Cell balancing method, cell balancing device, and charging system

Abstract

A cell balancing device (1) according to the present embodiment comprises, with respect to the entirety of N-number of cells (20) connected in series: one transformer (T); one switching element (112); one current detector (113); N-number of rectifying elements (122); and one control unit (13). The control unit (13) causes the switching element (112) to perform on / off operations by outputting a PWM signal to the switching element (112), and in particular, when the current value of the current detected by the current detector (113) exceeds a predetermined upper limit threshold (TH), the control unit (13) sets the PWM signal to an off-level.

Description

Cell balancing method, cell balance device, and charging system

[0001] The present invention relates to a cell balancing method, a cell balancing device, and a charging system.

[0002] Conventionally, cell balancing devices that equalize the terminal voltages of multiple series-connected cells have been known. For example, Patent Document 1 listed below discloses a balancing circuit including a transformer having a first winding coupled to a first battery, a second winding coupled to a second battery, and a third winding, a first transistor coupled between the first battery and the first winding, a second transistor coupled between the second battery and the second winding, and a control circuit that turns on the first transistor so that a current flows in the first winding and so that an induced voltage is generated in the second and third windings.

[0003] Special table 2019-508002 publication

[0004] However, the above-mentioned conventional techniques have the problem that the number of required parts tends to be large. For example, the above-mentioned balancing circuit requires a transformer or the like to balance the first battery and the second battery, so as the number of series-connected cells increases, the number of transformers and the like also increases, and the number of required parts tends to increase.

[0005] In one aspect, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a cell balance device and the like that requires a reduced number of parts.

[0006] In order to solve the above-described problems, a cell balancing method according to one aspect of the present invention is a cell balancing method that causes a cell balance device to execute a process of equalizing the terminal voltages of a plurality of cells connected in series, the cell balance device including: a first circuit that is connected in series between the positive and negative poles of all of the plurality of cells and includes a primary winding of a transformer, a switching element, and a current detector; second circuits, the number of which is equal to the number of the plurality of cells, each of which is connected in series between the positive and negative poles of each of the plurality of cells and includes a secondary winding of the transformer and a rectifying element that is magnetically coupled to the primary winding; and second circuits, the number of which is equal to the number of the plurality of cells, each of which is connected in series between the positive and negative poles of each of the plurality of cells and includes a PW transformer for the switching element. and a control unit that outputs an M signal to cause the switching element to perform an on / off operation, wherein each of the multiple secondary windings of the transformer is wound with approximately the same number of turns, and the turn ratio between the primary winding and each of the multiple secondary windings is determined based on the number of the multiple cells and a voltage that is predicted to drop in the rectifying element, and the control unit executes the steps of acquiring, from the current detector, a current value of a current detected by the current detector, and turning the PWM signal to an off level when the current value acquired in the current value acquiring step exceeds a predetermined upper threshold.

[0007] According to the present invention, it is possible to provide a cell balancing device and the like that requires a reduced number of parts.

[0008] FIG. 1 is a block diagram showing a schematic configuration of a vehicle equipped with a cell balance device according to an embodiment. FIG. 2 is a schematic diagram showing an example of the configuration of the cell balance device according to the embodiment. FIG. 3 is a schematic diagram showing an example of the hardware configuration of a control unit included in the cell balance device illustrated in FIG. 2. FIG. 4 is a diagram showing an example of the relationship between the waveform of a PWM signal generated by the control unit illustrated in FIG. 3 and the current values ​​of the on and off currents. FIG. 5 is a diagram illustrating a comparison between a conventional balancing process and the balancing process according to the embodiment. FIG. 6 is a diagram explaining problems that may occur in the conventional balancing process and how the balancing process according to the embodiment solves these problems. FIG. 7 is a table illustrating a comparison between the settings of a conventional cell balance device and the settings of the cell balance device according to the embodiment. FIG. 8 is a diagram illustrating an example of the processing procedure of the balancing process executed by the cell balance device according to the embodiment.

[0009] An embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described below with reference to the drawings. However, the present embodiment described below is merely an example of the present invention in all respects. Needless to say, various improvements and modifications can be made without departing from the scope of the present invention. In other words, when implementing the present invention, specific configurations according to the embodiment may be appropriately adopted. Note that, although data appearing in the present embodiment are described in natural language, more specifically, they are specified using pseudo-language, commands, parameters, machine language, etc. that can be recognized by a computer.

[0010] §1 Application Example Fig. 1 is a block diagram showing a schematic configuration of a vehicle VH equipped with a cell balancing device (cell balancing device 1) according to this embodiment. The vehicle VH is an example of the "vehicle" of the present invention. As shown in Fig. 1, the vehicle VH includes, for example, a charging system Sys including the cell balancing device 1, a vehicle control ECU (Electronic Control Unit) 4, and an auxiliary battery 5.

[0011] The charging system Sys is an example of a "charging system" according to the present invention and includes a cell balancing device 1, a battery pack 2, and a charger 3. As shown in FIG. 1 , in the charging system Sys, the cell balancing device 1, the battery pack 2, and the charger 3 are electrically connected to one another. The battery pack 2 is configured by connecting multiple cells (single cells) 20 in series, each of which can be charged and discharged. The battery pack 2 is configured by connecting N cells 20 in series (N is an integer equal to or greater than 2). In the illustrated example, the cells 20(1), 20(2), ..., 20(N) are connected in series. In this embodiment, when there is no need to particularly distinguish between the cells 20(1), 20(2), ..., 20(N), they may be simply referred to as "cells 20." Each cell 20 constituting the battery pack 2 may be, but is not limited to, a lithium-ion battery, a lithium polymer battery, a nickel-metal hydride secondary battery, or the like.

[0012] The charger 3 is a charger that can supply a DC voltage to the battery pack 2 to charge the battery pack 2, and is, for example, a charger that can simultaneously charge a plurality of cells 20. The charger 3 may also be a traction motor of the vehicle VH that can supply regenerative power to the battery pack 2.

[0013] The cell balance device 1 is a cell balance device (voltage equalization device) that equalizes (in other words, equalizes) the voltages V (terminal voltages) of multiple cells 20 connected in series. In particular, when the voltages V of the cells 20 are misaligned (no longer uniform), the cell balance device 1 according to this embodiment equalizes the voltages of the cells 20 by using energy output from the charger 3 or the stored energy of a cell 20 whose voltage V is higher than that of the other cells 20. In other words, even when energy is not supplied from the charger 3, the cell balance device 1 can equalize the voltages V of the cells 20 by using the stored energy of a cell 20 whose voltage V is higher than that of the other cells 20.

