 2  is a view illustrating an internal structure of a power storage apparatus according to an example embodiment of the present invention. 
         FIG. 3  is a view illustrating a battery management system according to an example embodiment of the present invention. 
         FIG. 4  is a view illustrating a slave battery management system (BMS) to which driving power is applied according to an example embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating a method of driving a battery management system according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the present invention are shown. This present invention may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure is thorough, and will fully convey the scope of embodiments of the present invention to those skilled in the art. In the drawings, the size and relative sizes of elements may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     Hereinafter, a power storage system according to an example embodiment of the present invention and a method of driving the same will be described in detail. 
       FIG. 1  is a view illustrating a power storage system according to an example embodiment of the present invention. 
     A power storage system  100  according to an example embodiment of the present invention is coupled between a power generating system  200  and a commercial system  300 . 
     The power generating system  200  may include a system for generating electrical energy using new and renewable energy such as solar light, wind power, wave power, tidal power, geothermal heat, and/or the like. The power generating system  200  may include a commercial system as well as the new and renewable energy. 
     The commercial system  300  may include a power station for generating power through firepower, waterpower, and/or nuclear power generation and may include a substation and a transmission site for changing a property of a voltage or a current in order to transmit generated power through a power transmission line or a power distribution line. 
     In  FIG. 1 , the power storage system  100  is coupled to the commercial system  300 . However, the commercial system  300  may be replaced by a load. The load refers to various electrical devices that use power (e.g., home appliances or producing facilities of a factory). 
     As illustrated in  FIG. 1 , the power storage system  100  according to the example embodiment of the present invention includes a first power converting unit (or a first power converter)  120 , a second power converting unit (or a second power converter)  140 , and a power storage apparatus  160 . 
     The first power converting unit  120  is coupled to the power generating system  200  to convert first power generated by the power generating system  200  into second power and to transmit the second power to a node N 1 . The first power generated by the power generating system  200  may be direct current (DC) power or alternate current (AC) power, and power of the node N 1  is DC power. That is, the first power converting unit  120  may be a DC-DC converter for converting first DC power into DC power or an AC-DC converter for converting first AC power into DC power. 
     The second power converting unit  140  is coupled between the node N 1  and the commercial system  300 . The second power converting unit  140  converts the DC power of the node N 1  into AC power to transmit the AC power to the commercial system  300 . That is, the second power converting unit  140  may be a DC-AC converter for converting DC power into AC power. 
     The power storage apparatus  160  stores the DC power of the node N 1 . The power storage apparatus  160  may supply the stored DC power to the node N 1  again during power failure. During power failure, the DC power supplied from the power storage apparatus  160  to the node N 1  is converted by the second power converting unit  140  to be transmitted to the commercial system  300  or to a load. Therefore, even during power failure, stable power may be supplied (e.g., always supplied) to the commercial system  300  or to the load. 
     The power storage apparatus  160  may not only supply the DC power to the node N 1  during power failure, but may always supply the stored power to the node N 1 . In this case, the power supplied from the power generating system  200  may be transmitted (e.g., always transmitted) to the commercial system  300  or the load through the power storage apparatus  160 . 
     Referring to  FIG. 2 , the power storage apparatus  160  according to an example embodiment of the present invention will be described in detail. 
       FIG. 2  is a view illustrating an internal structure of the power storage apparatus  160  according to the example embodiment of the present invention. 
     As illustrated in  FIG. 2 , the power storage apparatus  160  according to the example embodiment of the present invention includes a plurality of battery racks  160 A and  1608 , an AC-DC converting unit (or AC-DC converter)  164 , a plurality of main switches S 1 A, S 1 B, S 2 A, and S 2 B, a plurality of charge switches S 3 A and S 3 B, and diodes D 1 A and D 1 B. In  FIG. 2 , flow paths of voltage and current are illustrated by solid lines and flows of a measuring signal and a switching control signal of a battery management system are illustrated by dotted lines. 
     The first battery rack  160 A includes a plurality of battery packs  161 A, a plurality of slave battery management systems  162 A (hereinafter, referred to as ‘slave BMS’), and a master battery management system  163 A (hereinafter, referred to as ‘master BMS’). 
