BATTERY MONITORING DEVICE, RESISTANCE VALUE DERIVATION METHOD, AND CELL VOLTAGE DERIVATION METHOD

A battery monitoring device includes an analog-digital converter and a plurality of cell selection switches that selectively connect any of battery cells to the analog-digital converter. Each of the cell selection switches includes a first switch part, a second switch part, a third switch part, and a resistor element. The first switch part is provided on a conduction path leading from one of the battery cells to the analog-digital converter, brings the conduction path into a conductive state in an on-state of the first switch part, and brings the conduction path into a non-conductive state in an off-state of the first switch part. The second switch part switches on and off of the first switch part. The third switch part switches a magnitude of a current flowing into the first switch part when the first switch part is in the on-state. The resistor element is provided on the conduction path.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japan application serial no. 2023-031330, filed on Mar. 1, 2023. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The disclosed technique relates to a battery monitoring device, a resistance value derivation method, and a cell voltage derivation method.

Related Art

The following techniques are known in relation to the technique of measuring the voltage of each battery cell in a battery pack composed of a plurality of battery cells connected in series.

Patent Document 1 (Japanese Patent Application Laid-Open No. 2013-223320) describes a battery charging device including: a battery pack in which a plurality of battery cells are connected in series; a transformer with a first winding and a plurality of second windings connected in parallel with the respective battery cells; a first switch connected in series with the first winding and connected in parallel with the battery pack; a plurality of second switches provided between wirings connecting the battery cells and the second windings; a measurement part that measures the voltage of each battery cell; a diode that flows a current of a constant direction from the second winding to the battery cell; and a control part that acquires a battery cell voltage measured by the measurement part, identifies the battery cell with the lowest voltage, brings the second switch corresponding to the identified battery cell into a connected state, brings the other second switches into a disconnected state, and performs control to repeat connection and disconnection of the first switch until the identified battery cell voltage is equal to or exceeds the average voltage of all the battery cells.

Patent Document 2 (Japanese Patent Application Laid-Open No. 2015-34750) describes a cell voltage monitoring device that measures the voltage of a stacked battery in which a plurality of battery cells are connected in series. The cell voltage monitoring device includes: a measurement circuit that measures the voltage of the stacked battery; a switch that has a plurality of input terminals to which the positive terminals of the plurality of battery cells are respectively connected and an output terminal to which the measurement circuit is connected, and switches the connection between the input terminal and the output terminal on a per-battery-cell basis; a control part that sequentially switches the connection of the switch to output the voltage of each battery cell to the measurement circuit at a predetermined cycle; and a monitoring part that monitors the power generation status of a predetermined battery cell based on the voltage of the predetermined battery cell measured by the measurement circuit at the predetermined cycle.

Patent Document 3 (Japanese Patent Application Laid-Open No. 2019-165583) describes a vehicle battery charging device including: a plurality of battery cell groups in which battery cells are connected in series; a total voltage detection part that detects a total voltage of the plurality of battery cell groups; a parallel connection switch that connects the plurality of battery cell groups in parallel; a series connection switch that connects the lowest-order cell in one battery cell group with the highest-order cell in another battery cell group among the plurality of battery cell groups; a switching control part that controls the parallel connection switch and the series connection switch to be off or on to switch the plurality of battery cell groups to parallel connection or series connection; and a welding determination part that determines welding of the series connection switch based on the total voltage during the period in which the series connection switch is controlled to the off-state by the switching control part.

A battery monitoring device that monitors the state of a battery cell of a battery pack composed of a plurality of battery cells connected in series has a function of measuring a two-terminal voltage (hereinafter referred to as a cell voltage) of each of the plurality of battery cells. To realize this function, the battery monitoring device includes an analog-digital converter and a plurality of cell selection switches that selectively connect any of the plurality of battery cells to the analog-digital converter. During cell voltage measurement, an operating current flows through the cell selection switch due to the operation of the cell selection switch. A low-pass filter composed of a resistor element and a capacitor is connected between the battery monitoring device and each battery cell to remove noise that is mixed in during cell voltage measurement. During cell voltage measurement, the operating current of the cell selection switch also flows through the resistor element constituting the low-pass filter, and a voltage drop occurs in the resistor element, so an error of a degree that cannot be ignored occurs in the measurement value of the cell voltage. For example, in the case where the resistance value of the resistor element constituting the low-pass filter is 1 kΩ and the operating current of the cell selection switch is 1 μA, an error of 1 mV occurs in the measurement value of the cell voltage. This error is not tolerable in vehicle-mounted battery monitoring devices developed in recent years. If it is possible to measure the resistance value of the resistor element constituting the low-pass filter, it will be possible to obtain the magnitude of the voltage drop in the resistor element during cell voltage measurement. By removing the component of the voltage drop in the resistor element from a digital value outputted from the analog-digital converter, an accurate cell voltage can be obtained. However, it is not easy to measure the resistance value of each resistor element provided corresponding to each of the plurality of battery cells.