[0014] Furthermore, the charging system Sys includes the cell balance device 1 and the charger 3, and is therefore able to charge the assembled battery 2 (each cell 20) using the charger 3 while equalizing the voltage V of each cell 20. Therefore, the charging system Sys can efficiently store a larger amount of electrical energy in the assembled battery 2 (each cell 20).

[0015] The vehicle control ECU 4 detects and controls the state of the vehicle VH. The vehicle control ECU 4 is also connected to the cell balancing device 1 (particularly, the control unit 13 described later using FIG. 2 ) via, for example, a Controller Area Network (CAN) or other in-vehicle LAN, and can transmit and receive information (communicate) with the cell balancing device 1. The vehicle control ECU 4 according to this embodiment detects whether the vehicle VH is started (ignition on) or not (ignition off), and outputs state information SI indicating the detection result to the cell balancing device 1.

[0016] The auxiliary battery 5 is a storage battery different from the battery pack 2, and is formed, for example, from a lead-acid battery. The auxiliary battery 5 supplies drive power DP to the cell balancing device 1 (particularly, the control unit 13, described later using FIG. 2 ) when the vehicle VH is not started (ignition off). As will be described in detail later, when the vehicle VH is not started, the auxiliary battery 5 intermittently supplies drive power DP to the control unit 13, and the control unit 13 performs balancing processing only when drive power DP is supplied. In other words, the control unit 13 performs intermittent operation by repeatedly outputting and pausing a PWM signal. By having the control unit 13 perform the balancing processing intermittently (e.g., periodically or intermittently), the cell balancing device 1 can reduce the power consumption of the control unit 13 that performs the balancing processing. Therefore, the cell balancing device 1 can prevent the auxiliary battery 5, which supplies drive power DP to the control unit 13, from running out (leading to a power shortage of the auxiliary battery 5) when the vehicle VH is not started.

[0017] The cell balancing device 1 according to this embodiment includes one transformer T, one switching element 112, one current detector 113, N rectifying elements 122, and one control unit 13 for all N series-connected cells 20 (i.e., the entire battery pack 2). That is, the cell balancing device 1 requires only one transformer T, one switching element 112, one current detector 113, and one control unit 13, regardless of the number N of series-connected cells 20. Therefore, the cell balancing device 1 can reduce the number of required components compared to conventional balancing circuits. In particular, the cell balancing device 1 can reduce the number of required components even when the number N of series-connected cells 20 is large. As will be described in detail later, the transformer T includes a primary winding 111 and N secondary windings 121 wound with approximately the same number of turns nt (referred to as "nt2" in this embodiment) and corresponding to the N cells 20, respectively. The cell balance device 1 then intermittently discharges, for example, "a cell 20 having a higher (e.g., the highest) voltage V than the other cells 20" among the multiple cells 20, and passes an on / off current through the primary winding 111 of the transformer T. As a result, approximately the same voltage (induced voltage IV) is induced in each secondary winding 121 of the transformer T, and "a cell 20 having a lower (e.g., the lowest) voltage V than the other cells 20" is charged. The cell balance device 1, whose overview has been explained above, will be explained in detail below using FIGS. 2 to 8.

[0018] §2 Configuration Example Fig. 2 schematically illustrates an example of the configuration of the cell balance device 1 according to this embodiment. As shown in Fig. 2, the cell balance device 1 according to this embodiment includes a first circuit 11, a plurality of second circuits 12 (the same number as the number N of cells 20 connected in series), and a control unit 13. Fig. 2 shows an example in which the number N of cells 20 constituting the battery pack 2 is "2," and the battery pack 2 is formed by connecting cells 20(1) and 20(2) in series.

[0019] The first circuit 11 includes a primary winding 111 of a transformer T, a switching element 112, and a current detector 113, which are connected in series between the positive and negative electrodes of all of the cells 20 (i.e., between the positive and negative electrodes of the battery pack 2). In the illustrated example, the primary winding 111, the switching element 112, and the current detector 113 are connected in series between the positive and negative electrodes of the battery pack 2, in this order. For example, if the voltages V of the cells 20 become misaligned (uneven), the stored energy of a cell 20 with a higher voltage V than the other cells 20 is discharged through the first circuit 11. In the example shown in FIG. 2 , a discharge current DC from the cell 20 with a higher voltage V than the other cells 20 flows clockwise through the first circuit 11 at a total voltage Vall, which is the sum of the voltages V of the series-connected cells 20. The total voltage Vall is, for example, the sum of the voltages V of the cells 20 at the start of the balancing process (output of the PWM signal).

[0020] The primary winding 111 is a coil wound around a core (iron core) of the transformer T and magnetically coupled to a plurality (N) of secondary windings 121, which will be described later. In the illustrated example, one end of the primary winding 111 is connected to the overall positive electrode of the plurality of cells 20, i.e., connected to the positive electrode of the assembled battery 2. The other end of the primary winding 111 is connected to a switching element 112, and in the illustrated example, the other end of the primary winding 111 is connected to the overall negative electrode of the plurality of cells 20 via the switching element 112 and a current detector 113, i.e., connected to the negative electrode of the assembled battery 2. In this embodiment, the number of turns nt of the primary winding 111 is set to "nt1."

[0021] The switching element 112 is an element for switching on and off the current flowing through the primary winding 111, and is, for example, a switching transistor. Specifically, the switching element 112 may be configured as a bipolar transistor, an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor-field effect transistor (MOS-FET), or the like. In this embodiment, the switching element 112 is configured as a normally-off switch whose contacts are maintained in an open state (open state, off state, or open state) when it does not receive a PWM signal from the control unit 13. A normally-off switch is also called a normally-open switch or a normally-off switch. The switching element 112 configured as a normally-off switch performs on / off operations in response to the PWM signal output from the control unit 13. By performing on / off operations of the switching element 112, the stored energy of "cells 20 having a higher voltage V than the other cells 20" among the multiple cells 20 intermittently flows through the primary winding 111 at a total voltage Vall. In the illustrated example, the switching element 112 performs an on / off operation, causing a discharge current DC of the total voltage Vall to flow intermittently through the primary winding 111, that is, an on / off current of the total voltage Vall flows through the primary winding 111.

