Patent Publication Number: US-2021184474-A1

Title: Cell supervising circuit and battery management system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT International Application No. PCT/JP2019/033491 filed on Aug. 27, 2019, designating the United States of America, which is based on and claims priority of Japanese Patent Application No. 2018-160104 filed on Aug. 29, 2018. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to a cell supervising circuit and a battery management system including such a cell supervising circuit. 
     BACKGROUND 
     Patent Literature (PTL) 1 discloses a battery voltage monitoring device capable of ensuring the reliability of operation by improving the redundancy of an operating power source. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2011-163847 
     SUMMARY 
     Technical Problem 
     The present disclosure provides a cell supervising circuit capable of suppressing the collapse of cell balance while suppressing the addition of components related to communication, and a battery management system using such a cell supervising circuit. 
     Solution to Problem 
     A cell supervising circuit according to one aspect of the present disclosure includes: a measurement circuit which measures a state of charge of a power storage cell; an insulation element which is provided for the measurement circuit to contactlessly receive power supply from a power source different from the power storage cell; and a communication circuit which transmits, via the insulation element to a management device which manages a status of the power storage cell, information indicating the state of charge measured by the measurement circuit. 
     A battery management system according to another aspect of the present disclosure includes: a management device which manages a status of a power storage cell; and a cell supervising circuit, wherein the cell supervising circuit includes: a measurement circuit which measures a state of charge of the power storage cell; an insulation element which is provided for the measurement circuit to contactlessly receive power supply from a power source different from the power storage cell; and a communication circuit which transmits, to the measurement circuit via the insulation element, information indicating the state of charge measured by the measurement circuit. 
     Advantageous Effects 
     The present disclosure realizes a cell supervising circuit capable of suppressing the collapse of cell balance while suppressing the addition of components related to communication and a battery management system using such a cell supervising circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG. 1  is a diagram schematically illustrating a functional configuration of a BMS according to Embodiment 1. 
         FIG. 2  is a diagram schematically illustrating a functional configuration of a BMS according to Embodiment 2. 
         FIG. 3  is a diagram schematically illustrating a functional configuration of a BMS according to Embodiment 3. 
         FIG. 4  is a diagram illustrating connection relation between a BMU and a plurality of cell supervising circuits in the BMS according to Embodiment 3. 
         FIG. 5  is a diagram illustrating frequency characteristics of the impedance of a transmission path. 
         FIG. 6  is a diagram illustrating a power spectrum of the transmission path. 
         FIG. 7  is a diagram schematically illustrating a functional configuration of a BMS according to a variation of Embodiment 3. 
         FIG. 8  is a diagram schematically illustrating a functional configuration of a BMS according to Embodiment 4. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     [Configuration] 
     Hereinafter, a battery management system (BMS) according to Embodiment 1 will be described. First, the configuration of the BMS according to Embodiment 1 will be described.  FIG. 1  is a diagram schematically illustrating a functional configuration of the BMS according to Embodiment 1. 
     BMS  100  according to Embodiment 1 is loaded on, for example, a vehicle such as an electric vehicle. BMS  100  includes: battery management unit (BMU)  10 ; a plurality of battery packs  20 ; and a plurality of cell supervising circuits (CSC)  30  corresponding to the plurality of battery packs  20 .  FIG. 1  illustrates two battery packs  20  and two cell supervising circuit  30 , but the total numbers of battery packs  20  and cell supervising circuits  30  are not limited to two, and thus the aforementioned numbers may be three or more. Moreover, BMS  100  may include one battery pack  20  and one cell supervising circuit  30 . 
     BMU  10  monitors the state of the plurality of battery packs  20  and controls the plurality of battery packs  20 . Battery pack  20  includes a plurality of secondary battery cells  21  but is only required to include at least one secondary battery cell  21 . Secondary battery cell  21  is one example of a power storage cell. More specifically, secondary battery cell  21  is a lithium ion battery but may be any other battery such as a nickel hydrogen battery. The plurality of secondary battery cells  21  are, for example, serially connected but may be partially or entirely connected in parallel. 
     Note that it is not essential that battery pack  20  be formed by one or more secondary battery cells  21  and battery pack  20  may be formed by one or more storage capacitor cells. The storage capacitor cell is another one example of the power storage cell. More specifically, the storage capacitor cell is an electric double layer capacitor but may be, for example, a lithium ion capacitor. 
     More specifically, BMU  10  includes a plurality of communication circuits  11 , a plurality of primary power supply circuits  12 , and control microcomputer  13 .  FIG. 1  illustrates two communication circuits  11  and two primary power supply circuits  12 , but BMU  10  is only required to include the total numbers of communication circuits  11  and primary power supply circuits  12  corresponding to the total number of cell supervising circuits  30 . Note that BMU  10  is only required to include at least control microcomputer  13 . 
