Power storage device

A power storage device includes a plurality of series-connected battery cells and a balance circuit board. The balance circuit board includes: a heat-generating element (1121) that is provided for each of the plurality of battery cells, and is connected with the corresponding battery cell; and a first temperature sensor that is arranged within a range sandwiched between heat-generating elements positioned at both ends in an arrangement direction of a plurality of heat-generating elements (1121).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of International Application No. PCT/JP2016/065264 entitled “Power Storage Device” filed on May 24, 2016, which claims priority to Japanese Patent Application No. 2015-105596 filed on May 25, 2015, the disclosures of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a technique of controlling a cell balance operation of a power storage device.

BACKGROUND ART

A power storage device configured by including a plurality of battery cells such as lithium-ion batteries is equipped with a balance circuit for equalizing voltages among the battery cells, and performs circuit control for reducing a level difference in cell voltages. There are a balance circuit that uses a passive balance type and a balance circuit that uses an active balance type. The balance circuit of a passive balance type operates to equalize voltages among battery cells by connecting, to a bypassed resistance circuit and the like, a battery cell having a relatively high voltage, and discharging the battery cell alone.

An example of a technique relating to a balance circuit of a passive balance type is disclosed in PTL 1 described below, for example. PTL 1 described below discloses a power storage device provided with a balance circuit of a passive balance type including a resistor, and a temperature sensor, for each of a plurality of power storage elements (battery cells). PTL 1 also discloses a technique of controlling an ON/OFF state of a switch of each balance circuit in such a way that a temperature of the resistor detected by the temperature sensor is maintained at a maximum use temperature during an operation of each balance circuit. In addition, a technique of measuring a maximum temperature of a surface of a heat-generating object is disclosed in PTL 2 described below, for example.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In a balance operation of a balance circuit of a passive balance type, a resistance circuit1and the like generate Joule heat by consuming electric power. This heat becomes a factor of decrease in reliability and service life of a battery cell neighboring the balance circuit and other components. Thus, in terms of reliability and service life of components of a balance circuit, it is desirable that an operation of a balance circuit is controlled based on a temperature of the balance circuit. Accordingly, in order to accurately perform such control, a technique of accurately detecting heat generation of a balance circuit is desired.

An object of the present invention is to provide a technique of accurately detecting heat generation of a balance circuit.

Solution to Problem

The present invention provides a power storage device including a plurality of series-connected battery cells and a circuit board, wherein the circuit board includes: a heat-generating element that is provided for each of the plurality of battery cells, and is connected with the corresponding battery cell; and a first temperature sensor that is arranged within a range sandwiched between the heat-generating elements positioned at both ends in an arrangement direction of a plurality of the heat-generating elements.

Advantageous Effects of Invention

The present invention enables to accurately detect heat generation of a balance circuit.

DESCRIPTION OF EMBODIMENTS

Example embodiments of the present invention will be described below by using the drawings. Note that like components are assigned with like reference numerals throughout all the drawings, and description therefor will be omitted as appropriate.

Note that components illustrated in each drawing are implemented by an arbitrary combination of hardware and software, mainly by a central processing unit (CPU) of an arbitrary computer, a memory, a program loaded on a memory for implementing the components in the drawing, a storage medium for storing the program such as a hard disk, and an interface for network connection. Accordingly, a method and a device for implementing the components include various modification examples.

First Example Embodiment

FIG. 1is a diagram conceptually illustrating a configuration of a power storage device according to a first example embodiment. As illustrated inFIG. 1, the power storage device according to the present example embodiment includes a battery string3of a plurality of series-connected battery modules31, and a balance circuit board1that is connected with the battery string3through a connector2. The connector2includes an electrode terminal and a cell temperature sensor terminal on a battery cell311basis. Each of the battery modules31includes a plurality of series-connected battery cells311(four battery cells311in an example inFIG. 1). As illustrated inFIG. 1, each of the battery modules31may include a cell temperature sensor312that measures a temperature of each battery cell311.

The balance circuit board1includes a balance circuit block11and a control circuit block12. The balance circuit block11includes a balance circuit112for equalizing voltages of the battery cells311, and a first temperature sensor111for measuring a temperature of the balance circuit112. The control circuit block12includes a control unit121for controlling an operation of the balance circuit112, and a communication unit122for communicating with a not-illustrated external device and the like.