SUMMARY

A battery monitoring device according to an embodiment of the disclosed technique includes: an analog-digital converter; and a plurality of cell selection switches that selectively connect any of a plurality of battery cells to the analog-digital converter. Each of the cell selection switches includes a first switch part, a second switch part, a third switch part, and a resistor element. The first switch part is provided on a conduction path leading from one of the plurality of battery cells to the analog-digital converter, brings the conduction path into a conductive state in an on-state of the first switch part, and brings the conduction path into a non-conductive state in an off-state of the first switch part. The second switch part switches on and off of the first switch part. The third switch part switches a magnitude of a current flowing into the first switch part with the first switch part being in the on-state. The resistor element is provided on the conduction path.

A resistance value derivation method according to an embodiment of the disclosed technique derives, using the battery monitoring device described above, a resistance value of the resistor element. The resistance value derivation method includes the following. A first digital value is acquired, the first digital value being an output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the off-state. A second digital value is acquired, the second digital value being the output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the on-state. The resistance value of the resistor element is derived based on the first digital value and the second digital value.

A cell voltage derivation method according to an embodiment of the disclosed technique derives, based on a resistance value derived by the resistance value derivation method described above, a cell voltage of the battery cell. The cell voltage derivation method includes the following. A component of a voltage drop occurring in the resistor element at a time of acquiring the first digital value or the second digital value is derived based on the resistance value of the resistor element. A value obtained by removing the component of the voltage drop from the first digital value or the second digital value is derived as the cell voltage.

DESCRIPTION OF EMBODIMENTS

Embodiments of the disclosed technique enable measurement of a resistance value of a resistor element provided between a battery cell and a cell selection switch.

Hereinafter, an example of an embodiment of the disclosed technique will be described with reference to the drawings. In the drawings, the same or equivalent components and parts will be labeled with the same reference signs, and repeated descriptions will be omitted.

FIG.1is a diagram showing an example of a configuration of a battery monitoring device10according to an embodiment of the disclosed technique. The battery monitoring device10has a function of measuring a two-terminal voltage (hereinafter referred to as a cell voltage) of each battery cell50in a battery pack51composed of a plurality of battery cells50connected in series. The battery monitoring device10is composed of an integrated circuit provided on a semiconductor substrate. The battery monitoring device10includes a plurality of cell selection switches20, a polarity inversion circuit30, an analog-digital converter (ADC)40, and a control part12. The battery monitoring device10has a plurality of connection terminals11provided corresponding to the respective battery cells50. The positive electrode of a corresponding battery cell50is connected to the connection terminal11via a low-pass filter60.

The plurality of low-pass filters60are provided corresponding to the respective battery cells50, and each include a resistor element61and a capacitor62. One terminal of the resistor element61is connected to the positive electrode of the corresponding battery cell50, and the other terminal of the resistor element61is connected to the corresponding connection terminal11and one terminal of the capacitor62. The other terminal of the capacitor62is connected to the connection terminal11corresponding to the next-lower-order battery cell50. One terminal of the capacitor62constituting the low-pass filter60corresponding to the lowest-order battery cell50is connected to the corresponding connection terminal11, and the other terminal of this capacitor62is connected to ground.

The plurality of cell selection switches20are provided corresponding to the respective battery cells50. The plurality of cell selection switches20selectively connect any of the plurality of battery cells50to the analog-digital converter40. One terminal of the cell selection switch20is connected to the positive electrode of the corresponding battery cell50via the connection terminal11and the low-pass filter60, and the other terminal of the cell selection switch20is connected to an integrated node n1or n2. One of two cell selection switches20adjacent to each other is connected to one of the integrated nodes n1and n2, and the other cell selection switch20is connected to the other of the integrated nodes n1and n2.