     Communication circuit  11  is a circuit for BMU  10  to make communication with cell supervising circuit  30 . More specifically, communication circuit  11  includes, for example, a transmission circuit, a filter, and an amplifier circuit for signal transmission; and a reception circuit, a filter, and an amplifier circuit for signal reception. 
     Primary power supply circuit  12  forms switching power supply circuit  40  together with transformer  38  and secondary power supply circuit  39 . Switching power supply circuit  40  is a power supply circuit for performing contactless power feed to cell supervising circuit  30  through a path different from battery pack  20 . That is, cell supervising circuit  30  is operated not by battery pack  20  but by an electric power supplied by switching power supply circuit  40 . 
     Control microcomputer  13  monitors the state of the plurality of battery packs  20  and controls the plurality of battery packs  20 . 
     The plurality of cell supervising circuits  30  are circuits respectively corresponding to the plurality of battery packs  20 . Cell supervising circuit  30  is a circuit module and is formed by mounting circuit components on a substrate. More specifically, cell supervising circuit  30  includes measurement circuit  31 , communication circuit  37 , transformer  38 , and secondary power supply circuit  39 . 
     Measurement circuit  31  measures respective states of charge of the plurality of secondary battery cells  21  included in battery pack  20 . More specifically, measurement circuit  31  measures, as parameters indicating the states of charge of secondary battery cells  21 , respective voltage values of the plurality of secondary battery cells  21  included in battery pack  20 . Measurement circuit  31  includes: a plurality of switching elements  32  corresponding to the plurality of secondary battery cells  21 ; multiplexer  33 , AD converter  34 , storage unit  35 , and control circuit  36 . Measurement circuit  31  may measure the parameters directly or indirectly indicating the states of charge. 
     The plurality of switching elements  32  are switched on to individually discharge the states of charge of the corresponding plurality of secondary battery cells  21  for adjustment. 
     Multiplexer  33  is a selection circuit which selects voltage values at both ends of the plurality of secondary battery cells  21 . 
     AD converter  34  converts the analog voltage values selected and inputted by multiplexer  33  into digital voltage values. 
     Storage unit  35  is, for example, a non-volatile semiconductor memory, which stores an address for discriminating cell supervising circuit  30  from other cell supervising circuits  30  (in other words, discrimination information or recognition sign). The address can also be assumed as discrimination information for discriminating battery pack  20  from other battery packs  20 . Note that, in an example of  FIG. 1 , storage unit  35  is illustrated as part of measurement circuit  31  (that is, included in measurement circuit  31 ) but may be provided as a component separate from measurement circuit  31 . 
     Control circuit  36  generates information obtained by providing the address stored in storage unit  35  to the digital voltage value outputted from AD converter  34  (also written as information indicating the state of charge measured by measurement circuit  31 ), and outputs the generated information to communication circuit  37 . In other words, control circuit  36  is a control logic circuit. 
     Communication circuit  37  transmits, via transformer  38 , information indicating the states of charge measured by measurement circuit  31  to BMU  10  which manages the state of battery packs  20 . More specifically, communication circuit  37  includes: for example, a transmission circuit, a filter, and an amplifier circuit for signal transmission; and a reception circuit, a filter, and an amplifier circuit for signal reception. 
     Transformer  38  is an insulation element for measurement circuit  31  to contactlessly receive the power supply from a power source (more specifically, a primary power supply circuit) different from battery pack  20 . Transformer  38 , secondary power supply circuit  39 , and the primary power supply circuit form switching power supply circuit  40 . Note that cell supervising circuit  30  may include another coil element instead of transformer  38  as an insulation element. 
     Switching power supply circuit  40  is a power supply circuit for contactlessly feeding power to cell supervising circuit  30  through a path different from battery pack  20 . Measurement circuit  31  and communication circuit  37  are operated not by battery pack  20  but by the power supplied by switching power supply circuit  40 . In other words, switching power supply circuit  40  is an insulated DC-DC converter. 
     Note that the switching frequency of switching power supply circuit  40  is, for example, approximately 350 kHz, and the frequency band of the carrier wave in communication between cell supervising circuit  30  (communication circuit  37 ) and BMU  10  (communication circuit  11 ) is higher than 350 kHz. 
     Effects, Etc. 
