Patent Publication Number: US-2023147469-A1

Title: Battery monitor system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Patent Application No. PCT/JP2021/029907 filed on Aug. 16, 2021, which designated the U.S. and claims the benefit of priority from Japanese Patent Application No. 2020-149046 filed on Sep. 4, 2020. The entire disclosures of all of the above applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a system that monitors a plurality of battery cells forming a battery assembly. 
     BACKGROUND 
     In recent years, electric vehicles using secondary batteries and the like are spreading, and a demand for a battery monitor system (i.e., Battery Management System or BMS) for safely using secondary batteries is increasing. As for the secondary battery, by measuring an AC impedance and the like thereof, it is possible to estimate the internal state including the remaining battery capacity (i.e., State of Charge or SOC). For example, according to a conceivable technique, a signal excitation unit that causes a current to flow through a measurement target, a current measurement unit, and a voltage measurement unit that measures the response voltage from the battery are arranged with respect to each battery cell, and the impedance is measured using the current value and the voltage value obtained from these units. In the measurement using this AC impedance method, only the signal of the frequency component equal to the measurement frequency is detected, so the noise removal capability is high and measurement with a good signal-to-noise ratio (i.e., SNR) is possible. 
     SUMMARY 
     According to an example a battery monitor system may include: a reference signal generation unit; an excitation signal generation unit; a current generation unit for an excitation current; a current measurement unit for the excitation current; a voltage measurement unit for a voltage of each battery cell; an impedance measurement unit for an impedance of each battery cell; a noise measurement unit for a noise voltage; and a control unit. The control unit selects one or more battery cells not a measurement target, and the noise measurement unit measures the noise voltage near a measurement frequency equal to a frequency of an orthogonal reference signal while operating the voltage measurement unit without operating the current generation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG.  1    is a functional block diagram showing the configuration of a battery monitor device in a first embodiment; 
         FIG.  2    is a diagram showing the configuration of an excitation signal processing unit; 
         FIG.  3    is a diagram showing the configuration of a current excitation unit; 
         FIG.  4    is a diagram showing the configuration of a current measurement unit; 
         FIG.  5    is a diagram showing the configuration of a voltage measurement unit; 
         FIG.  6    is a diagram showing an example of a form of communication performed between a plurality of battery monitor devices and a battery control device; 
         FIG.  7    is a diagram showing waveforms of orthogonal reference signals; 
         FIG.  8    is a diagram showing the waveform and frequency spectrum of the excitation current; 
         FIG.  9    is a diagram showing the frequency spectrum of the excitation AC voltage in an ideal state and the frequency spectrum of the voltage output from the voltage measurement unit; 
         FIG.  10    is a diagram equivalent to  FIG.  9    when noise current is superimposed; 
         FIG.  11    is a flowchart showing a measurement process by the battery monitor device; 
         FIG.  12    is a diagram equivalent to  FIG.  10    when only the voltage measurement unit is operated; 
         FIG.  13    is a functional block diagram showing the configuration of a battery monitor device in a second embodiment; 
         FIG.  14    is a functional block diagram showing the configuration of a battery monitor device in a third embodiment; 
         FIG.  15    is a timing chart showing an example of a control mode when impedance and noise are measured by the battery monitor device of the third embodiment according to the fourth embodiment; 
         FIG.  16    is a functional block diagram showing the configuration of a battery monitor device in a fifth embodiment; 
         FIG.  17    is a flowchart showing a measurement process by the battery monitor device; 
         FIG.  18    is a diagram for explaining impedance measurement in an impedance measurement unit; 
         FIG.  19    is a diagram showing an example of a data table indicating impedance values and noise values at each frequency; 
         FIG.  20    is a functional block diagram showing the configuration of a battery monitor device in a sixth embodiment; and 
         FIG.  21    is a functional block diagram showing the configuration of a battery monitor device in a seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     For example, a battery pack mounted on an electric vehicle or a hybrid vehicle is connected to an inverter for driving a motor, and when the vehicle is running, the drive current of the inverter is superimposed on the battery current as noise current. In the impedance measurement of the conceivable technique, an error may occur in the impedance measurement result when a noise current including frequency components that are the same as or near the measurement frequency is superimposed on the excitation current. Therefore, accurate impedance measurement may not be performed while the vehicle is running, and an error may occur in the estimation of the internal state. 