The polarity inversion circuit30includes four switches30A,30B,30C, and30D composed of semiconductor elements such as transistors. One terminal of the switch30A is connected to the integrated node n1and the other terminal of the switch30A is connected to the analog input of the analog-digital converter40. One terminal of the switch30B is connected to the integrated node n2and the other terminal of the switch30B is connected to the analog input of the analog-digital converter40. One terminal of the switch30C is connected to the integrated node n2and the other terminal of the switch30C is connected to the reference input of the analog-digital converter40. One terminal of the switch30D is connected to the integrated node n1and the other terminal of the switch30D is connected to the reference input of the analog-digital converter40. The analog-digital converter40outputs a digital value corresponding to a difference between the voltage inputted into the analog input and the voltage inputted into the reference input.

The control part12controls on and off of the plurality of cell selection switches20and the switches30A to30D constituting the polarity inversion circuit30. For example, in the case of measuring the cell voltage of a battery cell501, the control part12controls cell selection switches20nand20n-1to the on-state, controls the switches30B and30D of the polarity inversion circuit30to the on-state, and controls the switches30A and30C of the polarity inversion circuit30to the off-state. Accordingly, the positive electrode of the battery cell50nis connected to the analog input of the analog-digital converter40, the negative electrode of the battery cell50nis connected to the reference input of the analog-digital converter40, and a digital value corresponding to the cell voltage of the battery cell50nis outputted from the analog-digital converter40.

The control part12derives the resistance value of the resistor element61based on the digital value outputted from the analog-digital converter40. The control part12further derives the cell voltage based on the resistance value of the resistor element61. The control part12is composed of a computer including a central processing unit (CPU) and a memory. The control part12is an example of a “resistance value derivation part” and a “cell voltage derivation part” in the disclosed technique.

FIG.2is a diagram showing an example of the configuration of the cell selection switch20n. The configurations of each of the plurality of cell selection switches20are the same as each other. InFIG.2, illustration of the polarity inversion circuit30is omitted. The cell selection switch20nincludes a first switch part21, a second switch part22, a third switch part23, a current mirror circuit24, a Zener diode25, a first current source26A, and a second current source26B.

The first switch part21is provided on a conduction path P1leading from the positive electrode of the battery cell50nto the analog-digital converter40. The first switch part21includes two P-channel metal-oxide-semiconductor field-effect transistors (MOSFETs)1A and1B connected in series. The MOSFETs1A and1B are power MOSFETs with a so-called double-diffused MOSFET (DMOS) configuration. The source of the MOSFET1A is connected to the positive electrode of the corresponding battery cell50nvia the connection terminal11and the resistor element61, and the drain of the MOSFET1A is connected to the source of the MOSFET1B. The drain of the MOSFET1B is connected to the analog input of the analog-digital converter40. The gates of the MOSFETs1A and1B are each connected to a node n3to which the anode of the Zener diode25is connected.

With both the MOSFETs1A and1B turning into the on-state, the first switch part21turns into the on-state, and the conduction path P1turns into a conductive state. With the next-lower-order cell selection switch20n-1also turning into the on-state, the positive electrode and the negative electrode of the battery cell50nare connected to the analog-digital converter40, and a digital value corresponding to the cell voltage of the battery cell50nis outputted from the analog-digital converter40. In contrast, with both the MOSFETs1A and1B turning into the off-state, the first switch part21turns into the off-state, and the conduction path P1turns into a non-conductive state.

The current mirror circuit24includes two P-channel MOSFETs2A and2B. The sources of the MOSFETs2A and2B are each connected to a node n4, which is a connection point of the MOSFETs1A and1B. The gates of the MOSFETs2A and2B are each connected to the drain of the MOSFET2A. The drain of the MOSFET2A is connected to a node n5, and the drain of the MOSFET2B is connected to the node n3. The anode of the Zener diode25is connected to the drain (node n3) of the MOSFET2B, and the cathode of the Zener diode25is connected to the source (node n4) of the MOSFET2B.