     In order to suppress heat generation, ignition, explosion, and deterioration caused by overcharging of battery pack  20  and maximize the state of charge of secondary battery cell  21  through charging, a typical BMS performs cell balance processing of equalizing the states of charge (SOC) of the plurality of secondary battery cells  21  included in battery pack  20  to charge the battery pack. At this point, BMU carries out daisy (bead) communication with the plurality of cell supervising circuits in order to manage the states of charge of the secondary battery cells (in other words, the voltage values of the secondary battery cells). 
     Typically, each of the plurality of cell supervising circuits in the typical BMS receives power supply from the battery pack (secondary battery cell) targeted for monitoring by the aforementioned cell supervising circuit. With such a configuration, a variation in the operating power of the plurality of cell supervising circuits serves as a factor contributing to the collapse of cell balance. In particular, the variation in the operating power due to a difference in the frequency of communication of the plurality of cell supervising circuits serves as a factor largely contributing to the collapse of cell balance. 
     There is a possible method for supplying a power from another power source (for example, a 12V battery in a case where the BMS is provided to be loaded on a vehicle) different from the battery pack to the cell supervising circuit for the purpose of suppressing the collapse of cell balance. In the method, the aforementioned another power source and the battery pack need to be galvanically isolated from each other. More specifically, the method for supplying the power from another power source to the cell supervising circuit includes, for example, power feed to the cell supervising circuit by an insulated DC-DC converter using a transformer. 
     However, in a case where the method for supplying the power from another power source to the cell supervising circuit is applied to the typical BMS, a power supply path (for example, a wire or a harness) needs to be provided between the plurality of cell supervising circuits and the BMU. Thus, there arise new problems such as an increase in the total number of components and a weight increase. 
     On the contrary, since BMS  100  also uses the power supply path (power line  50  and transformer  38 ) formed by switching power supply circuit  40  as a communication path of BMU  10  and cell supervising circuit  30 , a power supply path does not have to be provided separately. That is, BMS  100  can suppress the increase in the total number of components, the weight increase, etc. and also can suppress the collapse of cell balance due to the variation in the operating power of cell supervising circuit  30 . 
     [Method for Discriminating Plural Cell Supervising Circuits] 
     Next, the significance of the address stored in storage unit  35  will be described. In BMS  100 , it is required to manage the state of charge for each secondary battery cell  21 , and thus a mechanism of specifying individual secondary battery cells  21  by BMU  10  serving as a host system of cell supervising circuits  30  is required. 
     Here, the daisy communication is performed in the typical BMS and communication is performed in a bucket relay style in the daisy communication, so that addresses respectively specific to the plurality of cell supervising circuits  30  are sequentially provided at time of start of the communication. On the contrary, the plurality of cell supervising circuits  30  parallelly perform communication with BMU  10  in BMS  100 , which therefore brings about a problem that the method for sequentially providing the addresses cannot be used. 
     Thus, the address specific to the cell supervising circuit  30  is stored into storage unit  35  included in aforementioned cell supervising circuit  30  in BMS  100 . Cell supervising circuit  30  transmits, to BMU  10 , information including the address of aforementioned cell supervising circuit  30  provided to the voltage value measured by measurement circuit  31 . Consequently, BMU  10  can perform management while discriminating from which cell supervising circuit  30  the voltage value acquired through the communication has been transmitted (to which battery pack  20  the voltage value corresponds). The voltage values of the plurality of secondary battery cells  21  included in one battery pack  20  are transmitted in sequence, and thus discrimination between the plurality of secondary battery cells  21  included in one battery pack  20  is performed based on, for example, the aforementioned sequence. 
     Embodiment 2 
     [Configuration] 
     Hereinafter, a BMS according to Embodiment 2 will be described. First, the configuration of the BMS according to Embodiment 2 will be described.  FIG. 2  is a diagram schematically illustrating a functional configuration of the BMS according to Embodiment 2. Note that the description of Embodiment 2 will be provided, focusing on a difference from Embodiment 1, and the description of those already described in Embodiment 1 will be omitted or simplified as appropriate. 
     BMS  100   a  according to Embodiment 2 includes: BMU  10   a ; a plurality of battery packs  20 ; and a plurality of cell supervising circuits  30   a  corresponding to the plurality of battery packs  20 . 
     A difference between BMU  10  and BMU  10   a  lies in that BMU  10   a  includes AC power source  12   a  instead of primary power supply circuit  12 . AC power source  12   a  supplies an AC power to cell supervising circuits  30   a  via AC power line  50   a . That is, an AC power is supplied from BMU  10   a  to cell supervising circuit  30   a  over a galvanic isolation border in BMS  100   a.    
     A difference between cell supervising circuit  30  and cell supervising circuit  30   a  lies in that cell supervising circuit  30   a  includes conversion circuit  39   a  instead of secondary power supply circuit  39 . 