     The plurality of battery packs  161 A are serially coupled to each other to be coupled to a positive potential output end (+) and a negative potential output end (−) of the battery rack  160 A. Power lines are coupled to the positive potential output end (+) and the negative potential output end (−) of the battery rack  160 A, respectively. That is, the plurality of serially coupled battery packs  161 A output power to the power lines through the positive potential output end (+) and the negative potential output end (−) of the battery rack  160 A. 
     The battery packs  161 A include a plurality of cells serially coupled to each other or coupled to each other in parallel. Here, a cell as a rechargeable secondary battery may include a nickel-cadmium battery, a lead battery, a nickel metal hydride battery, a lithium ion battery, and/or a lithium polymer battery. 
     The plurality of slave BMSs  162 A manage charge and discharge of the battery packs  161 A, respectively, and the master BMS  163 A manages charge and discharge of the battery rack  160 A. In  FIG. 2 , the slave BMSs  162 A are provided in the battery packs  161 A, respectively. However, the slave BMSs  162 A may be provided to manage charge and discharge of the plurality of battery packs  161 A. 
     The slave BMSs  162 A may measure states of the plurality of battery packs  161 A. The slave BMSs  162 A may measure voltages, currents, or temperatures of the cells included in the battery packs  161 A, respectively. The slave BMSs  162 A may transmit information (hereinafter, referred to as battery state information) on measured states of the battery packs  161 A to the master BMS  163 A. 
     The master BMS  163 A may estimate states of charge (SOC) and states of health (SOH) of the respective cells or battery packs through the battery state information received from the respective slave BMSs  162 A. Thus, the master BMS  163 A may control charge and discharge of the battery rack  160 A. 
     In addition, the master BMS  163 A may control the cells included in the battery packs  161 A to perform cell balancing operations using the battery state information. 
     To be specific, the master BMS  163 A may output a cell balancing signal to the respective slave BMSs  162 A. Then, the slave BMSs  162 A may perform cell balancing operations using a passive cell balancing method of emitting power of cells having relatively high states of charge (SOC) through balancing resistance (i.e., resistance). The slave BMSs may also perform cell balancing operations using an active cell balancing method of supplying power of cells having relatively high SOC to cells in relatively low SOC. 
     Furthermore, the master BMS  163 A may detect whether or not there is something wrong with voltages and currents of the respective battery packs  161 A or the battery rack  160 A through the battery state information transmitted from the respective slave BMSs  162 A. The master BMS  163 A transmits a switching control signal to the main switches S 1 A and S 2 A so that the main switches S 1 A and S 2 A are blocked (e.g., open) when it is determined that there is something wrong with the respective battery packs  161 A or the battery rack  160 A to protect a battery. 
     Then, the master BMS  163 A generates a switching control signal for controlling turn-on/turn-off of the charge switch S 3 A to transmit the switching control signal to the charge switch S 3 A. The master BMS  163 A according to the example embodiment of the present invention turns on the charge and discharge switch S 3 A when the battery rack  160 A is to be charged and turns off the charge and discharge switch S 3 A when a charge operation is completed. 
     Furthermore, when there is something wrong with the master BMS  163 A, one of the plurality of slave BMSs  162 A may function as the master BMS  163 A. The slave BMS  162 A that functions as the master BMS  163 A may detect whether there is something wrong with the voltage and current of the battery rack  160 A to control the main switches S 1 A and S 2 A. 
     The second battery rack  160 B includes a plurality of battery packs  161 B, a plurality of slave BMSs  162 B, and a master BMS  163 B like the first battery rack  160 A. Since an internal structure of the second battery rack  1608  and functions of the respective elements of the second battery rack  160 B are the same or substantially the same as those of the first battery rack  160 A, detailed description thereof may be omitted. 
     The AC-DC converting unit  164  receives an AC voltage from the commercial system and converts the received AC voltage into a DC voltage VSS to transmit the DC voltage VSS to the master BMS  163 A and the master BMS  163 B. The DC voltage VSS output from the AC-DC converting unit  164  is used as operating power for operating the master BMS  163 A and the master BMS  163 B. In addition, the DC voltage VSS may be transmitted to the respective slave BMSs  162 A and  162 B to be used as operating power for operating the respective slave BMSs  162 A and  162 B. 