The second switch part22includes switches3A and3B composed of semiconductor elements such as transistors. One terminal of the switch3A is connected to the node n5, and the other terminal of the switch3A is connected to the first current source26A. One terminal of the switch3B is connected to the node n3, and the other terminal of the switch3B is connected to the second current source26B. The first current source26A draws a constant current of a current value I1. The second current source26B draws a constant current of a current value I2. In this embodiment, I1=I2. However, it may also be possible that I1≠I2.

The second switch part22switches on and off of the first switch part21. The second switch part22includes switches3A and3B, which turn on and off complementarily according to control performed by the control part12. With the switch3A turning into the on-state and the switch3B turning into the off-state, a current of the current value I1flows in the current mirror circuit24. Accordingly, the gate-source voltages of the MOSFETs1A and1B constituting the first switch part21becomes approximately 0 V, and the MOSFETs1A and1B turn into the off-state (that is, the first switch part21turns into the off-state). In contrast, with the switch3A turning into the off-state and the switch3B turning into the on-state, the Zener diode25breaks down, a Zener current of the current value I2flows in the Zener diode25, and a Zener voltage is generated across two terminals of the Zener diode25. Accordingly, the gate-source of the MOSFETs1A and1B constituting the first switch part21is biased by the Zener voltage, and the MOSFETs1A and1B turn into the on-state (that is, the first switch part21turns into the on-state). The path leading from the node n4to the node n3via the Zener diode25is referred to as a branch path P2.

The third switch part23includes a switch4composed of a semiconductor element such as a transistor. One terminal of the switch4is connected to the node n3, and the other terminal of the switch4is connected to the high potential side of the first current source26A. The third switch part23switches the magnitude of the current flowing into the first switch part21when the first switch part21is in the on-state. In the case where the first switch part21is in the on-state and the third switch part23is in the off-state, between the first current source26A and the second current source26B, only the second current source26B is connected to the branch path P2. Thus, in the case where an outflow current Ioutflowing out from the first switch part21is zero, the current value of an inflow current Iinflowing into the first switch part21becomes I2(=I1). In contrast, in the case where the first switch part21is in the on-state and the third switch part23is in the on-state, both the first current source26A and the second current source26B are connected to the branch path P2. Thus, in the case where the outflow current Ioutflowing out from the first switch part21is zero, the current value of the inflow current Iinflowing into the first switch part21becomes I1+I2(=2I2).

In the battery monitoring device10, in the case of measuring the cell voltage of the battery cell50, by bringing the first switch part21into the on-state, the conduction path P1is brought into the conductive state, so the inflow current Iinflows into the first switch part21. The inflow current Iinalso flows through the resistor element61constituting the low-pass filter60, and a voltage drop occurs in the resistor element61, so an error of a degree that cannot be ignored occurs in the measurement value of the cell voltage. If it is possible to measure the resistance value of the resistor element61, it will be possible to obtain the magnitude of the voltage drop in the resistor element61at the time of cell voltage measurement. By removing the component of the voltage drop in the resistor element61from the digital value outputted from the analog-digital converter40, it is possible to obtain an accurate cell voltage. However, it is not easy to measure the resistance value of each resistor element61provided corresponding to each of the plurality of battery cells50. Nonetheless, according to the battery monitoring device10in this embodiment, it is possible to easily derive the resistance value of the resistor element61.

FIG.3is a flowchart showing an example of a flow of the process performed in the control part12in the case where the control part12functions as a resistance value derivation part deriving the resistance value of each resistor element61. In the case of deriving the resistance value of a resistor element61n, in the next-lower-order cell selection switch20n-1, the first switch part21is maintained in the on-state, and the third switch part23is maintained in the off-state.

In step S1, in the cell selection switch20n, the control part12controls the first switch part21to the on-state by controlling the switch3A of the second switch part22to the off-state and controlling the switch3B to the on-state. In step S2, the control part12controls the third switch part23to the off-state.

Due to the switch control in steps S1and S2, the positive electrode of the battery cell50nis connected to the analog input of the analog-digital converter40, and the negative electrode of the battery cell50nis connected to the reference input of the analog-digital converter40. At this time, in the case where the outflow current Ioutflowing out from the first switch part21is zero, the current value of the inflow current Iinflowing into the first switch part21becomes I2(=I1). The inflow current Iinalso flows through the resistor element61nconstituting the low-pass filter60n, and a voltage drop occurs in the resistor element61n. A first digital value D1n, which is the output value of the analog-digital converter40in the case of performing switch control in steps S1and S2, is represented by the following equation (1). In equation (1), Vcell_nis the cell voltage of the battery cell50n, Rnis the resistance value of the resistor element611, and Rn-1is the resistance value of the resistor element61n-1. The first digital value D1ncorresponds to the cell voltage of the battery cell50n, but includes an error due to the voltage drop in the resistor elements61nand61n-1.