     Conversion circuit  39   a  converts, into a DC power, an AC power supplied from AC power source  12   a  via transformer  38  and supplies the DC power to measurement circuit  31  and communication circuit  37 . That is, transformer  38  is an insulation element for measurement circuit  31  to contactlessly receive the power supplied from AC power source  12   a  different from battery pack  20  in Embodiment 2. More specifically, conversion circuit  39   a  is formed by a full-wave rectifier circuit, a smoothening circuit, a regulator, etc. The frequency of AC power source  12   a  is, for example, 350 kHz and an effective value of AC power source  12   a  is, for example, 5V. Note that the frequency and the effective value of AC power source  12   a  are not specifically limited. 
     Effects and Others 
     In switching power supply circuit  40  described in Embodiment 1, a switching frequency and a switching pulse width fluctuate, so that a frequency component of switching noise may be spread over a wide range in some cases. That is, it may be difficult to ensure communication quality. 
     On the contrary, communication is performed via AC power line  50   a  to which the AC power is transmitted in BMS  100   a . If the waveform of the AC power is a sine wave with a single frequency, the frequency component is hardly spread, which therefore provides effect of easily ensuring the communication quality. Moreover, since the frequency component is hardly spread, the degree of freedom of a frequency band where the communication is performed is also improved. 
     Embodiment 3 
     [Configuration] 
     Hereinafter, a BMS according to Embodiment 3 will be described. First, the configuration of the BMS according to Embodiment 3 will be described.  FIG. 3  is a diagram schematically illustrating a functional configuration of the BMS according to Embodiment 3. Note that the description of Embodiment 3 will be provided, focusing on a difference from Embodiments 1 and 2, and the description of those already described in Embodiments 1 and 2 will be omitted or simplified as appropriate. 
     BMS  100   b  according to Embodiment 3 includes: BMU  10   b ; a plurality of battery packs  20 ; a plurality of cell supervising circuits  30   b  corresponding to the plurality of battery packs  20 . 
     A difference between BMU  10   a  and BMU  10   b  lies in that BMU  10   b  includes one communication circuit  11  and one AC power source  12   a  and further includes transformer  14 . Note that BMU  10   b  may include another coil element as an insulation element in place of transformer  14 . 
     Communication circuit  11  of BMU  10   b  is connected to AC power line  50   b  via transformer  14 . More specifically, a primary coil of transformer  14  is connected to communication circuit  11  and a secondary coil of transformer  14  is connected to AC power line  50   b.    
     AC power line  50   b  is commonly used by the plurality of cell supervising circuits  30   b . More specifically, each of the plurality of cell supervising circuits  30   b  is connected to AC power line  50   b  common to the plurality of cell supervising circuits  30   b  via transformer  38  included in aforementioned cell supervising circuit  30   b . More specifically, the primary coil of transformer  38  is connected to AC power line  50   b  and the secondary coil of transformer  38  is connected to communication circuit  37  and conversion circuit  39   a.    
     With such connection relation, BMU  10   b  can perform communication with each of the plurality of cell supervising circuits  30   b  via AC power line  50   b . AC power line  50   b  is a daisy communication line. 
     A difference between cell supervising circuit  30   a  and cell supervising circuit  30   b  lies in that cell supervising circuit  30   b  includes clock generation circuit  39   b.    
     Clock generation circuit  39   b  generates a clock signal synchronized with the frequency of an AC power. Measurement circuit  31  measures the voltage value of secondary battery cell  21  based on the clock signal generated. More specifically, clock generation circuit  39   b  is realized by a phase synchronization circuit (in other words, a phase locked loop (PLL) circuit). 
     Effects and Others 
     In BMS  100   b , single AC power source  12   a  can be used to supply an AC power to the plurality of cell supervising circuits  30   b  over the galvanic isolation border. Moreover, communication can be easily performed by dedicated AC power line  50   b.    
     Assumed as a configuration similar to that of BMS  100   b  is a configuration such that a power supply line (also called a bus bar) connecting together the plurality of battery packs  20  is used to perform power line communication (PLC). Such a configuration can eliminate a communication line. 
     Here, since the bus bar is typically a single wire without a GND wire, the use of the bus bar as a transmission line path for communication brings about a problem that noise resistance is low. Moreover, a large current intermittently flows to the bus bar, which therefore generates very large disturbance noise. Thus, there also arise a problem that it is difficult to ensure the communication quality. 
     However, another AC power line  50   b  different from the bus bar is used to perform communication in BMS  100   b . More specifically, a power is supplied from AC power source  12   a  to the plurality of cell supervising circuits  30   b  over the galvanic isolation border and AC power line  50   b  is used to perform communication in BMS  100   b . Consequently, it is possible to perform communication with high communication quality without newly providing a power supply path (more specifically, a wire, a harness, or the like) between the plurality of cell supervising circuits  30   b.    