[0022] The current detector 113 detects the discharge current DC flowing through the cell balancing device 1 (particularly the first circuit 11) and outputs (notifies) the detected discharge current DC (the current value of the discharge current DC) to the control unit 13. In the illustrated example, the current detector 113 is connected in series between the switching element 112 and the negative electrodes of all of the cells 20 (i.e., the negative electrode of the battery pack 2). The current detector 113 may include a shunt resistor (not shown).

[0023] As described above, the cell balance device 1 includes N second circuits 12. In FIG. 2 , which illustrates the configuration of the cell balance device 1 when "N=2," the cell balance device 1 includes two second circuits 12, specifically, second circuits 12(1) and 12(2). In this embodiment, when there is no need to particularly distinguish between the second circuits 12(1), 12(2), ..., 12(N), they may be simply referred to as "second circuits 12."

[0024] Each second circuit 12 includes a secondary winding 121 of a transformer T and a rectifying element 122 connected in series between the positive and negative electrodes of each cell 20. In the illustrated example, the rectifying element 122(1) and the secondary winding 121(1) of the transformer T are connected in series, in this order, between the positive and negative electrodes of cell 20(1). Similarly, the rectifying element 122(2) and the secondary winding 121(2) of the transformer T are connected in series, in this order, between the positive and negative electrodes of cell 20(2). In this embodiment, when there is no need to particularly distinguish between the secondary windings 121(1), 121(2), ..., 121(N), they may be simply referred to as the "secondary winding 121." Similarly, when there is no need to particularly distinguish between the rectifying elements 122(1), 122(2), ..., 122(N), they may be simply referred to as the "rectifying element 122."

[0025] Each secondary winding 121 is a coil wound around the core (iron core) of the transformer T and magnetically coupled to the primary winding 111. In the illustrated example, one end of each secondary winding 121 is connected to the negative pole of each cell 20, and the other end is connected to each rectifying element 122, i.e., connected to the positive pole of each cell 20 via each rectifying element 122. Each of the secondary windings 121(1), 121(2), ..., 121(N) is wound with approximately the same number of turns, and as described above, the number of turns nt of each secondary winding 121 in this embodiment is "nt2."

[0026] The cell balance device 1 passes an on / off current of the total voltage Vall through the primary winding 111. Specifically, the control unit 13, which will be described later, sends a PWM signal to the switching element 112, causing the on / off current of the total voltage Vall to flow through the primary winding 111. Since the number of turns nt of the primary winding 111 is "nt1" and the number of turns nt of each secondary winding 121 is "nt2," an induced voltage IV of "Vall × (nt2 / nt1)" is induced in each secondary winding 121. As a result, a charging current CC flows through the second circuit 12 of a cell 20 whose voltage V is lower than the induced voltage IV induced in the secondary winding 121. In the illustrated example, the charging current CC flows counterclockwise through the second circuit 12.

[0027] Each rectifying element 122 is provided to prevent backflow discharge from each cell 20 and is configured, for example, by a diode. However, it is not essential for the cell balancing device 1 that each rectifying element 122 be configured by a diode. Each rectifying element 122 may be configured by a switching element such as an FET.

[0028] If the rectifying elements 122 are configured as diodes, the voltage drop across the diodes results in heat loss. Therefore, measures to prevent heat generation in the diodes (rectifying elements 122) are necessary, particularly when the charging current CC (the current value of the charging current CC) flowing through the diodes (rectifying elements 122) is large. To eliminate the need for such heat generation measures, each rectifying element 122 may be configured as a switching element such as an FET. Configuring the rectifying elements 122 as switching elements eliminates the need for the above-described heat generation measures (in other words, measures to prevent voltage drop across the rectifying elements 122). When switching elements are used as the rectifying elements 122, detecting the flow of the charging current CC through the second circuit 12 and turning on the switching element (rectifying element 122) can eliminate the voltage drop across the rectifying elements 122 and reduce heat generation to zero. Note that configuring the rectifying elements 122 as diodes eliminates the need for the above-described control required when the rectifying elements 122 are configured as switching elements, and the cell balancing device 1 can be configured inexpensively.

[0029] (Regarding the Turns Ratio of the Primary Winding and the Secondary Winding) The induced voltage IV is desirably adjusted so that the charging current CC flows through the second circuit 12 only when the voltages V of the multiple cells 20 are not uniform. In other words, if the induced voltage IV is set too high, the charging current CC flows through the second circuit 12 even when the voltages V of the cells 20 are uniform, resulting in energy loss. Therefore, in this embodiment, the turns ratio (nt2 / nt1) between the primary winding 111 and each secondary winding 121 is determined based on the number N of cells 20 connected in series and the voltage V (drop voltage Vd) predicted to drop across the rectifying element 122. The drop voltage Vd is a value determined in advance by the characteristics (e.g., physical characteristics) of the rectifying element 122. For example, if the rectifying element 122 is configured as a diode, the value is determined in advance by the characteristics of the diode. Furthermore, as described above, if the rectifying element 122 is configured as a switching element, "Vd = 0."

[0030] For example, when the cell balance device 1 performs the balancing process at the full charge voltage Vfc of each cell 20, the turns ratio (nt2 / nt1) may be determined by the following calculation formula 1. That is, the turns ratio (nt2 / nt1) may be calculated by "Calculation formula 1: (nt2 / nt1) = {(1 / N) + Vd / (Vfc × N)}". In the example shown in FIG. 2 , when "Vfc = 3.0" and "Vd = 0.6", "N = 2", and therefore the turns ratio (nt2 / nt1) may be determined to be "6 / 10" by the above calculation formula 1.

[0031] Furthermore, if the voltage V of the cell 20 with the highest voltage V among the multiple cells 20 is defined as "Vmax" and the voltage V of the cell 20 with the lowest voltage V is defined as "Vmin," the turns ratio (nt2 / nt1) between the primary winding 111 and each secondary winding 121 may be determined so that the induced voltage IV satisfies the following conditional expression 1, taking into account the voltage drop Vd in the rectifying element 122. That is, the turns ratio (nt2 / nt1) may be determined so as to satisfy "conditional expression 1: Vmax > (induced voltage IV - voltage drop Vd) > Vmin." As described above, since "IV = {Vall × (nt2 / nt1)}," rearranging conditional expression 1 with respect to the turns ratio (nt2 / nt1) yields the following conditional expression 2. That is, "Conditional Formula 2: {(Vmax / Vall) + (Vd / Vall)} > (nt2 / nt1) > {(Vmin / Vall) + (Vd / Vall)}" is obtained. Note that the value obtained by dividing the average voltage Vav of the multiple cells 20 by the total voltage Vall is "1 / N," and this "1 / N" naturally satisfies "(Vmax / Vall) > (1 / N) > (Vmin / Vall)." Therefore, when "(Vfc × N)" is sufficiently close to the total voltage Vall, the turns ratio (nt2 / nt1) determined by the above-described calculation formula 1 is considered to satisfy conditional formula 2.