In step S3, the control part12acquires the first digital value D1noutputted from the analog-digital converter40and associates it with individual identification information of the battery cell50nto save to a memory (not shown).

In step S4, the control part12controls the third switch part23to the on-state while maintaining the first switch part21in the on-state. At this time, in the case where the outflow current Ioutflowing out from the first switch part21is zero, the current value of the inflow current Iinflowing into the first switch part21becomes I1+I2(=2I2). The inflow current Iinalso flows through the resistor element61nconstituting the low-pass filter60n, and a voltage drop occurs in the resistor element61n. A second digital value D2n, which is the output value of the analog-digital converter40in the case of performing switch control in step S4, is represented by the following equation (2). The second digital value D2ncorresponds to the cell voltage of the battery cell50n, but includes an error due to the voltage drop in the resistor elements61nand61n-1.

In step S5, the control part12acquires the second digital value D2noutputted from the analog-digital converter40and associates it with individual identification information of the battery cell50nto save to the memory (not shown).

In step S6, the control part12derives the resistance value Rnof the resistor element61nbased on the first digital value D1nacquired in step S3and the second digital value D2nacquired in step S5. Specifically, the control part12derives the resistance value Rnof the resistor element61nby calculating the following equation (3). The control part12associates the derived resistance value Rnwith individual identification information of the battery cell50nto save to the memory (not shown).

In step S7, the control part12determines whether derivation of the resistance value has been completed for all the resistor elements61. The control part12repeats the process from step S1to step S6until derivation of the resistance value is completed for all the resistor elements61.

FIG.4is a flowchart showing an example of a flow of the process performed in the control part12in the case where the control part12functions as a cell voltage derivation part deriving the cell voltage of each battery cell50.

In step S11, the control part12reads, from the memory (not shown), the resistance value Rnof the resistor element61nand the resistance value Rn-1of the resistor element61n-1derived in step S6of the flowchart shown inFIG.3.

In step S12, the control part12reads the first digital value D1nused to derive the resistance value Rnfrom the memory (not shown).

In step S13, based on the resistance values Rnand Rn-1of the resistor elements61nand61n-1, the control part12derives the component of the voltage drop occurring in the resistor elements61nand61n-1at the time of acquiring the first digital value D1n, and derives, as the cell voltage Vcell_n, a value obtained by removing the component of the voltage drop from the first digital value D1n. Specifically, the control part12derives the cell voltage Vcell_nof the battery cell50nby performing calculation represented by following equation (4) based on the resistance values Rnand Rn-1read in step S11and the digital value D1nread in step S12. The current value I2is assumed to be known.

In step S14, the control part12determines whether derivation of the cell voltage has been completed for all the battery cells50. The control part12repeats the process from step S11to step S13until derivation of the cell voltage is completed for all the battery cells50.

Although the above description exemplifies the case where the cell voltage Vcell_nof the battery cell50nis derived using the digital value D1n, it is also possible to derive the cell voltage Vcell_nof the battery cell50using the digital value D2n. That is, the control part12may also read the digital value D2nin step S12, and derive the cell voltage Vcell_nof the battery cell50nby performing calculation represented by the following equation (5) based on the resistance values Rnand Rn-1and the digital value D2nin step S13.

As described above, the battery monitoring device10according to the disclosed technique includes the analog-digital converter40and the plurality of cell selection switches20which selectively connect any of the plurality of battery cells50to the analog-digital converter40. Each of the cell selection switches20includes: the first switch part21which is provided on the conduction path P1leading from one of the plurality of battery cells50to the analog-digital converter40, brings the conduction path P1into the conductive state in the on-state of the first switch part21, and brings the conduction path P1into the non-conductive state in the off-state of the first switch part21; the second switch part22which switches on and off of the first switch part21; the third switch part23which switches the magnitude of the inflow current Iinflowing into the first switch part21when the first switch part21is in the on-state; and the resistor element61provided on the conduction path P1. Furthermore, the battery monitoring device10includes: the first current source26A connected to the branch path P2branched from the conduction path P1when the first switch part21is in the on-state; and the second current source26B connected to the branch path P2when the first switch part21and third switch part23are both in the on-state.