     Furthermore, in relation to the operating power of the slave BMSs  162 A, before performing the above-described cell balancing operation or while performing the above-described cell balancing operation, use of the DC voltage VSS as the operating power of the slave BMSs  162 A may be stopped and power stored in the battery packs  161 A managed by the slave BMSs  162 A may be used. 
     To be specific, the master BMS  163 A outputs an operating power changing signal so that the power stored in the respective battery packs  161 A is used as operating power of the respective slave BMSs  162 A. At this time, power of battery packs  161 A having relatively high voltages may be used as the operating power of the slave BMSs so that voltages of the plurality of battery packs managed by the master BMS  163 A are uniform. At this time, it is assumed that a voltage of a battery pack is the sum of voltages of battery cells included in the battery pack  161 A. 
     The master BMS  163 A may compare voltages or SOC of the plurality of battery packs  161 A with each other using the battery state information. The master BMS  163 A may generate the operating power changing signal using a comparison result. 
     The master BMS  163 A may output the operating power changing signal to the first slave BMS  162 A having a relatively high voltage or SOC (e.g., coupled to a battery pack  161 A having a relatively high voltage or SOC) so that the voltages or SOC of the plurality of battery packs  161 A are uniform or substantially uniform. 
     Then, the first slave BMS  162 A may stop using the operating power of the DC voltage VSS received from the master BMS  163 A and may use power stored in the first battery pack  161 A as operating power in accordance with or in response to the operating power changing signal. Hereinafter, operating power changing processes of the above-described slave BMSs will be described by battery pack balancing. 
     One end of the main switch S 1 A is coupled to the positive potential output end (+) of the first battery rack  160 A. One end of the charge switch S 3 A may be coupled to the other end of the main switch S 1 A and the other end of the charge switch S 3 A may be coupled to the node N 1 . An anode of a diode D 1 A may be coupled to one end of the charge switch S 3 A and a cathode of the diode D 1 A may be coupled to the other end of the charge switch S 3 A. The main switch S 2 A may be coupled between the negative potential output end (−) of the first battery rack  160 A and the node N 1 . 
     Here, each of the main switches S 1 A and S 2 A may be maintained in a turn-on state during charge/discharge to form a charge channel and a discharge channel. The main switches S 1 A and S 2 A may be turned off in order to block a voltage and a current output from the positive potential output end (+) and the negative potential output end (−) of the first battery rack  160 A when there is something wrong with the first battery rack  160 A. 
     Since the first battery rack  160 A to which the plurality of battery packs  161 A are serially coupled may output a high voltage and a high current of about 1 kV and 300 A, respectively, the main switches S 1 A and S 2 A may be realized by semiconductor devices capable of blocking the high voltage and the high current. Furthermore, since each of the main switches S 1 A and S 2 A form the charge channel and the discharge channel, the main switches S 1 A and S 2 A may be realized by back-to-back switches whose drains are coupled to each other. 
     The charge switch S 3 A is turned on during a charge operation of the first battery rack  160 A to form a charge channel and may be turned off when charge is completed. The diode D 1 A may form a discharge channel during a discharge operation of the first battery rack  160 A. 
     Furthermore, one end of the main switch S 1 B may be coupled to a positive potential output end (+) of the second battery rack  1608 . One end of the charge switch S 3 B may be coupled to the other end of the main switch S 1 B and the other end of the charge switch S 3 B may be coupled to the node N 1 . An anode of a diode D 1 B may be coupled to one end of the charge switch S 3 B and a cathode of the diode D 1 B may be coupled to the other end of the charge switch S 3 B. The main switch S 2 B may be coupled between a negative potential output end (−) of the second battery rack  1608  and the node N 1 . 
     Here, each of the main switches S 1 B and S 2 B may be maintained in a turn-on state during charge/discharge to form a charge channel and a discharge channel. The main switches S 1 B and S 2 B may be turned off in order to block a voltage and a current output from the positive potential output end (+) and the negative potential output end (−) of the second battery rack  160 B when there is something wrong with the second battery rack  160 B. 
     Since the second battery rack  160 B may output a high voltage and a high current, the main switches S 1 B and S 2 B may be realized by semiconductor devices capable of blocking the high voltage and the high current. Furthermore, since each of the main switches S 1 B and S 2 B must form the charge channel and the discharge channel, the main switches S 1 B and S 2 B may be realized by back-to-back switches whose drains are coupled to each other. 