     The present embodiments have been made in view of the circumstances described above, and an object thereof is to provide a battery monitor system capable of accurately measuring the impedance of a secondary battery even in an environment where noise current flows. 
     According to the battery monitor system, the excitation signal generation unit generates the excitation signal by processing the in-phase signal of the orthogonal reference signal generated by the reference signal generation unit, and the current generation unit generates the excitation current based on the excitation signal to energize the battery cell. The impedance measurement unit measures the alternating-current impedance of the battery cell based on the excitation current measured by the current measurement unit and the voltage of the battery cell measured by the voltage measurement unit. The noise measurement unit measures noise superimposed on the battery cell as a noise voltage based on the voltage measured by the voltage measurement unit, and estimates the noise current. 
     The control unit selects one or more battery cells whose AC impedance is not to be measured as a measurement target from among the plurality of battery cells, and measures the noise voltage near the measurement frequency equal to the frequency of the orthogonal reference signal using the noise measurement unit under a condition that only the voltage measurement unit connected to the selected battery cell is operated without operating the current generation unit connected to the selected battery cell. By controlling in this way, it is possible to measure the impedance of the battery cell and measure the noise voltage in parallel without affecting each other. Therefore, even when the battery assembly is supplying power to the load, it is possible to measure the impedance and the noise voltage with high accuracy. 
     According to the battery monitor system, the cell voltage measurement unit measures the voltage of the battery cell, and the resistance voltage measurement unit measures the voltage of the resistance element connected in series to the plurality of battery cells. Then, the control unit measures the noise voltage in the same manner as in claim  1  while operating the resistance voltage measurement unit without operating the current generation unit. Even in this configuration, it is possible to measure the impedance of the battery cell and the noise voltage in parallel without affecting each other, as in claim  1 . 
     According to the battery monitor system, the control unit transmits the noise voltage measured by the noise measurement unit to the higher level system together with the measurement result of the AC impedance by the impedance measurement unit. This allows the higher level system to evaluate the AC impedance measurement result based on the noise voltage level. 
     According to the battery monitor system, the control unit switches the battery cells whose noise voltage is to be measured as a measurement target in a time division manner, measures the AC impedance of all the battery cells within a certain period of time, and transmits these measurement results to the higher level system. As a result, the higher level system can grasp the AC impedance measurement results of all battery cells within a certain period of time. 
     First Embodiment 
     As shown in  FIG.  1   , the battery assembly  1  is configured by connecting a plurality of, for example, four battery cells  2 ( 1 ) to  2 ( 4 ) in series. The battery cell  2  is, for example, a secondary battery such as a lithium ion battery. The battery assembly monitor device  3  connected to the battery assembly  1  includes a control unit  4 , a signal generation unit  5 , an excitation signal processing unit  6 , a current excitation unit  7 , a current measurement unit  8 , a voltage measurement unit  9 , an impedance measurement unit  10 , a noise measurement unit  11 , a communication I/F  12 , and the like. A current excitation unit  7  , a current measurement unit  8  and a voltage measurement unit  9  are provided corresponding to each battery cell  2 . The communication I/F  12  is used by the battery assembly monitor device  3  to communicate with a later-described higher level system. 
     Voltage measurement units  8 ( 1 ) to  8 ( 4 ) are connected to the upper and lower electrodes of the battery cells  2 ( 1 ) to  2 ( 4 ), respectively. As shown in  FIG.  7   , the signal generation unit  5  generates orthogonal reference signals REFI and REFQ, which are sine waves and cosine waves having the same frequency as the measurement frequency fro. These orthogonal reference signals REFI and REFQ are output to the current measurement unit  8  and voltage measurement unit  9 . Only the reference signal REFI is input to the excitation signal processing unit  6 . 