The balance circuit112is provided at least on a battery module31basis. In the present example embodiment, the balance circuit112is provided for each of the plurality of battery cells311, and is connected with corresponding one of the battery cells311. The balance circuit112may be a balance circuit of a so-called passive balance type, or may be a balance circuit of a so-called active balance type. Note that, in the following respective example embodiments, a balance circuit of a passive balance type will be described as an example. However, the present invention is not limited to the balance circuit of the passive balance type.

Herein, the passive balance type is a type in which, when the plurality of battery cells311have a variation in voltage (discharge capacity), voltages among the plurality of battery cells are equalized by discharging a battery cell having a relatively high voltage with a battery cell having a lowest voltage as a reference. A balance circuit of the passive balance type includes a resistive element for discharging each of battery cells, and a switch element for controlling a balance operation by being switched ON/OFF. On the other hand, an active balance type is a type in which, when a plurality of battery cells have a variation in voltage, voltages among the plurality of battery cells are equalized by moving an electric charge from a battery cell having a large capacity to a battery cell having a small capacity. A balance circuit of the active balance type includes a capacitor or a transformer for moving an electric charge among battery cells, and a switch element for selecting a battery cell to be connected with the capacitor or the transformer. The resistive element and the transformer (inductor element) used in the balance circuit112mainly generate heat during a balance operation.

The first temperature sensor111is provided for every one or more of the balance circuits112, and measures a temperature of a corresponding balance circuit112. In addition, one or more first temperature sensors111are provided for a corresponding balance circuit112. For example, one or more first temperature sensors111are provided for each of the balance circuits112. When a width between the balance circuits112is narrow and the like, it may be difficult to provide the first temperature sensor111for every balance circuit112. In such a case, one or more first temperature sensors111may be provided for every predetermined number of the balance circuits112included in each battery module31.

The first temperature sensor111becomes less responsive to heat generated from a corresponding balance circuit112as the first temperature sensor111is more distant from the balance circuit112. Thus, in terms of temperature measurement accuracy, the first temperature sensor111is more preferably provided at a position that is closer to the corresponding balance circuit112. A position where the first temperature sensor111is placed will be described below.

FIG. 2is a schematic diagram representing a relationship between an operation state of a balance circuit and heat generation distribution in Y-Y′ direction. In an example inFIG. 2, eight balance circuits112respectively including a heat-generating element1121(a resistive element in the case of the passive balance type) and a switch element1122are disposed.FIG. 2(a)exemplifies temperature distribution in Y-Y′ direction when the balance circuits112of all channels perform balance operations. Note that a channel (CH) means a set of one battery cell311and a balance circuit112corresponding to the battery cell311. In the case ofFIG. 2(a), a peak of temperature appears at a central part since a heat dissipation performance at the central part is inferior to that at an end part, and the temperature becomes lower as coming closer to end part sides.FIG. 2(b)exemplifies temperature distribution in YY′ direction when the balance circuits112of upper-half channels perform balance operations. In the case ofFIG. 2(b), a peak of temperature appears at a central part of the upper-half channels.FIG. 2(c)exemplifies heat generation distribution in Y-Y′ direction when every other one of the balance circuits112performs a balance operation. In the case ofFIG. 2(c), temperature is low at parts of channels where no balance operation is performed, which results in wave-shaped temperature distribution. InFIG. 2(c), peaks of temperature appear at two points, which are between a third channel and a fourth channel from the top, and between a fifth channel and a sixth channel from the top. In this way, temperature distribution in Y-Y′ direction changes in response to an electrical conduction state of the heat-generating element1121, within a range sandwiched between the heat-generating elements1121at both ends. Thus, providing the first temperature sensor111in the range sandwiched between the heat-generating elements1121at both ends in Y-Y′ direction facilitates detection of heat generated from the heat-generating element1121. Note that, amongFIGS. 2(a) to (c), a highest temperature in Y-Y′ direction is detected in the case ofFIG. 2(a)in which the balance circuits112of all the channels operate. At least one first temperature sensor111is ideally arranged in such a way as to overlap a center of the range sandwiched between the heat-generating elements1121at both ends in Y-Y′ direction.

FIG. 3is a schematic diagram representing a relationship between an operation state of a balance circuit and heat generation distribution in X-X′ direction orthogonal to Y-Y′ direction. The switch element1122generally has small ON-resistance in comparison with that of the heat-generating element1121. Therefore, when the heat-generating element1121is in an electrical conduction state, a peak of temperature appears at a central part of the heat-generating element1121in X-X′ direction. Thus, the first temperature sensor111is ideally arranged in such a way as to overlap a center of the heat-generating element1121in X-X′ direction.