     The charge switch S 3 B is turned on during a charge operation of the second battery rack  160 B to form a charge channel and may be turned off when charge is completed. The diode D 1 B may form a discharge channel during a discharge operation of the second battery rack  160 B. 
     Next, referring to  FIG. 3 , structures of slave BMSs and a master BMS that perform battery pack balancing by changing operation power will be described in detail. 
       FIG. 3  is a view illustrating a battery management system according to an example embodiment of the present invention. As illustrated in  FIG. 3 , a positive potential output end (+) and battery packs  161  are coupled to each other by a power line, and a current (charge that flows through the power line to charge the battery packs  161  or a current Idischarge for discharging power charged in the battery packs  161  to the outside may be blocked in accordance with an operation of a main switch S 1 . 
     A master BMS  163  may include a master switch unit (or a master switch)  1630 , a master communication unit (or a master communicator)  1632 , a master control unit (or a master controller)  1634 , a master power unit  1636 , and a master sensing unit (or a master sensor)  1638 . 
     First, the master power unit  1636  may receive the DC voltage VSS obtained by converting the AC voltage from the AC-DC converting unit  164  so that the DC voltage VSS may be used as the operating power of the master BMS  163 . 
     The master power unit  1636  may output the DC voltage VSS to a first voltage converter  165 . Then, the first voltage converter  165  may convert the DC voltage VSS into a voltage corresponding to operating power of slave BMSs  162  to supply the converted voltage to the respective slave BMSs  162 . The respective slave BMSs  162  may use the voltage supplied from the first voltage converter  165  as the operating power. 
     The master communication unit  1632  may receive battery state information from the slave BMSs  162  to output the received battery state information to the master control unit  1634 . 
     In addition, the master communication unit  1632  may transmit a signal generated by the master control unit  1634  to control the slave BMSs  162  to communication units  1626  of the slave BMSs  162 . 
     The master sensing unit  1638  may measure the current that flows through the power line. For example, the master sensing unit  1638  may directly measure the current that flows through the power line using resistance formed in the power line. 
     In another example, the master sensing unit  1638  may measure the current that flows through the power line using a hall sensor. Then, the master sensing unit  1638  may output a measured result to the master control unit  1634 . Units by which the master sensing unit  1638  measures the current that flows through the power line are not limited to those in the above example. 
     Next, the master control unit  1634  may estimate SOC and SOH of cells  1610  using the battery state information output from the master communication unit  1632 . For example, the master control unit  1634  may include a data table that represents a relationship between open circuit voltages (OCV) and SOC to calculate the SOC of the respective cells  1610  from detected data that represents a relationship between the OCVs and the SOC of the respective cells  1610 . 
     However, a method of calculating the SOC of the battery cells  1610  is not limited to the method of calculating the SOC of the battery cells  1610  from the OCVs. For example, various appropriate or suitable methods of calculating the SOC such as a current integrating method may be used. 
     The master control unit  1634  may receive the SOC and the SOH of the respective cells  1610  that are calculated by the slave BMSs  162  through the master communication unit  1632 . 
     The master control unit  1634  may generate a signal for changing the operating power of the slave BMSs  162  in which voltages or SOC of the battery packs  161  are relatively high using the battery state information to output the generated signal to the corresponding slave BMSs  162 . 
     That is, the master control unit  1634  may generate the operating power changing signal for performing a battery pack balancing operation to output the generated operating power changing signal to the slave BMSs  162 . 
     For example, the master control unit  1634  may calculate the voltages or SOC of the battery packs  161  including the respective cells  1610  using voltage values of the respective cells  1610 . Then, the master control unit  1634  may compare the voltages or SOC of the respective battery packs  161  with each other. 
     The master control unit  1634  may generate the operating power changing signal so that power charged in the battery packs  161  is used as the operating power of the slave BMSs  162  in which the voltages or SOC of the battery packs  161  are relatively high to output the generated operating power changing signal to the corresponding slave BMSs  162 . 
     For example, the first slave BMS  162  may manage the first battery pack  161 , the second slave BMS  162   c  may manage the second battery pack  161   c , the third slave BMS  162   d  may manage the third battery pack  161   d , and the master BMS  163  may manage the first slave BMS  162 , the second slave BMS  162   c , and the third slave BMS  162   d.    