[0032] Returning to the explanation of Fig. 2, the control unit 13 transmits a PWM signal to the switching element 112 to cause the switching element 112 to perform an on / off operation, that is, to cause an on / off current to flow through the primary winding 111.

[0033] (Hardware Configuration of Control Unit) Fig. 3 schematically illustrates an example of the hardware configuration of the control unit 13 according to this embodiment. As shown in Fig. 3, the control unit 13 according to this embodiment includes a CPU (Central Processing Unit) 131, a RAM (Random Access Memory) 132, and a non-volatile memory 133. The CPU 131, the RAM 132, and the non-volatile memory 133 constitute, for example, a microcomputer. In the illustrated example, the non-volatile memory 133 stores a balancing processing program PG and threshold information IT.

[0034] The balancing processing program PG is a program that causes the control unit 13 to execute information processing such as generating a PWM signal and transmitting the generated PWM signal to the switching element 112. The balancing processing program PG includes a series of instructions for the information processing. The CPU 131 executes the balancing processing program PG read from the nonvolatile memory 133 using the RAM 132 as a work area, thereby executing the above-mentioned information processing such as generating and transmitting a PWM signal.

[0035] The threshold information IT indicates a threshold value TH that the control unit 13 references when generating a PWM signal. The threshold value TH is an example of an "upper limit threshold" in the present invention. In this embodiment, the threshold value information IT indicates a non-start threshold value ITF and a startup threshold value ITN. The non-start threshold value ITF is an example of an "upper limit threshold" in the present invention, and is the threshold value TH that the control unit 13 references when the vehicle VH is not started (while the ignition is off). The startup threshold value ITN is an example of an "upper limit threshold" in the present invention, and is the threshold value TH that the control unit 13 references when the vehicle VH is started (while the ignition is on). As will be described in detail later, the non-start threshold value ITF is greater than the startup threshold value ITN.

[0036] (Regarding the PWM Signal Generated by the Control Unit) As the output performance of the cell balance device 1 is improved, the cell balance device 1 may be damaged if the difference in voltage V between the cells 20 becomes too large. For example, if the difference between the voltage Vmax of the cell 20 with the highest voltage V and the voltage Vmin of the cell 20 with the lowest voltage V becomes too large, a current exceeding the allowable value for the circuit may flow in at least one of the first circuit 11 and the second circuit 12, potentially damaging the cell balance device 1. Therefore, the cell balance device 1 (particularly the control unit 13) limits the current value of the current flowing in the first circuit 11 and the second circuit 12.

[0037] In this embodiment, the control unit 13 limits the current flowing through the cell balance device 1 by, for example, changing the duty ratio of the PWM signal according to the current value of the discharge current DC detected by the current detector 113 so that the discharge current DC flowing through the first circuit 11 is equal to or less than the threshold value TH. Specifically, when the current value of the discharge current DC flowing through the first circuit 11 exceeds the threshold value TH, the control unit 13 turns the PWM signal sent to the switching element 112 to the off level. That is, as illustrated in FIG. 4 , as the difference in voltage V between the cells 20 increases, the on / off current flowing through the primary winding 111 (in other words, the discharge current DC flowing through the first circuit 11) increases. Therefore, when the current value of the discharge current DC exceeds the threshold value TH, the control unit 13 turns the PWM signal sent to the switching element 112 to the off level and sends the PWM signal to the switching element 112. 4, the control unit 13 can limit the current value of the on / off current flowing through the primary winding 111 to the threshold value TH or less, even when the difference in voltage V between the cells 20 is large. Therefore, even when the difference in voltage V between the cells 20 becomes too large, the cell balance device 1 can prevent an overcurrent from flowing and damaging the cell balance device 1.

[0038] In this embodiment, the control unit 13 references the nonvolatile memory 133 and acquires the threshold information IT stored therein. The control unit 13 also acquires from the current detector 113 the "current value of the discharge current DC flowing through the first circuit 11" detected by the current detector 113. The control unit 13 then compares the acquired "current value of the discharge current DC" with the threshold value TH indicated by the threshold information IT, and when the current value of the discharge current DC exceeds the threshold value TH, the control unit 13 sets the PWM signal to the off level. In other words, when the current value of the discharge current DC exceeds the threshold value TH, the control unit 13 generates a PWM signal with a reduced duty ratio, i.e., a PWM signal with a reduced proportion of high (on) time. The control unit 13 then transmits the generated PWM signal to the switching element 112, thereby limiting the current values ​​of the on and off currents flowing through the primary winding 111 to be equal to or less than the threshold value TH.

[0039] (Regarding the timing at which the control unit executes the balancing process) Because differences in the voltages V of the cells 20 are undesirable, the balancing process for equalizing (leveling) the voltages V of the cells 20 has conventionally been performed constantly, in other words, continuously. Accordingly, when the vehicle VH is not running (when the ignition is off), the conventional cell balancing device that executes the balancing process is constantly supplied with drive power from an auxiliary battery, such as a lead-acid battery. Furthermore, when the vehicle VH is running (when the ignition is on), the conventional cell balancing device is constantly supplied with drive power from sources other than the auxiliary battery (for example, a generator (alternator), a high-voltage battery via a DC-DC converter, or the like).

[0040] In the example shown in FIG. 5A , a conventional cell balancing device that constantly performs balancing even when the ignition is off constantly flows an adjustment current (a current equivalent to at least one of the discharge current DC and the charge current CC) of a predetermined value (10 mA in the illustrated example). Accordingly, the auxiliary battery that supplies drive power to the conventional cell balancing device constantly consumes a current (power) of a predetermined value (200 mA in the illustrated example) while the ignition is off. Therefore, as illustrated in FIG. 6A , conventional balancing devices have the risk of the auxiliary battery running out of power (insufficient power) while the ignition is off, potentially preventing the balancing process from being performed (leading to a failure to perform the balancing process).