     [Frequency Band of Communication] 
     Next, the frequency band of a carrier wave of communication in BMS  100   b  will be described.  FIG. 4  is a diagram illustrating connection relation between BMU  10   b  and the plurality of cell supervising circuits  30   b  in BMS  100   b .  FIG. 5  is a diagram illustrating frequency characteristics of the impedance of a transmission path including AC power line  50   b.    
     As illustrated in  FIGS. 4 and 5 , the transmission path including AC power line  50   b  has characteristics suitable for the frequency of the AC power. More specifically, the resonance frequency of the transmission path is set to a substantially same frequency as the frequency of the AC power. The resonance frequency of the transmission path and the frequency of the AC power are, for example, several hundreds of kilohertz and 350 kilohertz in examples of  FIGS. 4 and 5 . Consequently, power supply with little loss is realized. 
     Moreover, the frequency band used in the communication is higher than the frequency of the AC power in BMS  100   b . That is, communication circuit  37  of cell supervising circuit  30   b  performs communication by use of the frequency band higher than the frequency of the AC power. The carrier wave frequency of communication is, for example, 20 MHz. 
     As described above, if the carrier wave frequency of the communication is set to a high frequency band distant from the resonance frequency of AC power line  50   b , BMS  100   b  can perform communication with a relatively low communication power by use of a frequency band in which the impedance in the impedance characteristics of the transmission path is high. Note that the way of setting such a frequency band is also applicable to Embodiments 1 and 2. 
     [Communication Channel] 
     Moreover, the frequency band used for the communication may be divided into a plurality of frequency bands.  FIG. 6  is a diagram illustrating a power spectrum of the transmission path including AC power line  50   b.    
     As illustrated in  FIG. 6 , BMS  100   b  divides the frequency band used for the communication into a plurality of communication channels  1  to n (where n is an integer of 2 or more). Each of the plurality of cell supervising circuits  30   b  is used as a communication channel having part of the frequency band assigned to cell supervising circuit  30   b  described above. Consequently, the communication speed and the communication quality can be improved. 
     For example, full dual communication is realized through discrimination between a transmission channel and a reception channel. Moreover, synchronous communication between the plurality of cell supervising circuits  30   b  and BMU  10   b  is realized through communication while different communication channels are respectively assigned to the plurality of cell supervising circuits  30   b . Moreover, BMS  100   b  can select the communication channel in the most favorable state from among the plurality of communication channels to perform the communication, and the two communication channels are used for the transmission of the same signal and signals received on a reception side are compared to each other to realize redundant communication. Note that the way of dividing such a frequency band into a plurality of communication channels is also applicable to Embodiments 1 and 2. 
     [Clock Generation Circuit] 
     Next, the significance of clock generation circuit  39   b  will be described. Typically, accurate calculation of the SOC and a state of health (SOH) of the battery pack requires obtaining an accurate open circuit voltage (OCV). However, it is difficult to measure the accurate OCV with a load actually set to zero in a state in which battery pack  20  is incorporated in a set. 
     Thus, there is a possible method for calculating the OCV through subtraction of an operating current and a voltage drop caused by the inner impedance from the measured cell voltage value by measuring the inner impedance of secondary battery cell  21 . To calculate the accurate OCV by the aforementioned method, the timing of measuring the voltages of the plurality of secondary battery cells  21  and the timing of measuring the current need to be brought into agreement with each other with high accuracy. 
     Moreover, the current measurement is typically performed on a BMU side, targeted on one portion of the bus bar, and each of the plurality of cell supervising circuits includes an individual clock oscillator and the plurality of cell supervising circuits independently operate without synchronizing with each other. In such a case, the timing of measuring the voltage is instructed from the BMU side through daisy communication, but it is impossible to accurately synchronize the timing of measuring the voltages and the timing of measuring the current with each other due to a delay in the communication time and a variation of clock generators of the plurality of cell supervising circuits. 
     On the contrary, an AC power is supplied from single AC power source  12   a  to the plurality of cell supervising circuits  30   b  in BMS  100   b . Therefore, generating a clock signal based on the frequency of the AC power by clock generation circuit  39   b  makes it possible to synchronize respective system clocks of the plurality of cell supervising circuits  30   b . Further, synchronizing the current measurement timing on BMU  10   b  side with the frequency of the AC power makes it possible to accurately synchronize the timing of measuring the voltage of secondary battery cell  21  by cell supervising circuit  30   b  and the timing of measuring the current on BMU  10   b  side. 