     The master control unit  1634  may determine that the first battery pack  161  has a higher voltage or SOC than that of the second battery pack  161   c  and the third battery pack  161   d.    
     The master control unit  1634  may output the operating power changing signal to the first slave BMS  162  so that voltages or SOC of the first battery pack  161 , the second battery pack  161   c , and the third battery pack  161   d  are uniform. 
     To be specific, the master controller  1634  may calculate differences in voltage or charge state between the battery packs  161  and  161   c  and between the battery packs  161  and  161   d . When the calculated voltage differences are no less than (e.g., greater than or equal to) a threshold voltage value or the calculated differences in charge state are no less than (e.g., greater than or equal to) a threshold charge state, the master controller  1634  may output the operating power changing signal to the slave BMS  162  that manages the battery pack  161  that has a higher voltage or is in a higher charge state. 
     For example, when the difference in charge state between a first battery pack  161  and the second battery pack  161   c  is no less than (e.g., greater than or equal to) 5%, the master controller  1634  may output the operating power changing signal to the slave BMS (e.g.,  162  or  162   c ) that manages the battery pack  161  that is in a higher charge state between the first battery pack  161  and the second battery pack  161   c.    
     As another example, when the voltage difference between the first battery pack  161  and the third battery pack  161   d  is no less than (e.g., greater than or equal to) 2V, the master controller  1634  may output the operating power changing signal to the slave BMS  162  that manages the battery pack  161  that has a higher voltage between the first battery pack  161  and the third battery pack  161   d.    
     Alternatively, the master controller  1634  may calculate deviations in voltage or charge state between the battery packs  161  and  161   c  and between the battery packs  161  and  161   d . When the calculated voltage deviations are no less than (e.g., greater than or equal to) a reference value (e.g., a predetermined value) or the calculated deviations in charge state are no less than (e.g., greater than or equal to) a reference value (e.g., a predetermined value), the master controller  1634  may output the operating power changing signal to the slave BMS  162  that manages the battery pack  161  that has a higher voltage or is in a higher charge state. 
     Then, the slave BMS  162  that receives the operating power changing signal may use the power charged in the battery pack  161  as the operating power of the slave BMS  162 . As the power charged in the battery pack  161  is used as the operating power of the slave BMS  162 , the voltage of the battery pack  161  or the power charged in the battery pack  161  may be reduced. Therefore, the voltages or SOC of the entire battery packs  161  may be uniform by the battery pack balancing operation. 
     Then, a variation in the voltages of the entire battery cells included in the entire battery packs or a variation in the SOC of the entire battery cells may be reduced. To be specific, since the power charged in the battery pack  161  managed by the slave BMS  162  that receives the operating power changing signal is used as the operating power of the slave BMS  162 , the power charged in the battery cells  1610  included in the corresponding battery pack  161  may be reduced. 
     Therefore, the voltages or SOC of the battery cells  1610  having relatively high voltages that are included in the corresponding battery pack  161  may be reduced so that a deviation in the voltages or SOC of the entire battery cells  1610  may be reduced. 
     When the deviation in the voltages or SOC of the entire battery cells  1610  is reduced to be no more than a reference level (e.g., a predetermined level), the master control unit  1634  may output an operating power change stopping signal and may generate a cell balancing signal. 
     The master control unit  1634  may output the cell balancing signal to the slave BMSs  162  so that a cell balancing operation is performed on the respective cells  1610 . Then, the slave BMSs  162  that receive the cell balancing signal may perform a balancing operation on the voltages of the battery cells  1610  included in the battery packs  161 . 
     In addition, after performing balancing, the master control unit  1634  may determine a balancing terminating condition and may output a cell balancing terminating signal to the slave BMSs  162 . The master control unit  1634  may determine whether a balanced cell voltage reaches a specific voltage value or a difference between the balanced cell voltage and a minimum cell voltage is no more than the specific voltage value. In addition, the master control unit  1634  may determine whether a cell temperature deviates from a reference range (e.g., a predetermined range). 
     For example, in a case of balancing during full-charge, the master control unit  1634  may terminate cell balancing when the cell voltage is no more than 3.8V or the difference between the balanced cell voltage and the minimum cell voltage is no more than 5 mV. In a case of balancing during overcharge, the master control unit  1634  may terminate cell balancing when the minimum cell voltage is no more than 2.2V or the difference between the balanced cell voltage and the minimum cell voltage is no more than 5 mV. In addition, the master control unit  1634  may terminate cell balancing when the cell temperature is no more than 0° C. or no less than 50° C. 