[0041] Therefore, the cell balance device 1 changes the execution of the balancing process from the conventional continuous execution (constant execution) to intermittent execution (intermittent or periodic execution) when the vehicle VH is not started (while the ignition is off). Specifically, while the ignition is off, the control unit 13 performs an intermittent operation in which it repeatedly outputs (transmits) a PWM signal and pauses the output. Accordingly, the auxiliary battery 5, which supplies drive power DP to the cell balance device 1 (specifically, the control unit 13) while the ignition is off, also intermittently supplies drive power DP. Specifically, the auxiliary battery 5 supplies drive power DP to the control unit 13 only while the control unit 13 is outputting a PWM signal, and does not supply drive power DP to the control unit 13 while the control unit 13 is pausing the output of the PWM signal. For example, the auxiliary battery 5 periodically (i.e., intermittently) supplies the driving power DP to the control unit 13 in accordance with the output cycle of the PWM signal from the control unit 13 .

[0042] Specifically, the control unit 13 first acquires status information SI from the vehicle control ECU 4. Then, when the status information SI indicates that the vehicle VH is not started (ignition off), the control unit 13 performs the intermittent operation described above (outputting and pausing the output of PWM signals). While the ignition is off, the balancing process is performed intermittently (the control unit 13 performs intermittent operation by repeatedly outputting and pausing the output of PWM signals), thereby reducing the power consumption of the cell balance device 1 and preventing the auxiliary battery 5 from running down.

[0043] 5B , for example, an adjustment current of a predetermined current value (10 A in the illustrated example) flows intermittently through the cell balance device 1, which intermittently performs balancing processing while the ignition is off. In the example shown in FIG. 5B , the control unit 13 performs an intermittent operation of outputting a PWM signal at a cycle of 100 ms for 100 seconds while the ignition is off, and an adjustment current of 10 A flows through the cell balance device 1 only during the period (cycle) when the control unit 13 is outputting the PWM signal. Therefore, the average value of the adjustment current (average adjustment current) flowing through the cell balance device 1 while the ignition is off is 10 mA.

[0044] Furthermore, the auxiliary battery 5 supplies drive power DP to the control unit 13 only during the period (cycle) when the control unit 13 outputs the PWM signal. In the example shown in FIG. 5B , the auxiliary battery 5 consumes 200 mA of current (power) at a 100-ms cycle over 100 seconds. Therefore, the average current consumption of the auxiliary battery 5 when the ignition is off is 0.2 mA. Therefore, compared to the conventional auxiliary battery shown in FIG. 5A , the average current consumption of the auxiliary battery 5 shown in FIG. 5B is significantly reduced, significantly reducing the possibility of a power shortage when the ignition is off. As shown in FIG. 6B , the cell balance device 1 intermittently performs the balancing process, so the auxiliary battery 5 does not experience a power shortage when the ignition is off, and the cell balance device 1 can perform the balancing process even when the ignition is off.

[0045] FIG. 7 is a table comparing the settings of the conventional cell balancing device and the conventional auxiliary battery with the settings of the cell balancing device 1 and the auxiliary battery 5 described above. While the ignition is off, the conventional cell balancing device constantly performs balancing, and the conventional auxiliary battery constantly consumes 200 mA of current. Therefore, the average power consumption of the conventional auxiliary battery while the ignition is off is 200 mA. In contrast, while the ignition is off, the cell balancing device 1 intermittently performs balancing at 100 ms intervals every 100 seconds. The auxiliary battery 5 consumes 200 mA of current only when the cell balancing device 1 performs balancing. Therefore, the average power consumption of the auxiliary battery 5 while the ignition is off is 0.2 mA, which is one thousandth of the average power consumption of the conventional auxiliary battery while the ignition is off.

[0046] (Regarding Maintaining Balancing Capability) As described above, the cell balance device 1 intermittently performs balancing processing while the ignition is off to prevent a power shortage in the auxiliary battery 5. Specifically, the control unit 13 performs intermittent operation by repeatedly outputting and pausing the output of a PWM signal. However, performing the balancing processing only intermittently rather than continuously (performing it all the time) may reduce the balancing capability of the cell balance device 1 (the capability to equalize the voltages V of the cells 20).

[0047] Therefore, the cell balance device 1 continuously performs the balancing process (always performs the balancing process) while the vehicle VH is running (while the ignition is on). For example, the cell balance device 1 is constantly supplied with drive power DP from a source other than the auxiliary battery 5 and continuously performs the balancing process; specifically, the control unit 13 continuously generates and outputs PWM signals (always). Furthermore, while the ignition is off, the cell balance device 1 increases the adjustment current (at least one of the discharge current DC and the charge current CC) to prevent a decrease in balancing capability even when the balancing process is performed only intermittently.

[0048] Specifically, while the ignition is off, the control unit 13 uses the non-start threshold ITF as the threshold TH to be referenced when generating a PWM signal, particularly as the threshold TH to be referenced when determining the duty ratio of the PWM signal. As described above, the non-start threshold ITF is greater than the start threshold ITN, which is used as the threshold TH when the ignition is on. Therefore, while the ignition is off, the control unit 13 allows a larger discharge current DC to flow through the first circuit 11 than when the ignition is on. In other words, the control unit 13 allows larger on and off currents to flow through the primary winding 111. By making the on and off currents during the ignition off larger than the on and off currents during the ignition on, the control unit 13 makes the balancing capability during the ignition off larger (higher) than the balancing capability during the ignition on. Therefore, the cell balance device 1 can prevent a decrease in balancing capability even when the ignition is off and the balancing process is performed only intermittently. In particular, the cell balance device 1 achieves the following effects by continuously performing the balancing process while the ignition is on and intermittently performing the balancing process with increased balancing capability while the ignition is off. That is, the cell balance device 1 can quickly adjust the difference in voltage V between the cells 20 and quickly equalize the voltages V of the cells 20 by performing the balancing process both while the ignition is on and while the auxiliary battery 5 is running out of power.

[0049] Here, increasing the adjustment current flowing through the cell balance device 1 (e.g., the discharge current DC flowing through the first circuit 11) can cause the following problems. For example, the voltage drop in a circuit through which the adjustment current flows (e.g., the first circuit 11) may increase, or a measurement error may occur in the "voltage V of each cell 20" measured by a voltage detection circuit (not shown) using the adjustment current. To suppress such errors, the cell balance device 1 increases the adjustment current only when the ignition is off, and performs balancing continuously using a small adjustment current when the ignition is on. Specifically, when the ignition is on, the control unit 13 uses a startup threshold ITN, which is smaller than the non-startup threshold ITF, as the threshold TH referenced when generating a PWM signal. When the ignition is on, the control unit 13 generates a PWM signal using the startup threshold ITN as the threshold TH (in other words, determines the duty ratio of the PWM signal). By this control, the control unit 13 (cell balance device 1) reduces the regulated power while the ignition is on, and can prevent large measurement errors, for example, in the "voltage V of each cell 20" measured using the regulated current.