     Thus, BMS  100   b  can accurately measure the inner impedance of secondary battery cell  21  to thereby calculate the accurate OCV and more accurately calculate the SOC and the SOH. 
     [Variation] 
     Cell supervising circuit  30   b  may target on only one secondary battery cell  21  for monitoring.  FIG. 7  is a diagram schematically illustrating a functional configuration of a BMS (BMS according to the variation of Embodiment 3) in which cell supervising circuit  30   b  targets on only one secondary battery cell  21  for the monitoring. Measurement circuit  31   c  does not include multiplexer  33  in BMS  100   c  illustrated in  FIG. 7 . That is, it is possible to omit multiplexer  33  in BMS  100   c . Note that cell supervising circuit  30  or cell supervising circuit  30   a  may target on only one secondary battery cell  21  for monitoring in Embodiments 1 and 2. 
     Embodiment 4 
     Hereinafter, a BMS according to Embodiment 4 will be described. First, a configuration of the BMS according to Embodiment 4 will be described.  FIG. 8  is a diagram schematically illustrating a functional configuration of the BMS according to Embodiment 4. Note that the description of Embodiment 4 will be provided, focusing on a difference from Embodiments 1 to 3, and those already described in Embodiments 1 to 3 will be omitted or simplified as appropriate. 
     BMS  100   d  according to Embodiment 3 includes: BMU  10   b ; a plurality of battery packs  20  (not illustrated in  FIG. 8 ); and a plurality of cell supervising circuits  30   d  corresponding to the plurality of battery packs  20 . 
     Cell supervising circuit  30   d  includes voltage detection circuit  39   d  at a later stage of conversion circuit  39   a . Although not illustrated, other configurations of cell supervising circuit  30   d  are the same as those of cell supervising circuit  30   b.    
     Voltage detection circuit  39   d  outputs a control signal for controlling the activation or stop of measurement circuit  31  in accordance with an output voltage of conversion circuit  39   a . More specifically, voltage detection circuit  39   d  is realized by, for example, a comparator circuit, and outputs a control signal (at a high level) when the output voltage of conversion circuit  39   a  is greater than or equal to a predetermined voltage. Voltage detection circuit  39   d  stops a control signal (at a low level) when the output voltage of conversion circuit  39   a  is less than the predetermined voltage. Note that the logic of the control signal may be reverse, and voltage detection circuit  39   d  may output the control signal (at a high level) when the output voltage of conversion circuit  39   a  is less than the predetermined voltage and may stop the control signal (at a low level) when the output voltage of conversion circuit  39   a  is greater than or equal to the predetermined voltage. 
     As described above, conversion circuit  39   a  converts the AC power provided from AC power source  12   a  into a DC power, and thus voltage detection circuit  39   d  can be said to output a control signal upon start of the supply of the AC power from AC power source  12   a  to cell supervising circuit  30   d . The control signal is, for example, a signal for controlling the activation or stop of cell supervising circuit  30   d  (for example, a power on reset signal), and more specifically is outputted to control circuit  36 . 
     Typically, when battery pack  20  is not in use, the cell supervising circuit needs to be shut down to suppress the power consumption of battery pack  20  at a maximum. For example, in a case where the BMS is loaded on an electric car, even when a state in which the electric car is stopped is maintained for approximately one to two years without performing the charging of battery pack  20 , it is necessary that battery pack  20  do not turn into a completely discharged state (the electric car needs to be operable). 
     In order to shut down the cell supervising circuit, the cell supervising circuit (more specifically, a battery management IC corresponding to the cell supervising circuit, in other words, a battery monitoring IC) has a shutdown mode, but a method for activating the cell supervising circuit in the shutdown mode is still to be reviewed. The BMU uses a power supply system different from a high voltage system of, for example, battery pack  20 , and thus the reference voltage differs from that of the cell supervising circuit and an activation signal needs to be provided to the cell supervising circuit over the galvanic isolation border in order to activate the cell supervising circuit by the BMU. 
     Possible methods for providing the activation signal to the cell supervising circuit include: for example, a method using a photocoupler; and a method for transmitting an activation signal through a daisy communication interface. The method using the photocoupler faces a problem that a wire or a harness for connection between the plurality of cell supervising circuits and the BMU is required. Moreover, the method for transmitting the activation signal through the daisy communication interface faces a problem that a reception circuit cannot be turned off in order to permit cell supervising circuit  30   d  to receive the activation signal even in the shutdown mode, which requires slight power consumption in the shutdown mode. 