Furthermore, in the battery monitoring device10, the control part12functions as a resistance value derivation part. The control part12functioning as the resistance value derivation part acquires the first digital value D1n, which is the output value of the analog-digital converter40in the case where the first switch part21is in the on-state and the third switch part23is in the off-state, acquires the second digital value D2n, which is the output value of the analog-digital converter40in the case where the first switch part21is in the on-state and the third switch part23is in the on-state, and derives the resistance value Rnof the resistor element61nbased on the first digital value D1nand the second digital value D2n.

Furthermore, in the battery monitoring device10, the control part12functions as a cell voltage derivation part. The control part12functioning as the cell voltage derivation part derives a component of a voltage drop occurring in the resistor elements61nand61n-1when acquiring the first digital value D1nor the second digital value D2n, based on the resistance values Rnand Rn-1of the resistor elements61nand61n-1, and derives, as the cell voltage Vcell_n, a value obtained by removing the component of the voltage drop from the first digital value D1nor the second digital value D2n.

According to the battery monitoring device10in the embodiment of the disclosed technique, it becomes possible to measure the resistance value of the resistor element61. Moreover, based on the measured resistance value, it becomes possible to perform measurement of the cell voltage with high precision.

In this embodiment, as an example, it has been illustrated that the battery monitoring device10performs derivation of the resistance value of the resistor element61and derivation of the cell voltage. However, the derivation of the resistance value of the resistor element61and the derivation of the cell voltage may also be performed by an external device cooperating with the battery monitoring device10. In that case, the battery monitoring device10supplies the first digital value D1nand the second digital value D2nto the external device.

Regarding the above embodiments, the following supplementary notes are further disclosed.

A battery monitoring device including:an analog-digital converter; anda plurality of cell selection switches that selectively connect any of a plurality of battery cells to the analog-digital converter, whereeach of the cell selection switches includes:a first switch part that is provided on a conduction path leading from one of the plurality of battery cells to the analog-digital converter, brings the conduction path into a conductive state in an on-state of the first switch part, and brings the conduction path into a non-conductive state in an off-state of the first switch part;a second switch part that switches on and off of the first switch part;a third switch part that switches a magnitude of a current flowing into the first switch part with the first switch part being in the on-state; anda resistor element that is provided on the conduction path.

The battery monitoring device according to Supplementary note 1, further including:a first current source that is connected to a branch path branched from the conduction path with the first switch part being in the on-state; anda second current source that is connected to the branch path with the first switch part and the third switch part both being in the on-state.

The battery monitoring device according to Supplementary note 1 or 2, further including:a resistance value derivation part that derives a resistance value of the resistor element, wherethe resistance value derivation part:acquires a first digital value which is an output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the off-state,acquires a second digital value which is the output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the on-state, andderives the resistance value of the resistor element based on the first digital value and the second digital value.

The battery monitoring device according to Supplementary note 3, further including:a cell voltage derivation part that derives a cell voltage of the battery cell, wherethe cell voltage derivation part:derives a component of a voltage drop occurring in the resistor element at a time of acquiring the first digital value or the second digital value, based on the resistance value of the resistor element, andderives, as the cell voltage, a value obtained by removing the component of the voltage drop from the first digital value or the second digital value.

A resistance value derivation method that derives, using the battery monitoring device according to any one of Supplementary notes 1 to 4, a resistance value of the resistor element,the resistance value derivation method including:acquiring a first digital value which is an output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the off-state;acquiring a second digital value which is the output value of the analog-digital converter with the first switch part brought into the on-state and the third switch part brought into the on-state; andderiving the resistance value of the resistor element based on the first digital value and the second digital value.

A cell voltage derivation method that derives, based on a resistance value derived by the resistance value derivation method according to Supplementary note 5, a cell voltage of the battery cell,the cell voltage derivation method including:deriving a component of a voltage drop occurring in the resistor element at a time of acquiring the first digital value or the second digital value, based on the resistance value of the resistor element, andderiving, as the cell voltage, a value obtained by removing the component of the voltage drop from the first digital value or the second digital value.