[0050] As described above, the non-start threshold ITF used as the threshold TH while the ignition is off is greater than the start threshold ITN used as the threshold TH while the ignition is on. While the ignition is off, the control unit 13 uses the non-start threshold ITF as the threshold TH to periodically (in other words, intermittently) generate a PWM signal and transmits the generated PWM signal to the switching element 112. That is, while the ignition is off, the control unit 13 periodically passes an ON / OFF current through the primary winding 111, i.e., periodically executes a balancing process. Therefore, the non-start threshold ITF may be determined in accordance with the "period in which the balancing process is executed." For example, the non-start threshold ITF may be set so that the "average value of the non-start threshold ITF per unit time" calculated from the non-start threshold ITF and the execution cycle of the balancing process matches the start threshold ITN. In other words, the adjustment current while the ignition is off may be set so that the average value of the adjustment current while the ignition is on is equal to the average value of the adjustment current while the ignition is off.

[0051] For example, if the activation threshold ITN is 10 mA and the "execution cycle of the balancing process while the ignition is off" is 100 ms / 100 seconds, the non-activation threshold ITF may be set to 10 A. In other words, as illustrated in Fig. 7, if the adjustment current of the balancing process that is continuously executed (always executed) while the ignition is on is 10 mA, and the balancing process is executed at a cycle of 100 ms / 100 seconds while the ignition is off, the adjustment current of the balancing process while the ignition is off may be set to 10 A.

[0052] By setting the non-startup threshold ITF so that the "average value per unit time of the non-startup threshold ITF," calculated from the execution cycle of the balancing process and the non-startup threshold ITF, matches the startup threshold ITN, the cell balancing device 1 (controller 13) achieves the following effect: That is, even when the ignition is off and the balancing process is executed only intermittently (periodically), the cell balancing device 1 can maintain the same balancing capacity per unit time as when the ignition is on and the balancing process is executed continuously.

[0053] In this embodiment, a first circuit 11 is configured to sandwich the entirety of the multiple cells 20 (i.e., the assembled battery 2), and a transformer T (more precisely, a primary winding 111 of the transformer T) is installed on the first circuit 11. A second circuit 12 including a secondary winding 121 of the transformer T is then configured for each cell 20 within the first circuit 11. The cell balancing device 1 (controller 13) outputs a PWM signal to the switching element 112 to cause the switching element 112 to perform an on / off operation, thereby causing an on / off current to flow through the primary winding 111. In the transformer T of the cell balancing device 1, the number of turns nt of each secondary winding 121 is "nt2," which is approximately equal to each other. Therefore, the induced voltages IV induced in each secondary winding 121 are also approximately equal to each other. Therefore, among the cells 20 corresponding to each secondary winding 121, a charging current CC flows through the cells 20 whose voltage V is lower than the induced voltage IV induced in the secondary winding 121. Therefore, the cell balance device 1 can take the discharge current DC for balancing from all of the multiple (N in this embodiment) series-connected cells 20, and can pass the charge current CC only to "cells 20 whose voltage V is lower than that of the other cells 20." In other words, the cell balance device 1 can take the discharge current DC for equalizing the voltage V of each cell 20 from all of the battery pack 2, and can pass the charge current CC only to "cells 20 whose voltage V is lower than that of the other cells 20."

[0054] In particular, only one of the transformer T, switching element 112, current detector 113, and control unit 13 is required in the cell balancing device 1, regardless of the number N of series-connected cells 20. Therefore, the cell balancing device 1 can reduce the number of required parts compared to conventional balancing circuits that require a transformer or the like to balance the first battery and the second battery.

[0055] In the transformer T, the turns ratio (nt2 / nt1) between the primary winding 111 and each secondary winding 121 is determined based on the number N of cells 20 connected in series and the voltage drop Vd across the rectifying element 122. For example, when the cell balancing device 1 performs balancing processing at the fully charged voltage Vfc of each cell 20, the turns ratio (nt2 / nt1) is determined by "Calculation Formula 1: (nt2 / nt1) = {(1 / N) + Vd / (Vfc × N)}."

[0056] In the example shown in FIG. 2 , when Vfc = 3.0 and Vd = 0.6, N = 2, and therefore the turns ratio (nt2 / nt1) is calculated as 6 / 10 according to the above-described formula 1. In this case, if the voltage V(1) of cell 20(1) is 3.0 and the voltage V(2) of cell 20(2) is 2.8, then the on / off current of the total voltage Vall = 5.8 (= 3.0 + 2.8) flows through the primary winding 111. Because the turns ratio (nt2 / nt1) is 6 / 10, the induced voltage IV induced in the secondary winding 121 is 3.48 (= 5.8 × (6 / 10)). Furthermore, because the voltage drop Vd of the rectifying element 122 is 0.6, a voltage of 2.88 (= 3.48 - 0.6) is applied to each cell 20. As a result, the charging current CC flows only through cell 20(2) where "V(2) = 2.8 < 2.88," and the charging current CC does not flow through cell 20(1) where "V(1) = 3.0 > 2.88." Therefore, for the entire N series-connected cells 20 (i.e., the assembled battery 2), the stored energy (discharge current DC) of "cells 20 with a higher voltage V than the other cells 20" among the N cells 20 flows to the primary winding 111 at a total voltage Vall. Furthermore, "cells 20 with a lower voltage V than the other cells 20" are charged with a charging current CC of "{Vall × (nt2 / nt1)} - Vd."

[0057] As described above, the cell balancing device 1 is a voltage equalizer that equalizes the voltages V (terminal voltages) of N series-connected cells 20. The cell balancing device 1 includes a transformer T having a primary winding 111 and N secondary windings 121 corresponding to each cell 20, a switching element 112 that supplies an on / off current to the primary winding 111, and a control unit 13 that outputs a PWM signal to the switching element 112 to cause the switching element 112 to perform an on / off operation. The cell balancing device 1 further includes N second circuits 12 that supply a charging current CC from each secondary winding 121 to each corresponding cell 20 via each rectifying element 122. By adopting this configuration, the cell balancing device 1 can reduce the number of required components compared to conventional balancing circuits.