In addition, it can be predicted fromFIGS. 2 and 3that a point that may reach a highest temperature on a plane of the balance circuit board1(hereinafter, also written as a maximum heat-generating point) is a point of intersection between a point that reaches a highest temperature in Y-Y′ direction and a point that reaches a highest temperature in X-X′ direction when all the corresponding balance circuits112operate. Note that it is possible to grasp the maximum heat-generating point also by carrying out a test operation or simulation of the balance circuit board1and measuring temperature distribution of the balance circuit board1during a balance operation, for example.

When considering a component interval, a wiring interval, an electrical insulation interval, and the like of electrical components such as the heat-generating element1121and the first temperature sensor111, the first temperature sensor111cannot always be provided at an ideal position with no error. The first temperature sensor111needs to be provided as close as possible to the maximum heat-generating point. As a specific example, when a distance in Y-Y′ direction between the heat-generating elements1121at both ends is denoted by W, a width in X-X′ direction of each heat-generating element1121is denoted by L, and a coordinate of the maximum heat-generating point that is specified as described inFIGS. 2 and 3is denoted by (Wymax, Lxmax), a range for providing the first temperature sensor111can be defined by the following Expression 1 and Expression 2, for example.
[Mathematical 1]
Placement Range inY-Y′ Direction=Wymax±0.3×W(Expression 1)
Placement Range inX-X′ Direction=Lxmax±0.3×L(Expression 2)

This can be illustrated as inFIG. 4.FIG. 4is a diagram exemplifying a range for providing the first temperature sensor111. A range indicated by slanting lines is defined based on the maximum heat-generating point (Wymax, Lxmax) that is specified as described usingFIGS. 2 and 3, the distance W between the balance circuits112positioned at both ends, and the width L of the balance circuit112. The first temperature sensor111is provided in such a way as to overlap the range indicated by the slanting lines.

FIG. 5is a diagram illustrating a first placement example of the first temperature sensor111.FIGS. 5(a), (b), and (c)respectively illustrate a top view, a sectional view in Y-Y′ direction, and a sectional view in X-X′ direction of the balance circuit board1in the first placement example. As illustrated inFIG. 5, for example, the first temperature sensor111can be provided on the balance circuit board1on a face at rear side of a face provided with the components (the heat-generating element1121and the like) of the balance circuit112. In this case, restriction on arrangement due to other components is reduced, and the first temperature sensor111can be provided in such a way as to overlap the maximum heat-generating point that is predicted as described inFIGS. 2 and 3or is specified by a test operation and the like.

FIG. 6is a diagram illustrating a second placement example of the first temperature sensor111.FIGS. 6(a), (b), and (c)respectively illustrate a top view, a sectional view in Y-Y′ direction, and a sectional view in X-X′ direction of the balance circuit board1in the second first placement example. It is assumed in the second placement example, a case in which a certain degree of a space is present at a central part of eight balance circuits112. In this case, the first temperature sensor111can be provided in the space at the central part, with a physical electrical insulation interval. Also with such a configuration, it becomes possible to provide the first temperature sensor111in such a way as to overlap the maximum heat-generating point that is specified as described inFIGS. 2 and 3. In addition, in the example inFIG. 6, the first temperature sensor111and the balance circuit112may be configured to be insulated from each other by providing an insulator between the first temperature sensor111and the balance circuit112.

FIG. 7is a diagram illustrating a third placement example of the first temperature sensor III.FIGS. 7 (a), (b), and (c)respectively illustrate a top view, a sectional view in Y-Y′ direction, and a sectional view in X-X′ direction of the balance circuit board1in the third placement example. In the third placement example, the first temperature sensor111is provided on an upper face of the heat-generating element1121while being covered with an insulator1111that is a coating film and the like formed of an insulating material. Also with such a configuration, it becomes possible to provide the first temperature sensor111in such a way as to overlap the maximum heat-generating point that is predicted as described inFIGS. 2 and 3or is specified by a test operation and the like.