     As shown in  FIG.  2   , the excitation signal processing unit  6  corresponding to the excitation signal generation unit level-converts the input reference signal REFI according to the target excitation current set by the control unit  4  by the level converter  21 , so that a DC offset is obtained, and further converts to an analog voltage signal by the DAC  22 . The analog voltage signal is input to the error amplifier  24  after the image component imparted by the demodulation processing is removed through the filter  23 . 
     The voltage signal IxSP from the current excitation unit  7  is input to the inversion input terminal of the error amplifier  24 , and the output signal VCSx is controlled so as to match the potential difference from the voltage signal IxSN with the voltage signal applied to the non-inversion input terminal as the control target value. Here, X is equal to 1 to 4. The excitation current output from the current excitation unit  7  is, as shown in  FIG.  8   , an AC current to which a DC offset is added, and its frequency components include a DC component and the measurement frequency f LO  component. 
     As shown in  FIG.  3   , the current excitation unit  7  corresponding to the current generation unit includes a series circuit of a resistance element RLx, an N-channel MOSFET MOSFET_Mx, and a resistance element RSx. The output signal VCSx of the excitation signal processing unit  6  is given to the gate of the FET FET_Mx, and both ends of the resistance element RSx are input to the excitation signal processing unit  6  and the current measurement unit  8  as voltage signals IxSP and IxSN, respectively. Both ends of the series circuit output as excitation current signals IxFP and IxFN. That is, the current excitation unit  7  generates the excitation current signals IxFP and IxFN such that the terminal voltage of the element RSx, which is the sense resistor, matches the control target value. 
     As shown in  FIG.  4   , the current measurement unit  8  includes a subtraction units  25 P and  25 N, an ADC  26 , a DC offset correction unit  27 , a subtraction unit  28 , a filter  29  and an orthogonal demodulator  30 . The voltage signals IxSP and IxSN are input to ADC  26  via the subtraction units  25 P and  25 N, respectively. The voltage data converted by the ADC  26  is input to the DC offset correction unit  27  and the subtraction unit  28 . The DC offset correction unit  27  generates a DC offset correction value according to the output data of the ADC  26  and inputs it to the subtraction units  25  and  28 . 
     The output data of the subtraction unit  28  is input to the orthogonal demodulator  30  via the filter  29 . The orthogonal demodulator  30  includes multipliers  311  and  31 Q and filters  321  and  32 Q. The output data of the filter  29  is input to multipliers  31 I and  31 Q. Reference signals REFI and REFQ are also input to the multipliers  31 I and  31 Q, respectively, and the orthogonal demodulation is performed by multiplying each input signal. The image components are removed from the output data of the multipliers  31 I and  31 Q through the filters  32 I and  32 Q, respectively, to generate data IxBI and IxBQ, which are input to the impedance measurement unit  10  and the noise measurement unit  11 , respectively. 
     The configuration of the voltage measurement unit  9  is symmetrical to that of the current measurement unit  8 , as shown in  FIG.  5   , and the corresponding configuration element has the same reference numeral. The terminal voltages VxSP and VxSN of the corresponding battery cells  2  are input to the voltage measurement unit  9 , and the orthogonal demodulation is performed in the same manner as the current measurement unit  8 , data VxBI and VxBQ are generated, and they are input to the impedance measurement unit  10  and noise measurement unit  11 . In the electronic monitor device  3 , the portion other than the current excitation unit  7  is constructed as an integrated circuit  33 . 
     When the excitation current is applied to the battery cell  2 , it is converted into voltage by the AC impedance. An ideal frequency spectrum of the excitation voltages VxSP and VxSN generated at both ends of the battery cell  2  produces a signal at the DC component and at the measurement frequency fLo as shown in  FIG.  9   . The DC component is the sum of the product of the voltage of the battery cell  2 , the impedance, and the DC offset of the excitation current, and an AC voltage is generated at the frequency f LO , which is the product of the AC impedance and the excitation AC current. At this time, the voltage output as the measurement result of the voltage measurement unit  9  is only the DC voltage of the battery cell  2 . 