     The master control unit  1634  may generate a signal for controlling the main switch S 1  in accordance with the battery state information transmitted from the slave BMSs  162  and the current measuring result of the master sensing unit  1638  to output the generated signal to the master switch unit  1630 . 
     Each of the slave BMSs  162  may include a sensing unit (or sensor)  1620 , a power unit  1622 , a control unit (or controller)  1624 , a communication unit  1626 , a switch unit  1628 , and a second voltage converter  1629 . The second voltage converter  1629  may be provided outside the slave BMS  162 . 
     First, the sensing unit  1620  may measure states of the battery pack  161  and the respective cells  1610  included in the battery pack  161 . For example, the sensing unit  1620  may measure an entire voltage or an intermediate voltage (i.e., a total voltage of the entire cells  1610  or voltages of the respective cells  1610 ) of the battery pack  161 , a temperature of the battery pack  161  or of the respective cells  1610 , and/or a current that flows through the battery pack  161  or the respective cells  1610 . 
     The voltage values measured by the sensing unit  1620  may include OCVs of the respective cells  1610  or voltages measured during charge and discharge. 
     The sensing unit  1620  may be coupled to nodes among the respective cells  1610  in order to measure the entire voltage or intermediate voltage of the battery pack  161 . That is, at least one wiring line for measuring the intermediate voltage of the battery pack  161  may be formed between the sensing unit  1620  and the cells  1610 . The measured voltage values, current values, and temperature values of the respective cells  1610  may be output to the control unit  1624 . 
     The communication unit  1626  and the power unit  1622  may transmit the battery state information to the master BMS  163  by control of (or in accordance with control signals received from) the control unit  1624 . The battery state information may include a value measured by the sensing unit  1620 . 
     In addition, the communication unit  1626  and the power unit  1622  may receive the operating power changing signal and the cell balancing signal output from the master BMS  163  to transmit the received operating power changing signal and cell balancing signal to the control unit  1624 . 
     The power unit  1622  may receive power for operating the slave BMS  162  to supply the received power to the respective elements of the slave BMS  162 . 
     The switch unit  1628  may apply the power supplied from the first power changing unit  165  or the power supplied from the battery pack  161  to the power unit  1622 . The switch unit  1628  may selectively apply the power supplied from the first power changing unit  165  or the power supplied from the battery pack  161  to the power unit  1622  in accordance with the control of (or in accordance with control signals received from) the control unit  1624 . 
     The second voltage converter  1629  may convert the voltage of the battery pack  161  to supply the converted voltage to the power unit  1622  through the switch unit  1628 . For example, the battery pack  161  may be coupled to one end of the second voltage converter  1629  and the switch unit  1628  may be electrically coupled to the other end of the second voltage converter  1629 . When the switch unit  1628  performs an operation for using the power stored in the battery pack  161  as the operating power of the slave BMS  162 , the second voltage converter  1629  may be electrically coupled to the power unit  1622  to convert the voltage of the battery pack  161  and to supply the converted voltage as the operating power of the slave BMS  162 . 
     The control unit  1624  may estimate the SOC or SOH of the battery pack  161  or the respective cells  1610  included in the battery pack  161  from the voltage values, current values, and temperature values of the respective cells  1610  that are output from the sensing unit  1620 . 
     In addition, the control unit  1624  may generate a signal for controlling an operation of the switch unit  1628  to output the generated signal to the switch unit  1628  in accordance with the operating power changing signal received from the communication unit  1626 . 
     For example, when the operating power changing signal is received, the control unit  1624  may provide a signal for operating the switch unit  1628  to the switch unit  1628  so that the switch unit  1628  and the second voltage converter  1629  are coupled to each other. Then, the voltage of the battery pack  161  may be converted by the second voltage converter  1629  to be supplied to the power unit  1622 . 
     In addition, when the cell balancing signal is received, the control unit  1624  may perform the cell balancing operation. For example, the control unit  1624  may turn on or off a balancing switch for a cell to be balanced among the battery cells  1610  to perform discharge through balancing resistance and to perform the cell balancing operation. 