[0058] In particular, when the current value of the discharge current DC detected by the current detector 113 exceeds the threshold value TH, the control unit 13 sets the PWM signal to be sent to the switching element 112 to the off level. Therefore, the control unit 13 can limit the current value of the on / off current flowing through the primary winding 111 to be equal to or less than the threshold value TH.

[0059] §3 Operation Example FIG. 8 is a flowchart showing an example of the processing procedure of the cell balance device 1 (particularly, the control unit 13) according to this embodiment. The processing procedure described below is an example of the processing procedure of a cell balancing method BM that causes a processor (e.g., the CPU of the control unit 13) to execute a process (balancing process) of "equalizing the voltages V of the plurality of cells 20 connected in series." However, the processing procedure described below is merely an example, and each step may be modified as much as possible. Furthermore, steps in the processing procedure described below may be omitted, replaced, or added as appropriate depending on the embodiment.

[0060] (Step S110) In step S110, the control unit 13 acquires the status information SI, and in this embodiment, acquires the status information SI from the vehicle control ECU 4. As described above, the status information SI indicates whether the vehicle VH is started (the ignition is on) or not started (the ignition is off).

[0061] (Step S120) In step S120, the control unit 13 determines whether the vehicle VH is started. The control unit 13 determines whether the status information SI acquired in step S110 indicates that the vehicle VH is started. If the control unit 13 determines that the vehicle VH is started (Yes in step S120), the control unit 13 continuously (always) executes the balancing process (output of PWM signals), and in the example shown in FIG. 8, the control unit 13 proceeds to step S130. If the control unit 13 determines that the vehicle VH is not started (No in step S120), the control unit 13 executes the balancing process intermittently (in other words, intermittently or periodically), and in the example shown in FIG. 8, the control unit 13 proceeds to step S160.

[0062] (Step S130) In step S130, the control unit 13 obtains driving power DP from a source other than the auxiliary battery 5 (for example, a generator (alternator), a high-voltage battery via a DC-DC converter, etc.). That is, in this embodiment, the control unit 13 is supplied with driving power DP from a source other than the auxiliary battery 5 while the ignition is on.

[0063] (Step S140) In step S140, the control unit 13 acquires the current value of the discharge current DC. In this embodiment, the control unit 13 acquires the current value of the discharge current DC detected by the current detector 113 from the current detector 113.

[0064] (Step S150) In step S150, the control unit 13 uses the startup threshold ITN as the threshold TH to generate a PWM signal and outputs (transmits) the generated PWM signal to the switching element 112. The control unit 13 references the nonvolatile memory 133 to acquire the threshold information IT stored therein and compares the "current value of the discharge current DC" acquired in step S140 with the startup threshold ITN indicated by the threshold information IT. When the current value of the discharge current DC exceeds the startup threshold ITN, the control unit 13 sets the PWM signal to an off level, i.e., generates a PWM signal with a reduced duty ratio (in other words, a reduced proportion of the on time). By outputting the generated PWM signal to the switching element 112, the control unit 13 controls (limits) the current value of the discharge current DC flowing through the first circuit 11 to be equal to or less than the startup threshold ITN. As described above, the control unit 13 continuously executes steps S130, S140, and S150 while the ignition is on (while the vehicle VH is started), that is, continuously (always) executes the balancing process (output of PWM signals).

[0065] (Step S160) In step S160, the control unit 13 periodically (at predetermined intervals) (i.e., intermittently) acquires driving power DP from the auxiliary battery 5. For example, in this embodiment, the control unit 13 receives intermittent supply of driving power DP from the auxiliary battery 5 while the ignition is off.

[0066] (Step S170) In step S170, the control unit 13 acquires the current value of the discharge current DC at a predetermined period, and in this embodiment, acquires the current value of the discharge current DC detected by the current detector 113 from the current detector 113 at a predetermined period.

[0067] (Step S180) In step S180, the control unit 13 periodically generates a PWM signal using the non-startup threshold ITF as the threshold TH and outputs the generated PWM signal to the switching element 112. The control unit 13 references the non-volatile memory 133 to acquire threshold information IT stored therein and compares the "current value of the discharge current DC" acquired in step S170 with the non-startup threshold ITF indicated by the threshold information IT. When the current value of the discharge current DC exceeds the non-startup threshold ITF, the control unit 13 turns the PWM signal to an off level, i.e., generates a PWM signal with a reduced duty ratio (in other words, a reduced proportion of on time). By outputting the generated PWM signal to the switching element 112, the control unit 13 controls (limits) the current value of the discharge current DC flowing through the first circuit 11 to be equal to or less than the non-startup threshold ITF. As described above, while the ignition is off (while the vehicle VH is not started), the control unit 13 intermittently executes steps S160, S170, and S180. That is, while the ignition is off, the control unit 13 performs an intermittent operation in which the execution and pause of steps S160, S170, and S180 are repeated at predetermined intervals (predetermined cycles), that is, the control unit 13 intermittently executes the balancing process (output of PWM signals).

[0068] [Features] As described above, the cell balancing device 1 according to this embodiment is a cell balancing device that equalizes the voltages V (terminal voltages) of multiple cells 20 connected in series. The cell balancing device 1 includes a first circuit 11, multiple second circuits 12 (e.g., the same number as the number N of the series-connected cells 20), and a control unit 13. The first circuit 11 includes a primary winding 111 of a transformer T connected in series between the positive and negative electrodes of all of the multiple cells 20 (i.e., between the positive and negative electrodes of the battery pack 2), a switching element 112, and a current detector 113. Each second circuit 12 includes a secondary winding 121 of the transformer T connected in series between the positive and negative electrodes of each cell 20 and magnetically coupled to the primary winding 111, and a rectifying element 122. The control unit 13 transmits a PWM signal to the switching element 112 to cause the switching element 112 to perform an on / off operation.

[0069] In this embodiment, the secondary windings 121 of the transformer T are wound with approximately the same number of turns. The turn ratio (nt2 / nt1) between the primary winding 111 and each secondary winding 121 is determined based on the number N of cells 20 connected in series and the voltage V (drop voltage Vd) predicted to drop across the rectifying element 122.