FIG. 8is a diagram illustrating a fourth placement example of the first temperature sensor111.FIGS. 8(a), (b), and (c)respectively illustrate a top view, a sectional view in Y-Y′ direction, and a sectional view in X-X′ direction of the balance circuit board1in the fourth placement example. In the fourth placement example, it is assumed that the balance circuit board1is a substrate having a multilayer structure. In such a balance circuit board1, as illustrated inFIG. 8, a wiring line1123to be connected with the heat-generating element1121may be provided on an inner layer. In this case, the first temperature sensor111may be provided at a position overlapping the wiring line1123in a planar view. InFIG. 8, the wiring line1123is connected to an end part of three series-connected heat-generating elements1121, and the first temperature sensor III is arranged in such a way as to overlap the wiring line1123in a planar view. The wiring line1123is a metal and has high thermal conductivity, which facilitates transfer of heat from the heat-generating element1121. Thus, heat from the heat-generating element1121can be detected relatively accurately by providing the first temperature sensor111right above the wiring line1123in this way.

FIG. 9is a diagram illustrating a fifth placement example of the first temperature sensor111. In the fifth placement example, illustrated is a case where twelve balance circuits112are provided separately in three blocks, and there exist a plurality of spaces in which the first temperature sensor111can be arranged between the blocks in Y-Y′ direction. In the present drawings, it is assumed that circuit components of the middle block are present at a center of a range sandwiched between the heat-generating elements1121at both ends, and there is no space for arranging the first temperature sensor111. In this case, the first temperature sensor111can be arranged in at least either a space between the upper block and the middle block or a space between the middle block and the lower block. Note thatFIG. 9illustrates an example in which the first temperature sensor111is provided in the upper space. Although not illustrated, when five blocks are aligned, the first temperature sensor111is preferably provided in at least either a space between the second block and the third block or a space between the third block and the fourth block, since these spaces are closer to a center of a range sandwiched between the heat-generating elements1121at both ends. Without limitation thereto, the first temperature sensor111may be provided in a space between the uppermost block and the second block, or in a space between the fourth block and the lowermost block.

Note that an arrangement position of the first temperature sensor111is not limited to the examples illustrated inFIGS. 5 to 9. For example, a plurality of first temperature sensors111may be provided across the entire balance circuit board1, within a range that is defined in consideration of heat generation distribution in Y-Y′ direction and X-X′ direction. In addition, for example, inFIGS. 6, 8, and 9, the first temperature sensor111may be provided on a rear face, as illustrated inFIG. 5.

In addition, the first temperature sensor111becomes less responsive to heat generated from a corresponding balance circuit112as the first temperature sensor111is more distant from the balance circuit112. Thus, in terms of temperature measurement accuracy, the first temperature sensor111is more preferably provided at a position that is closer to the corresponding balance circuit112. The first temperature sensor111can be mounted near a surface of the balance circuit board1by using a thermistor resistor, a semiconductor temperature sensor, a resistance temperature detector (RTD), and the like as the first temperature sensor111. Accordingly, it becomes possible to arrange the first temperature sensor111in proximity to the balance circuit112in a direction perpendicular to a face of the balance circuit board1, and measure a temperature of the balance circuit112with high accuracy.

The control unit121controls a balance operation of the balance circuit112corresponding to each of the battery cells311, based on a voltage of each of the battery cells311and a temperature (hereinafter, also written as a first measured temperature) measured by the first temperature sensor111.

A basic control of the balance circuit112performed by the control unit121will be described. The control unit121acquires a voltage of each of the battery cells311via a terminal (not illustrated) of the connector2connected to each of the battery cells311, and specifies a battery cell311having a lowest voltage (hereinafter, a lowest voltage cell) among the plurality of battery cells311. Then, the control unit121calculates, for each of the battery cells311other than the lowest voltage cell, a voltage difference ΔVB from the lowest voltage cell. Then, the control unit121actuates the balance circuit112corresponding to the battery cell311whose voltage difference ΔVB from the lowest voltage cell is equal to or more than a voltage difference that is a balance operation start condition (hereinafter, this voltage difference will be written as “ΔVBon”). The voltage difference ΔVB between the battery cell311corresponding to the balance circuit112and the lowest voltage cell is reduced by actuating the balance circuit112. Then, when the voltage difference ΔVB between the battery cell311and the lowest voltage cell becomes equal to or less than a voltage difference that is a balance operation end condition (hereinafter, this voltage difference will be written as “ΔVBoff”), the control unit121stops a balance operation of the balance circuit112corresponding to the battery cell311.