     On the other hand, as shown in  FIG.  10   , if the input voltage includes noise due to the noise current flowing, the voltage as the measurement result shows a frequency spectrum with a DC component and a small band around it. Conventionally, this has been a factor of error in impedance measurement. 
     Actually, as shown in  FIG.  6   , a plurality of battery assemblies  1  are connected in series, and a battery monitor device  3  is connected to each battery assembly  1 . A plurality of battery monitor devices  3  communicate with an ECU (i.e., a battery control device  34 ), which is a higher level system. The battery control device  34  and the communication I/F  12  of each battery monitor device  3  are connected in a daisy chain manner, for example. 
     Next, an operation of the present embodiment will be described. An example of measuring the impedance of battery cells  2 ( 1 ) to  2 ( 3 ) and measuring the noise in battery cell  2 ( 4 ) is shown. As shown in  FIG.  11   , the battery control device  34  transmits to the battery monitor device  3  the measurement frequency fro, the impedance measurement target, in this case, battery cells  2 ( 1 ) to  2 ( 3 ) and a measurement start command (at A 1 ). 
     Upon receiving the measurement start command, the control unit  4  of the battery monitor device  3  causes the excitation signal processing units  6 ( 1 ) to  6 ( 3 ) to generate VCSx as a DC voltage value. Then, the current excitation units  7 ( 1 ) to  7 ( 3 ) control the voltages IxFP and IxFN so as to apply a DC current corresponding to the voltage value VCSx (at B 1 ). 
     At this time, signals IxSP and IxSN as DC offsets corresponding to the DC current are input to the current measurement units  8 ( 1 ) to  8 ( 3 ) from the current excitation units  7 ( 1 ) to  7 ( 3 ). Similarly, terminal voltages VxSP and VxSN of battery cells  2 ( 1 ) to  2 ( 3 ) are input as DC offsets to voltage measurement units  9 ( 1 ) to  9 ( 3 ), respectively. Current measurement units  8 ( 1 ) to  8 ( 3 ) and voltage measurement units  9 ( 1 ) to  9 ( 3 ) remove the DC offset included in the input signal by DC offset correction unit  27  (at B 2 ). 
     Next, the signal generation unit  5  generates orthogonal reference signals REFI and REFQ. The excitation signal processing units  6 ( 1 ) to  6 ( 3 ) and the current excitation units  7 ( 1 ) to  7 ( 3 ) apply excitation currents according to the reference signal REFI (at B 3 ). The current measurement units  8 ( 1 ) to  8 ( 3 ) measure currents flowing through the sense resistors RS of the current excitation units  7 ( 1 ) to  7 ( 3 ), and the voltage measurement units  9 ( 1 ) to  9 ( 4 ) measure the voltages of the corresponding battery cells  2 ( 1 ) to  2 ( 4 ) (at B 4 ). 
     In this state, the impedance measurement unit  10  measures the impedance of the battery cells  2 ( 1 ) to  2 ( 3 ), and the noise measurement unit  11  measures the noise of the battery cell  2 ( 4 ) (at B 5 ).  FIG.  12    shows the voltage measured in this state and the frequency spectrum of the signal output from the noise measurement unit  11 . Then, the control unit  4  transmits the measured impedance and noise to the battery control device  34  via the communication I/F  12  (at B 6 ). 
     The battery control device  34  stores the impedance and noise received from the battery monitor device  3  in a table for storing the latest measurement results (at A 2 ). Then, the accuracy of the measurement result is determined according to the noise level (at A 3 ). When the determination value of the accuracy level is less than the predetermined value, the received impedance and noise measurement results are written and updated in the data storage table together with the determination value. On the other hand, when the determination value is equal to or greater than the predetermined value, the data storage table is not updated (at A 4 ). 