     On the contrary, BMU  10   b  can control the turning off and on of the control signal (that is, the activation signal) provided from voltage detection circuit  39   d  to measurement circuit  31  as a result of turning on and off of AC power source  12   a  in BMS  100   d . That is, installing voltage detection circuit  39   d  which detects the AC power on cell supervising circuit  30   d  side makes it possible for BMU  10   b  to easily activate cell supervising circuit  30   d.    
     More specifically, note that conversion circuit  39   a  which converts the AC power into a DC power can be formed by a diode bridge and a smoothening capacitor, so that a circuit current is no longer required. Moreover, since voltage detection circuit  39   c  operates based on the DC power source provided by rectifying the AC power, power supply from battery pack  20  side does not have to be received. Therefore, with the configuration like that of cell supervising circuit  30   d , it is possible to set, to almost zero, the power supplied from battery pack  20  and consumed by cell supervising circuit  30   d  in the shutdown mode. 
     SUMMARY 
     As described above, cell supervising circuit  30  includes: measurement circuit  31  which measures a state of charge of secondary battery cell  21 ; transformer  38  for measurement circuit  31  to contactlessly receive power supply from a power source different from secondary battery cell  21 ; and communication circuit  37  which transmits, via transformer  38  to BMU  10  which manages the state of secondary battery cell  21 , information indicating the state of charge measured by measurement circuit  31 . Secondary battery cell  21  is one example of a power storage cell and transformer  38  is one example of an insulation element. BMU  10  is one example of a management device. Note that cell supervising circuits  30   a ,  30   b , and  30   d  also have the same configuration. 
     Such cell supervising circuit  30  uses, as a communication path formed with BMU  10 , a power supply path from a power source different from a power source of secondary battery cell  21 , and thus it is possible to suppress the collapse of cell balance due to variation in the operating power of cell supervising circuit  30  while suppressing the addition of components related to communication. 
     Moreover, for example, in BMS  100   b , the aforementioned power source is AC power source  12   a  and cell supervising circuit  30   b  further includes conversion circuit  39   a  which converts, into a DC power, the AC power supplied from AC power source  12   a  via transformer  38  and supplies the DC power to measurement circuit  31 . 
     Such cell supervising circuit  30   b  can covert the AC power into a DC power for operation. 
     Moreover, for example, transformer  38  is connected to AC power source  12   a  via AC power line  50   b  in BMS  100   b . AC power line  50   b  is connected with another cell supervising circuit  30   b  different from aforementioned cell supervising circuit  30   b  and BMS  100   b.    
     Such cell supervising circuit  30   b  can use AC power line  50   b  common to another cell supervising circuit  30   b  to perform communication with BMU  10 . 
     Moreover, for example, communication circuit  37  uses a frequency band higher than the frequency of the AC power to perform communication in BMS  100   b.    
     Such cell supervising circuit  30   b  can use a frequency band higher than the impedance in the impedance characteristics of the transmission path to perform communication with a relatively low communication power. 
     Moreover, for example, communication circuit  37  uses part of the frequency band as a communication channel assigned to cell supervising circuit  30   b  in BMS  100   b.    
     Such cell supervising circuit  30   b  can utilize the communication channel to thereby improve the communication speed and the communication quality. 
     Moreover, for example, cell supervising circuit  30   d  further includes voltage detection circuit  39   d  which outputs a control signal for controlling the activation or stop of measurement circuit  31  in accordance with the output voltage of conversion circuit  39   a.    
     With such cell supervising circuit  30   d , BMU  10   b  can easily activate cell supervising circuit  30   d.    
     Moreover, for example, cell supervising circuit  30   b  further includes clock generation circuit  39   b  which generates a clock signal synchronized with the frequency of the AC power. Measurement circuit  31  measures the state of charge of secondary battery cell  21  based on the clock signal generated. 
     Consequently, for example, when the AC power is supplied from single AC power source  12   a  to the plurality of cell supervising circuits  30   b , the AC power can be used to synchronize respective system clocks of the plurality of cell supervising circuits  30   b.    
     Moreover, for example, cell supervising circuit  30   b  further includes storage unit  35  which stores an address for discriminating cell supervising circuit  30   b  from another cell supervising circuit  30   b . The address is one example of discrimination information. 
     With such cell supervising circuit  30   b , the transmission of the state of charge provided with the address of cell supervising circuit  30   b  makes it possible for BMU  10   b  to discriminate which cell supervising circuit  30   b  has transmitted the state of charge acquired through the communication for the purpose of management. 
     Moreover, BMS  100  includes: BMU  10  which manages the state of secondary battery cell  21 ; and cell supervising circuit  30 . Cell supervising circuit  30  includes: measurement circuit  31  which measures the state of charge of secondary battery cell  21 ; transformer  38  which is provided for measurement circuit  31  to contactlessly receive the power supply from a power source different from secondary battery cell  21 ; and communication circuit  37  which transmits, to BMU  10  via transformer  38 , information indicating the state of charge measured by the measurement circuit. 