Herein, the balance circuit112generates heat through a balance operation. For example, a balance circuit of the passive balance type includes a resistive element, and during execution of a balance operation, discharge energy of the battery cell311is consumed by the resistive element and the resistive element generates heat accordingly. This heat generation of the resistive element may cause a temperature of the balance circuit112to be higher than that of the battery cell311. For example, when a power storage device is used, a temperature of the battery cell311is about 40° C. even after rising, whereas a temperature of the balance circuit may rise up to approximately 85° C. through a balance operation. Then, this heat transfers to the battery cell311and other components within the power storage device via a medium such as the balance circuit board1and thereby causes temperature rise. This may adversely affect service life, operation reliability, and the like of the battery cell311and the other components.

In view of the above, when a temperature measured by the first temperature sensor111provided for the balance circuit112(hereinafter, also written as a first measured temperature) is equal to or more than an upper reference temperature, the control unit121stops an operation of the balance circuit112corresponding to the first temperature sensor111, regardless of the voltage difference ΔVB of the battery cell311relative to the lowest voltage cell described above. When the balance circuit112corresponding to the first temperature sensor111is stopped because the first measured temperature measured by the first temperature sensor111becomes the upper reference temperature, heat generation of the balance circuit112calms down and the first measured temperature begins to decrease. Then, when the first measured temperature measured by the first temperature sensor111becomes equal to or less than a lower reference temperature that is lower than the upper reference temperature, the control unit121restarts the operation of the balance circuit112corresponding to the first temperature sensor111. The control unit121holds, for example, information indicating a correspondence relationship between the first temperature sensor111and the balance circuit112in a not-illustrated storage region in advance. When there is the first temperature sensor111measuring a temperature equal to or more than the upper reference temperature, by using the information in the storage region, the control unit121identifies the balance circuit112of which a balance operation is to be stopped. In addition, by using the information in the storage region, the control unit121identifies the balance circuit112of which a balance operation is to be restarted, when there is the first temperature sensor111measuring a temperature equal to or more than the upper reference temperature and then the temperature drops to measure a temperature equal to or less than the lower reference temperature.

The upper reference temperature is a threshold temperature at which an operation of the balance circuit112is to be stopped. When the upper reference temperature is set lower, heat generation of the balance circuit can be suppressed and the battery cell311and the components can be protected with higher accuracy. Meanwhile, the balance circuit112is more frequently stopped, and it may take a longer time for voltages of the battery cells311to be equalized. The lower reference temperature is a threshold temperature at which an operation of the balance circuit112is to be restarted. When the lower reference temperature is set lower, a temperature of the balance circuit112decreases sufficiently and the battery cell311and the components can be protected with higher accuracy. Meanwhile, a time for a balance operation to be restarted is prolonged, and it takes a longer time for voltages of the battery cells311to be equalized. The upper reference temperature and the lower reference temperature are respectively adjusted to appropriate values, based on uses, performance requirements, and the like of a power storage device, for example, and are set in a not-illustrated storage region such as a memory of the control unit121. For example, the upper reference temperature and the lower reference temperature are set as 85° C. and 80° C., respectively, in the storage region of the control unit121. When a difference (temperature hysteresis) between the upper reference temperature and the lower reference temperature is 5° C. or more, a balance operation of each balance circuit112can be stably controlled. In addition, when a difference between the upper reference temperature and the lower reference temperature is, for example, 10° C. or less, it is possible to reduce an adverse effect caused by heat generation of the balance circuit112and the plurality of battery cells311can be balanced without taking a longer time. However, a range of the difference is not limited to the range exemplified herein.

When an operation stop time of the balance circuit112due to the first measured temperature becoming equal to or more than the upper reference temperature exceeds a reference value per unit time, or when an operation stop number-of-times of the balance circuit112due to the first measured temperature becoming equal to or more than the upper reference temperature exceeds a reference number-of-times per unit time, the communication unit122outputs, to outside, information capable of specifying the balance circuit or a battery cell corresponding to the balance circuit.