     As described above, according to the battery assembly monitor device  3  of the present embodiment, the excitation signal processing unit  6  processes the in-phase signal REFI of the orthogonal reference signal generated by the signal generation unit  5  to generate the excitation signal VCSx, and the current excitation unit  7  generates an excitation current based on the excitation signal VCSx by using the voltage signals IxSP and IxSN, and energizes the battery cell  2 . The impedance measurement unit  10  measures the alternating-current impedance of the battery cell  2  based on the excitation current measured by the current measurement unit  8  and the voltage of the battery cell  2  measured by the voltage measurement unit  9 . The noise measurement unit  11  measures the noise superimposed on the battery cell  2  as a noise voltage based on the same excitation current and the same voltage. 
     The control unit  4  selects the battery cell  2 ( 4 ), the AC impedance of which is not to be measured as a measurement target, from among the battery cells  2 ( 1 ) to  2 ( 4 ), and measures the noise voltage near the measurement frequency equal to the frequency fro of the orthogonal reference signal using the noise measurement unit  11  under a condition that only the voltage measurement unit  8  is operated without operating the current excitation unit  7  connected to the battery cell  2 ( 4 ). By controlling in this way, it is possible to measure the impedance of the battery cell  2  and measure the noise voltage in parallel without affecting each other. Therefore, even when the battery assembly  1  is supplying power to the load, it is possible to measure the impedance and the noise voltage with high accuracy. 
     Then, the control unit  4  transmits the noise voltage of the battery cell  2 ( 4 ) to the battery control device  34  together with the measurement result of the AC impedance by the impedance measurement unit  10 . The battery control device  34  determines the accuracy level of the measurement result according to the level of the noise voltage. When the determination value of the accuracy level is smaller than the predetermined value, the battery control device  34  writes the received measurement result of the impedance and noise together with the determination value in the table for data storage to update the table. But, when the determination value is equal to or greater than the predetermined value, the battery control device  34  does not update the table. In this manner, the battery control device  34  determines whether or not to update the data storage table according to the accuracy level of the measurement result, thereby improving the accuracy of the measurement result. 
     Second Embodiment 
     Hereinafter, the identical parts as those in the first embodiment will be designated by the same reference numerals for simplification of the description. Only differences from the first embodiment will be described below. As shown in  FIG.  13   , the battery monitor device  41  of the second embodiment includes a pair of an excitation signal processing unit  6 ( 1 ) and a current excitation unit  7 ( 1 ) that energizes the battery cells  2 ( 1 ) and  2 ( 2 ) with the excitation current, and a pair of an excitation signal processing unit  6 ( 2 ) and a current excitation unit  7 ( 2 ) that energizes the battery cells  2 ( 3 ) and  2 ( 4 ) with the excitation current. Then, the current measurement unit  8 ( 1 ) measures the excitation current flowing through the battery cells  2 ( 1 ) and  2 ( 2 ), and the current measurement unit  8 ( 2 ) measures the excitation current flowing through the battery cells  2 ( 3 ) and  2 ( 4 ). A portion of the battery monitor device  41  excluding the current excitation unit  7  is configured as an integrated circuit  42 . 
     According to the second embodiment configured as described above, the excitation current can be energized to the battery cells  2 ( 1 ) to  2 ( 4 ) and measured only by using two sets of the excitation signal processing unit  6 , the current excitation unit  7 , and the current measurement unit  8 , so that the circuit area can be reduced. 
     Third Embodiment 
     As shown in  FIG.  14   , in the battery monitor device  43  of the third embodiment, the battery monitor device  41  is modified, and only measurement units  9 ( 1 ) and  9 ( 2 ) are used for voltage measurement. Then, the selector  44 ( 1 ) is arranged between the battery cells  2 ( 1 ) and  2 ( 2 ) and the voltage measurement unit  9 ( 1 ), and the selector  44 ( 2 ) is arranged between the battery cells  2 ( 3 ) and  2 ( 4 ) and the voltage measurement unit  9 ( 2 ). Switching of the selector  44  is controlled by the control unit  4 . 