     Hereinafter, referring to  FIG. 4 , the slave BMS  162  whose operating power is converted by driving the switch unit  1628  will be described. 
       FIG. 4  is a view illustrating the slave BMS  162  to which the operating power is applied according to an example embodiment of the present invention. As illustrated in  FIG. 4 , the switch unit  1628  may include relays  2 A and  2 B and may drive the relays  2 A and  2 B in accordance with the signal of the control unit  1624 . For example, the respective relays may be driven by currents applied from the control unit  1624 . 
     First, the control unit  1624  may drive the relay  2 B to supply the power whose voltage is converted by the first voltage converter  165  to the power unit  1622 . 
     When the operating power changing signal is received from the master BMS, the control unit  1624  may output a signal for stopping driving of the relay  2 B and driving the relay  2 B to the switch unit  1628 . Then, the relay  2 A of the switch unit  1628  is driven and driving of the relay  2 B may be stopped. 
     The switch unit  1628  may further include a capacitor for preventing coupling of a voltage to be applied to the power unit  1622  when both the relays  2 A and  2 B are driven or driving of both the relays  2 A and  2 B is stopped. 
     When the relay  2 B operates, the power charged in the battery pack  161  may be applied to the power unit  1622  through the second voltage converter  1629 . The second voltage converter  1629  may convert the voltage of the battery pack  161  into a voltage suitable for a driving voltage of the slave BMS  162 . 
     When the operating power change stopping signal is received from the master BMS, the control unit  1624  may output a signal for stopping driving of the relay  2 A and driving the relay  2 B to the switch unit  1628 . Then, the relay  2 B of the switch unit  1628  may be driven and driving of the relay  2 A may be stopped. When the relay  2 B operates, the power of which voltage is converted by the first voltage converter  165  may be supplied to the power unit  1622 . 
     Hereinafter, referring to  FIG. 5 , a method of changing the operating power of the slave BMS  162  will be described. 
       FIG. 5  is a flowchart illustrating a method of driving a battery management system according to an example embodiment of the present invention. As illustrated in  FIG. 5 , the slave BMS  162  measures values related to the state of the battery pack  161  S 100 . For example, the sensing unit  1620  of the slave BMS  162  may measure the voltages, currents, or temperatures of the respective battery cells  1610  and the battery pack  161 . 
     Then, the slave BMS  162  transmits state information on the battery pack  161  to the master BMS  163  S 110 . For example, the communication unit  1626  of the slave BMS  162  may convert the battery state information into a CAN communication type signal to output the CAN communication type signal to the master BMS  163  in accordance with the control of (or in accordance with control signals received from) the control unit  1624 . 
     The slave BMS  162  measuring the battery state information S 100  and the slave BMS  162  transmitting the measured battery state information to the master BMS S 110  may be periodically performed while the slave BMS  162  operates. 
     Next, the control unit  1624  of the master BMS  163  calculates the voltage or SOC of the battery pack  161  using the battery state information output from the slave BMS  162  S 120 . 
     The master BMS  163  compares the voltages or SOC of the respective battery packs  161  with each other S 130 . Then, a signal for changing the operating power of the slave BMS  162  of the battery pack  161  having a relatively high SOC or voltage is generated S 140 . 
     Then, the master BMS  163  transmits the operating power changing signal to the slave BMS  162  S 150 . 
     The slave BMS that receives the operating power changing signal controls the operation of the switch unit so that the operating power is supplied from the battery pack S 160 . Then, the slave BMS  162  may convert the power stored in the battery pack  161  into the operating power to use the operating power. 
     While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. Therefore, a person of an ordinary skill in the art may easily make a selection and replacement. In addition, a person of an ordinary skill in the art may omit parts of the constituent elements described in the present specification without deteriorating performance or may add constituent elements in order to improve performance. In addition, a person of an ordinary skill in the art may change an order of the processes of the method described in the present specification in accordance with process environment or equipment. Therefore, scope of the present invention must be determined not by the described example embodiments but by the appended claims and equivalents thereof. 
     DESCRIPTION OF SYMBOLS 
     
         
           100 : power storage system 
           200 : power generating system 
           300 : commercial system 
           120 : first power converting unit 
           140 : second power converting unit 
           160 : power storage apparatus 
           161 : battery pack 
           162 : slave BMS 
           163 : master BMS