Specifically, the control unit121holds, in a not-illustrated storage region, an operation stop time of the balance circuit112due to the first measured temperature becoming equal to or more than the upper reference temperature, or an operation stop number-of-times of the balance circuit112due to the first measured temperature becoming equal to or more than the upper reference temperature. Then, the control unit121calculates, by using held information, the operation stop time of the balance circuit112per unit time or the operation stop number-of-times of the balance circuit112per unit time. Then, the control unit121determines whether or not there is the balance circuit112of which the calculated operation stop time is equal to or more than a preset reference value per unit time, or whether or not there is the balance circuit112of which the calculated operation stop number-of-times is equal to or more than a preset reference number-of-times per unit time. For example, the control unit121determines whether or not the operation stop time per unit time of the balance circuit112occupies half or more of the unit time, or whether or not the balance circuit112has stopped operating for a fixed number of times or more per unit time. When the operation stop time per unit time is longer than a predetermined reference value, or when the operation stop number-of-times per unit time is large, there is a possibility of occurrence of an abnormality, such as abnormal heat generation of the balance circuit112and failure of the first temperature sensor111. In view of this, when there is the balance circuit112of which the operation stop time exceeds the predetermined reference value or of which the operation stop number-of-times exceeds a predetermined reference number-of-times, the control unit121generates information capable of specifying the balance circuit or the battery cell311corresponding to the balance circuit112, and causes the communication unit122to output the information.

A configuration of the power storage device according to the present example embodiment will be described by usingFIG. 10.FIG. 10is a diagram schematically illustrating a circuit configuration example of the power storage device according to the first example embodiment. In the present figure, an example in which one first temperature sensor111is provided for one balance circuit112is illustrated.

As illustrated inFIG. 10, each of the battery cells311included in the battery module31is connected with a corresponding balance circuit112through a terminal of the connector2. The balance circuit112is provided in the balance circuit block11of the balance circuit board1, and includes the heat-generating element1121and the switch element1122. In the case of a passive balance type, the heat-generating element1121is a resistive element that consumes discharge energy of the battery cell311corresponding to each of the balance circuits112. Further, the switch element1122is a switching transistor, a metal oxide semiconductor field effect transistor (MOS-FET), and the like, for example.

The control unit121is connected with each of the switch elements1122through a control line. The control unit121transmits a control signal to each of the switch elements1122through the control line, and switches ON/OFF states of each switch element1122. When the switch element1122is switched to an ON state by the control unit121, the battery cell311and the balance circuit112corresponding to the battery cell311forms a closed loop, and a balance operation is executed. On the other hand, when the switch element1122is switched to an OFF state by the control unit121, the closed loop formed by the battery cell311and the balance circuit112corresponding to the battery cell311is released, and the balance operation is stopped.

The control unit121controls ON/OFF states of the switch element1122of each of the balance circuits112, based on a voltage difference between the plurality of battery cells311and a temperature detected by each of the first temperature sensors111.

The control unit121is connected to terminals of the connector2, and acquires a voltage of each of the battery cells311from a voltage between the terminals. The control unit121compares the acquired voltages of the battery cells311, and specifies a lowest voltage cell. Then, the control unit121calculates a voltage difference ΔVB from the lowest voltage cell for each of the battery cells311other than the lowest voltage cell. Herein, when the voltage difference ΔVB between the lowest voltage cell and a certain battery cell311is ΔVBon, the control unit121transmits a control signal for switching the switch element1122corresponding to the battery cell311to an ON state. Accordingly, a balance operation by the balance circuit112is executed. On the other hand, when the voltage difference ΔVB between the lowest voltage cell and the certain battery cell311becomes equal to or less than ΔVBoffby executing the balance operation, the control unit121transmits a control signal for switching the switch element1122corresponding to the battery cell311to an OFF state. Accordingly, the balance operation by the balance circuit112is stopped. A voltage difference between the battery cell311and the lowest voltage cell is reduced as the heat-generating element1121consumes discharge energy of the battery cell311whose voltage difference ΔVB from the lowest voltage cell is equal to or more than ΔVBon. A voltage of each of the plurality of battery cells311becomes close to a voltage of the lowest voltage cell by performing a balance operation for each of the plurality of battery cells311whose voltage difference ΔVB from the lowest voltage cell is equal to or more than ΔVBon. As a result, voltages of the plurality of battery cells311are equalized.

In addition, the control unit121is connected with each of the first temperature sensors111through a signal line. The control unit121acquires an output voltage from each of the first temperature sensors111and converts the output voltage into a temperature (a first measured temperature). When the heat-generating element1121generates heat through a balance operation and the first measured temperature becomes equal to or more than an upper reference temperature, the control unit121transmits a control signal for setting the switch element1122of the balance circuit112corresponding to the first temperature sensor111to an OFF state, regardless of the voltage difference ΔVB. Accordingly, a balance operation of the balance circuit112corresponding to the first temperature sensor111measuring a temperature equal to or more than the upper reference temperature is stopped. When heat generation of the heat-generating element1121calms down and the first measured temperature becomes equal to or less than a lower reference temperature, the control unit121transmits a control signal for setting the switch element1122to an ON state, and restarts the balance operation by the balance circuit112.