[0070] Furthermore, the control unit 13 according to this embodiment acquires from the current detector 113 the current value of the discharge current DC detected by the current detector 113. Then, when the acquired current value of the discharge current DC exceeds the threshold value TH, the control unit 13 sets the PWM signal to be transmitted to the switching element 112 to the off level.

[0071] The cell balancing method BM according to this embodiment is a cell balancing method in which the cell balance device 1 executes a process (balancing process) to equalize the voltages V of a plurality of series-connected cells 20. In the cell balancing method BM, the control unit 13 executes step S140 or step S170 and step S150 or step S180, as illustrated in FIG. 8 . In step S140 or step S170, the control unit 13 acquires from the current detector 113 the current value of the discharge current DC detected by the current detector 113. In step S150 or step S180, when the current value of the discharge current DC acquired in step S140 or step S170 exceeds a threshold value TH, the control unit 13 sets the PWM signal to be sent to the switching element 112 to the off level.

[0072] As described above, the cell balancing device 1 includes one transformer T, one switching element 112, one current detector 113, N rectifying elements 122, and one control unit 13 for all N series-connected cells 20 (i.e., the entire battery pack 2). In other words, only one transformer T, one switching element 112, one current detector 113, and one control unit 13 are required in the cell balancing device 1, regardless of the number N of series-connected cells 20. Therefore, the cell balancing device 1 can reduce the number of required components compared to conventional balancing circuits, etc. In particular, the cell balancing device 1 can reduce the number of required components even when the number N of series-connected cells 20 is large.

[0073] Furthermore, when the current value of the discharge current DC flowing through the first circuit 11 exceeds the threshold value TH, the control unit 13 sets the PWM signal to be sent to the switching element 112 to the off level. That is, when the current value of the discharge current DC exceeds the threshold value TH, the control unit 13 generates a PWM signal with a reduced duty ratio (proportion of on time) and sends the generated PWM signal to the switching element 112. Through this control, the control unit 13 can limit the current values ​​of the on and off currents flowing through the primary winding 111 to equal to or less than the threshold value TH, and can prevent overcurrent from flowing and damaging the cell balancing device 1 even if the difference in voltage V between the cells 20 becomes too large.

[0074] §4 Modifications Although the embodiments of the present invention have been described above in detail, the above description is merely an example of the present invention in every respect. It goes without saying that various improvements and modifications can be made without departing from the scope of the present invention. For example, the following modifications are possible. Note that, in the following, the same reference numerals are used for components similar to those in the above embodiment, and descriptions of similar points to those in the above embodiment are omitted where appropriate. The following modifications can be combined as appropriate.

[0075] In the above embodiment, an example has been described in which the vehicle control ECU 4 and the control unit 13 of the cell balancing device 1 are configured as separate computers. However, the configuration of the control unit according to this embodiment is not limited to this example and may be determined appropriately depending on the embodiment. For example, the vehicle control ECU 4 and the control unit 13 of the cell balancing device 1 may be integrated into one computer. Furthermore, at least one of the vehicle control ECU 4 and the control unit 13 of the cell balancing device 1 may be configured as multiple computers.

[0076] REFERENCE SIGNS LIST 1...cell balancing device, 3...charger, 5...auxiliary battery, 11...first circuit, 12...second circuit, 13...controller, 20...cell, 111...primary winding, 112...switching element, 113...current detector, 121...secondary winding, 122...rectifier element, BM...cell balancing method, DP...driving power (power), ITF...non-start threshold (non-start upper limit threshold), ITN...start threshold (start upper limit threshold), SI...status information (vehicle status information), Sys...charging system, T...transformer, TH...threshold (upper limit threshold), VH...vehicle

Claims

1. A cell balancing method for causing a cell balance device to execute a process of equalizing the terminal voltages of a plurality of cells connected in series, the cell balance device comprising: a first circuit including: a primary winding of a transformer connected in series between the positive and negative poles of all of the plurality of cells; a switching element; and a current detector; second circuits, the number of which is the same as the number of cells, including: secondary windings of the transformer connected in series between the positive and negative poles of each of the plurality of cells and magnetically coupled to the primary winding; and rectifying elements; and a control unit that outputs a PWM signal to the switching element to cause the switching element to perform an on / off operation, wherein each of the plurality of secondary windings of the transformer is wound with approximately the same number of turns, and the turn ratio between the primary winding and each of the plurality of secondary windings is determined based on the number of the plurality of cells and the voltage predicted to drop at the rectifying element; and the control unit performs the steps of: acquiring from the current detector a current value of a current detected by the current detector; a step of setting the PWM signal to an off level when the current value acquired in the step of acquiring the current value exceeds a predetermined upper threshold.

2. The cell balancing method of claim 1, wherein the plurality of cells and the cell balance device are both mounted on a vehicle, and the control unit further executes a step of acquiring vehicle state information indicating whether the vehicle is started or not, and when the vehicle state information acquired in the step of acquiring vehicle state information indicates that the vehicle is not started, the control unit performs intermittent operation in which output of the PWM signal and pause of output are repeated at predetermined intervals, and an auxiliary battery separate from the plurality of cells supplies power to the control unit only during a period when the control unit is outputting the PWM signal during the intermittent operation, and does not supply power to the control unit during a period when the control unit is pausing output of the PWM signal during the intermittent operation.

3. The cell balancing method according to claim 2, wherein the non-start upper limit threshold used as the upper limit threshold when the vehicle is not started is greater than the start upper limit threshold used as the upper limit threshold when the vehicle is started.

4. A cell balancing device that equalizes the terminal voltages of a plurality of cells connected in series, comprising: a first circuit including: a primary winding of a transformer connected in series between the positive and negative poles of all of the plurality of cells; a switching element; and a current detector; second circuits, the number of which is the same as the number of cells, including: secondary windings of the transformer magnetically coupled to the primary windings and rectifying elements, each connected in series between the positive and negative poles of each of the plurality of cells; and a control unit that outputs a PWM signal to the switching elements to cause the switching elements to perform on / off operations, wherein each of the plurality of secondary windings of the transformer is wound with approximately the same number of turns, and the turn ratio between the primary winding and each of the plurality of secondary windings is determined based on the number of the plurality of cells and the voltage predicted to drop at the rectifying element; and the control unit obtains from the current detector the current value of the current detected by the current detector, When the acquired current value exceeds a predetermined upper threshold, the cell balancing device sets the PWM signal to an off level.

5. A charging system comprising: the cell balancing device according to claim 4; and a charger that charges the plurality of cells.

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