     That is, the voltage measurement of the battery cells  2 ( 1 ) and  2 ( 2 ) is performed by the voltage measurement unit  9 ( 1 ) by switching the selector  44 ( 1 ), and the voltage measurement of the battery cells  2 ( 3 ) and  2 ( 4 ) is performed by the voltage measurement unit  9 ( 2 ) by switching the selector  44 ( 2 ). A portion of the battery monitor device  43  excluding the current excitation unit  7  is configured as an integrated circuit  45 . According to the third embodiment configured as described above, the circuit area can be further reduced. 
     Fourth Embodiment 
     A fourth embodiment shown in  FIG.  15    is an example of a control mode when impedance and noise are measured by the battery monitor device  43  of the third embodiment. “Impedance measurements # 1  to # 4 ” indicate that all measurements are performed in the same manner, and only the contents of “impedance measurement # 1 ” are shown. There are four measurement phases. In the first phase, the control unit  4  measures the impedance of the battery cell  2 ( 1 ) with the voltage measurement unit  9 ( 1 ) and measures the noise of the battery cell  2 ( 3 ) with the voltage measurement unit  9 ( 2 ). In the next second phase, the voltage measurement unit  9 ( 1 ) measures the impedance of the battery cell  2 ( 2 ), and the voltage measurement unit  9 ( 2 ) similarly measures the noise of the battery cell  2 ( 3 ). 
     In the subsequent third phase, the voltage measurement unit  9 ( 1 ) measures the noise of the battery cell  2 ( 1 ), and the voltage measurement unit  9 ( 2 ) measures the impedance of the battery cell  2 ( 3 ). In the next fourth phase, the voltage measurement unit  9 ( 1 ) measures the noise of the battery cell  2 ( 2 ), and the voltage measurement unit  9 ( 2 ) similarly measures the impedance of the battery cell  2 ( 4 ). The measured impedances and noise voltages of the battery cells  2 ( 1 ) to  2 ( 4 ) are sent to the battery control device  34 . This feature of measurement is repeated in sequence. 
     As described above, according to the fourth embodiment, the control unit  4  switches the battery cells  2  as the measurement target of the noise voltage in a time division manner, measures the AC impedance of all the battery cells  2  within a certain period of time, and transmits these measurement results to the battery control device  34 . Thereby, the battery control device  34  can grasp the measurement results of the AC impedance and the noise voltage of all the battery cells  2  within a certain period of time. 
     Fifth Embodiment 
     As shown in  FIG.  16   , the battery monitor device  46  of the fifth embodiment is a modification of the battery monitor device  41 , in which the output signal of the noise measurement unit  11  is input into the noise subtraction units  47 ( 1 ) and  47 ( 2 ), and the output signals of the noise subtraction units  47 ( 1 ) and  47 ( 2 ) are input to the impedance measurement unit  48 . A portion of the battery monitor device  46  excluding the current excitation unit  7  is configured as an integrated circuit  49 . 
     Next, operation of the fifth embodiment will be described. An example of measuring the impedance of battery cell  2 ( 1 ) and measuring the noise in battery cell  2 ( 3 ) is shown. As shown in  FIG.  17   , the battery control device  34  transmits the measurement frequency fro, the information about the measurement target, in this case, the battery cells  2 ( 1 ) and  2 ( 3 ), the latest information of the battery cell  2 ( 1 ), that is the previous impedance measurement value of the battery cell  2 ( 1 ), and the measurement start command to the battery monitor device  3  (at A 5 ). 
     Upon receiving the measurement start command, the control unit  4  of the battery monitor device  46  causes the excitation signal processing units  6 ( 1 ) to generate VCSx as a DC voltage value. Then, the current excitation unit  7 ( 1 ) control the voltages IxFP and IxFN so as to apply a DC current corresponding to the voltage value VCSx (at B 7 ). Note that the latest impedance measurement value is transferred to the noise subtraction unit  47 . Similar to the first embodiment, the current measurement unit  8 ( 1 ) and voltage measurement unit  9 ( 1 ) remove the DC offset included in the input signal by DC offset correction unit  27  (at B 8 ). 