In addition, the control unit121holds, in a not-illustrated storage region, a time for which each balance circuit112stops operating, or the number of times each balance circuit112stops operating. Then, the control unit121calculates an operation stop time or an operation stop number-of-times per unit time of each balance circuit112by using information held in the storage region. Then, the control unit121determines whether or not there is the balance circuit112of which the calculated operation stop time per unit time is equal to or more than a predetermined reference value, or the balance circuit112of which the calculated operation stop number-of-times per unit time is equal to or more than a predetermined reference number-of-times. When there is the balance circuit112concerned, the control unit121generates information capable of specifying the balance circuit112or the battery cell311corresponding to the balance circuit112, and outputs the information to a communicably connected external monitoring device and the like, for example, via the communication unit122. Accordingly, the external monitoring device can display the information transmitted from the communication unit122on a display, and an operator of the external monitoring device is able to specify the balance circuit112or the battery cell311that seems to have an abnormality of some kind.

The control unit121includes, for example, a not-illustrated storage region such as a read only memory (ROM) and a random access memory (RAM), and stores programs for implementing the above-described functions in the storage region. In addition, the control unit121includes a not-illustrated central processing unit (CPU), and implements the above-described functions by executing the programs stored in the storage region by use of the CPU.

Further, the control unit121may be configured to execute a balance operation by monitoring a voltage difference between the plurality of battery cells311all the times, or may be configured to execute a balance operation only when charging.

Although not illustrated inFIG. 10, the plurality of battery modules31are connected in series between an external positive electrode terminal4and an external negative electrode terminal5of the power storage device. The power storage device is connected to a not-illustrated external device and a not-illustrated external power supply through the external positive electrode terminal4and the external negative electrode terminal5, and performs discharging or charging.

An example of a balance operation in the power storage device inFIG. 10will be described by usingFIG. 11.

Note that, in the following description, when a certain channel has a voltage difference ΔVB of equal to or more than ΔVBon, the control unit121executes a balance operation at a predetermined balance frequency FBAL. The balance frequency FBAL includes an ON period Tonduring which the switch element1122is in an ON state, and an OFF period Toffduring which the switch element1122is in an OFF state. The ON period Tonand the OFF period Toffare alternately repeated. The control unit121can manage a lapse of these periods by using a timer, for example.

FIG. 11is a diagram exemplifying a specific flow of a balance operation of each channel in the power storage device inFIG. 10.

First, a voltage difference ΔVB of CH1reaches ΔVBonat time t1. Then, the control unit121sets the switch element1122of CH1to an ON state, and starts a balance operation of CH1. Accordingly, the voltage difference ΔVB of CH1begins to decrease. In addition, a first measured temperature begins to rise due to heat generated from the heat-generating element1121of CH1. Note thatFIG. 11illustrates a behavior of the linearly rising first measured temperature, but, in fact, the first measured temperature may exhibit various behaviors upon receiving influence such as operation states of surrounding channels.

Thereafter, the first measured temperature continues to rise, and the first measured temperature reaches an upper reference temperature TRU at time t2. Then, the control unit121sets the switch element1122of CH1corresponding to the first temperature sensor111to an OFF state, and stops the balance operation of CH1. Since the balance operation is stopped in CH1, heat generation of the heat-generating element1121of CH1calms down, and the first measured temperature begins to decrease. Note thatFIG. 11illustrates a behavior of the linearly decreasing first measured temperature, but in fact, the first measured temperature may exhibit various behaviors upon receiving influence such as operation states of surrounding channels. In addition, when the balance operation of the balance circuit112is stopped, load impedance is increased and voltage rebound occurs. As illustrated inFIG. 11, a voltage of the battery cell311of CH1exhibits an upward tendency because of this voltage rebound.

Thereafter, the first measured temperature drops down to a lower reference temperature TRL at time t3. Then, the control unit121determines whether a balance frequency FBAL of CH1corresponding to the first temperature sensor111is an ON period Tonor an OFF period Toff. The control unit121restarts the balance operation of CH1in the case of the ON period Ton. In an example inFIG. 11, since CH1is in the ON period Tonat time t3, the control unit121sets the switch element1122of CH1to an ON state, and the balance operation is restarted in CH1. Consequently, the voltage difference ΔVB of CH1begins to decrease again, and the first measured temperature begins to rise again.