     Such BMS  100  uses, as the communication path of BMU  10  and cell supervising circuit  30 , a power supply path from another power source different from secondary battery cell  21 . Thus, BMS  100  can suppress the collapse of cell balance due to the variation in the operating power of cell supervising circuit  30  while suppressing the addition of components related to the communication. 
     Moreover, for example, BMS  100   b  includes a plurality of cell supervising circuits  30   b  corresponding to the plurality of secondary battery cells  21 . Each of the plurality of cell supervising circuits  30   b  is connected to AC power line  50   b  common to the plurality of cell supervising circuits  30   b  via transformer  38  included in aforementioned cell supervising circuit  30 . BMU  10   b  performs communication with each of the plurality of cell supervising circuits  30   b  via AC power line  50   b.    
     In such BMS  100   b , BMU  10   b  can perform communication with each of the plurality of cell supervising circuits  30   b  by use of AC power line  50   b  common to the plurality of cell supervising circuits  30   b.    
     Moreover, for example, BMU  10   b  performs communication by use of a frequency band higher than the frequency of the AC power through AC power line  50   b  in BMS  100   b . Each of the plurality of cell supervising circuits  30   b  uses part of the frequency band as the communication channel assigned to aforementioned cell supervising circuit  30   b.    
     Such BMS  100   b  can utilize the communication channel to thereby improve the communication speed and the communication quality. 
     Moreover, for example, in BMS  100   b , each of the plurality of cell supervising circuits  30   b  further includes storage unit  35  which stores the address for discriminating another cell supervising circuit  30   b  different from aforementioned cell supervising circuit  30   b . The information having the address of aforementioned cell supervising circuit  30   b  provided to the measured state of charge is transmitted to BMU  10   b.    
     In such BMS  100   b , the transmission of the information provided with the address of cell supervising circuit  30   b  by cell supervising circuit  30   b  permits BMU  10   b  to perform management while discriminate which cell supervising circuit  30   b  has transmitted the state of charge acquired through the communication. 
     Other Embodiments 
     The embodiments have been described above, but the present disclosure is not limited to the embodiments described above. 
     For example, the embodiments described above illustrate the transformer as the insulation element, but the insulation element may be another insulation element such as an electromagnetic resonance coupler. 
     Moreover, the battery pack used in an electric car is subjected to the management in the embodiments described above, but the BMS may manage a battery provided for any purpose. 
     Moreover, the circuit configurations described in the embodiments above are each one example and the present disclosure is not limited to the aforementioned circuit configurations. That is, the present disclosure also includes a circuit which can, similarly to the aforementioned circuit configuration, realize the characteristic functions of the present disclosure. For example, the present disclosure also includes those in which elements such as a switching element (transistor), a resistive element, and a capacitive element are connected in series or in parallel to a certain element within a range in which same functions as those of the aforementioned circuit configurations can be realized. 
     Moreover, the components included in the cell supervising circuit may be integrated in any manner in the embodiments described above. For example, the measurement circuit and the communication circuit may be realized as a single integrated circuit or may be realized as mutually different integrated circuits. 
     Moreover, the cell supervising circuit is realized by hardware in the embodiments described above. However, part of the components included in the cell supervising circuit may be realized by executing a software program suitable for the aforementioned components. Part of the components included in the cell supervising circuit may be realized by reading and executing a software program recorded on a recording medium such as a hard disc or a semiconductor memory by a program execution unit such as a central processing unit (CPU) or a processor. 
     Moreover, the processing executed by the specific processor in the embodiments described above may be executed by another processor. Moreover, the sequence of a plurality of processes may be changed or the plurality of processes may be performed in parallel in the operation described in the embodiments above. 
     In addition, the present disclosure also includes a mode obtained by making various modifications conceivable to those skilled in the art to the embodiments or a mode realized by combining the components and the functions in the embodiments in a desired manner within a scope not departing from the spirits of the present disclosure. 
     For example, the present disclosure may be released as a BMU, a power storage capacitor management system, or a power storage capacitor management unit, or the like. The present disclosure may be realized as a vehicle such as an electric car on which the cell supervising circuit or the BMS according to any of the embodiments described above is loaded. The present disclosure may be realized as a device other than a vehicle on which the cell supervising circuit or the BMS according to any of the embodiments described above is loaded. 
     Although only some exemplary embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The cell supervising circuit and the BMS using such a cell supervising circuit of the present disclosure are widely usable for in-vehicle uses, etc.