     Next, the signal generation unit  5  generates orthogonal reference signals REFI and REFQ. The excitation signal processing unit  6 ( 1 ) and the current excitation unit  7 ( 1 ) apply excitation currents according to the reference signal REFI (at B 9 ). The current measurement unit  8 ( 1 ) measures the current flowing through the sense resistor RS of the current excitation unit  7 ( 1 ), selects the battery cell  2 ( 1 ) with the selector  44 ( 1 ), and selects the battery cell  2 ( 3 ) with the selector  44 ( 2 ). The voltage measurement units  9 ( 1 ) and  9 ( 2 ) measure voltages V 1  and V 3  of battery cells  2 ( 1 ) and  2 ( 3 ), respectively (at B 10 ). 
     In this state, the noise measurement unit  11  measures the noise of the battery cell  2 ( 3 ) (at B 11 ). The noise subtraction unit  47  calculates the noise voltage of the battery cell  2 ( 1 ) from the latest impedance measurement values Z 1  and Z 3  notified from the battery control device  34 , and subtracts it from the measured voltage V 1  (at B 12 ). Processing here will be described with reference to  FIG.  18   . 
     The impedances of the battery cells  2 ( 1 ) and  2 ( 3 ) are defined as Z 1  and Z 3 , the excitation current is defined as Imeas, and the noise current is defined as In. The voltages V 1  and V 3  of the battery cells  2 ( 1 ) and  2 ( 3 ) are given below. 
         V 1 =Z 1 ( In+Imeas ) 
         V 3 =Z 3× In  
 
     The product of the impedance Z 1  and the noise current In is expressed as follows from the latest measured values. 
         Z 1 ×In=V 3 ×Z 1 /Z 3 
     When subtracting the product of (Z 1 ·In) from the measured voltage V 1 , the product of the impedance Z 1  and the excitation current Imeas is obtained. 
         V 1 −V 3 ×Z 1 /Z 3 =V 1= Z 1( In+Imeas )−V3× Z 1 /Z 3= Z 1× Imeas  
 
     In subsequent step B 13 , the impedance measurement unit  10  obtains the impedance Z 1  of the battery cell  2 ( 1 ) by dividing the product of (Z 1 ·Imeas) by the excitation current Imeas. Then, the control unit  4  transmits the measured impedance and noise to the battery control device  34  via the communication I/F  12  (at B 6 ). 
     After the battery control device  34  executes steps A 2  and A 3 , the received impedance and noise measurement results are written and updated in the data storage table together with the accuracy level determination value, but, when the determination value is equal to or larger than the predetermined value, the data storage table is not updated (at A 6 ). 
     Based on the frequency list, the battery control device  34  executes the processing shown in  FIG.  17    while changing the frequency, thereby creating a data table showing impedance values and noise values at each frequency as shown in  FIG.  19   . Thereby, the battery control device  34  can obtain the frequency with a high noise level. 
     As described above, according to the fifth embodiment, the noise subtraction unit  47  is arranged between the voltage measurement unit  9  and the impedance measurement unit  48 , and the noise subtraction unit  47  subtracts the value of (V 3 ·Z 1 /Z 3 ), corresponding to the result obtained by multiplying a certain battery cell  2  as the measurement target by the measurement result Z 1  of the AC impedance measured last time, from the voltage V 1  output from the voltage measurement unit  9 ( 1 ). Thereby, the impedance measurement unit  48  can obtain the impedance Z 1  of the battery cell  2 ( 1 ) by eliminating the influence of the noise current In. 
     Sixth Embodiment 
     As shown in  FIG.  20   , the battery monitor device  50  of the sixth embodiment is prepared by modifying the battery monitor device  46 . A resistance element  51  for noise measurement is connected to the low potential side of the battery cell  2 ( 4 ), and the terminal voltage of the resistance element  51  is measured by the voltage measurement unit  9 ( 3 ). The measurement result of the voltage measurement unit  9 ( 3 ) is input to the noise measurement unit  11 . A portion of the battery monitor device  46  excluding the current excitation unit  7  is configured as an integrated circuit  52 . The voltage measurement units  9 ( 1 ) and  9 ( 2 ) correspond to the cell voltage measurement unit, and the voltage measurement unit  9 ( 3 ) corresponds to the resistance voltage measurement unit. 
     According to the sixth embodiment configured as described above, the voltage measurement units  9 ( 1 ) and  9 ( 2 ) measure the voltage of the battery cells  2 , and the voltage measurement unit  9 ( 3 ) measures the voltage of the resistance element  51  connected in series to the plurality of battery cells  2 . Then, the control unit  4  operates the voltage measurement unit  9  ( 3 ) without operating the current excitation unit  7  to measure the noise voltage. Even in this configuration, it is possible to measure the impedance of the battery cell  2  and the noise voltage in parallel without affecting each other. 
     Seventh Embodiment 
     As shown in  FIG.  21   , the battery monitor device  53  of the seventh embodiment is prepared by modifying the battery monitor device  50 . The selectors  54 ( 1 ) and  54 ( 2 ) are arranged in place of the selectors  44 ( 1 ) and  44 ( 2 ). Each terminal voltage of the battery cells  2 ( 1 ) to  2 ( 4 ) can be switched and input into the selectors  54 ( 1 ) and  54 ( 2 ), respectively. A portion of the battery monitor device  53  excluding the current excitation unit  7  is configured as an integrated circuit  55 . 
     With this configuration, for example, it is possible to control such that the selector  54 ( 1 ) selects the battery cell  2 ( 1 ), constantly measures the impedance of the battery cell  2 ( 1 ), and the selector  54 ( 2 ) switches and measures the impedance of the other battery cells  2 ( 2 ) to  2 ( 4 ) in a time-sharing manner. In this way, for example, when the impedance at a frequency of 100 Hz varies by 20% or more from its average value, it is possible to select the battery cell  2  to be monitored intensively and constantly measure the impedance. 
     As described above, according to the seventh embodiment, two sets of the voltage measurement unit  9  and the selector  54  are provided, and each of the selectors  54 ( 1 ) and  54 ( 2 ) switches the voltage of all the battery cells  2 ( 1 ) to  2 ( 4 ) to be measured. As a result, for example, it is possible to execute the measurement control such that the selector  54 ( 1 ) fixedly selects the cell  2 ( 1 ) to be monitored intensively and performs high-speed measurement, and the voltages of the other normal battery cells  2 ( 2 ) to  2 ( 4 ) are sequentially switched by the selector  54 ( 2 ) to measure the voltage with low speed. 
     Other Embodiments 
     The number of battery cells  2  may not be limited to “4” and may be any number as long as the number is more than one. 
     The feature of communication performed between a plurality of battery monitor devices and the battery control device may not be limited to the daisy chain connection, and may employ a bus method, a round robin method, wireless communication, or the like. 
     The selection of the measurement mode of the battery cell  2  using the selectors  54 ( 1 ) and  54 ( 2 ) performed in the seventh embodiment may be applied to other embodiments. 
     The controllers and methods described in the present disclosure may be implemented by a special purpose computer created by configuring a memory and a processor programmed to execute one or more particular functions embodied in computer programs. Alternatively, the controllers and methods described in the present disclosure may be implemented by a special purpose computer created by configuring a processor provided by one or more special purpose hardware logic circuits. Alternatively, the controllers and methods described in the present disclosure may be implemented by one or more special purpose computers created by configuring a combination of a memory and a processor programmed to execute one or more particular functions and a processor provided by one or more hardware logic circuits. The computer programs may be stored, as instructions being executed by a computer, in a tangible non-transitory computer-readable medium. 
     It is noted that a flowchart or the processing of the flowchart in the present application includes sections (also referred to as steps), each of which is represented, for instance, as A 1 . Further, each section can be divided into several sub-sections while several sections can be combined into a single section. Furthermore, each of thus configured sections can be also referred to as a device, module, or means. 
     While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.