Thereafter, the predetermined ON period Ton elapses at time t4. The control unit121sets the switch element1122of CH1to an OFF state with timing at which time elapsed after the balance operation turned into the ON state reaches Ton, and stops the balance operation. Herein, although not illustrated, the control unit121completes the balance operation of CH1when the voltage difference ΔVB becomes equal to or less than ΔVBoffduring the ON period Ton.

In addition, although not illustrated, the control unit121completes the balance operation of CH1when the voltage difference ΔVB after occurrence of voltage rebound is equal to or less than a predetermined threshold value (an intermediate value between ΔVBonand ΔVBoff, for example) at a time of entering a next ON period Tonafter a lapse of the OFF period Toff. Note that in this case, a threshold value for completing the balance operation is set to a desirable value according to a use environment and the like of the power storage device. When the voltage difference ΔVB is more than the predetermined threshold value, the control unit121continues the balance operation of the channel in a next cycle.

FIG. 12is a flowchart illustrating a flow of processing of the balance operation inFIG. 11.

The control unit121specifies, based on a voltage between the terminals of the connector2, a lowest voltage cell from among the plurality of battery cells311, and calculates a voltage difference ΔVB between the lowest voltage cell and each of the other battery cells311(S101). Then, the control unit121determines, for each of the battery cells311, whether or not the voltage difference ΔVB between the battery cell311and the lowest voltage cell is equal to or more than ΔVBonthat is a balance operation start condition (S102). When the voltage difference ΔVB from the lowest voltage cell is less than ΔVBon(S102: NO), it is not necessary to execute a balance operation in a channel of the battery cell311. Thus, the control unit121does not actuate the balance circuit112and ends the processing. On the other hand, when the voltage difference ΔVB from the lowest voltage cell is equal to or more than ΔVBon(S102: YES), the control unit121starts a balance operation of a channel of the battery cell311(S103).

The control unit121determines, for the channel performing the balance operation, whether or not the voltage difference ΔVB from the lowest voltage cell becomes equal to or less than ΔVBoffthat is one of balance operation completion conditions (S104). When the voltage difference ΔVB from the lowest voltage cell becomes equal to or less than ΔVBoffthrough the balance operation (S104: YES), the control unit121completes the balance operation of the channel.

On the other hand, when the voltage difference ΔVB from the lowest voltage cell is not equal to or less than ΔVBoff(S104: YES), the control unit121determines whether or not a first measured temperature acquired from the corresponding first temperature sensor111of the channel is equal to or more than an upper reference temperature TRU (S105).

When the first measured temperature is less than the upper reference temperature TRU (S105: NO), the control unit121determines whether a balance frequency FBAL of the channel is an ON period Tonor an OFF period Toff(S106). When the balance frequency FBAL is the ON period Ton(S106: Ton), the above-described processing from S104is repeated. On the other hand, when the balance frequency FBAL is the OFF period Toff(S106: Toff), the control unit121sets the switch element1122of the channel to an OFF state, and stops the balance operation (S107). Then, after waiting until the OFF period Toffelapses, the control unit121determines whether or not the voltage difference ΔVB when a next cycle starts (at a time when a next ON period Tonstarts) is equal to or less than a predetermined threshold value (an intermediate value between ΔVBonand ΔVBoff, for example) (S108). Herein, when the voltage difference ΔVB is equal to or less than the threshold value (S108: YES), the control unit121completes the balance operation of the channel. On the other hand, when the voltage difference ΔVB is more than the threshold value (S108: NO), a balance operation of the next cycle is started (S103).

On the other hand, when the first measured temperature is equal to or more than the upper reference temperature TRU (S105: YES), the control unit121stops the balance operation of the channel (S109). Then, the control unit121holds a state in which the balance operation of the channel is stopped until the first measured temperature becomes equal to or less than a lower reference temperature TRL (S110: NO). When the first measured temperature becomes equal to or less than the lower reference temperature TRL (S110: YES), the control unit121determines whether a balance frequency FBAL of the channel is an ON period Tonor an OFF period Toff(S111). When the balance frequency FBAL is the ON period Ton(S111: Ton), the control unit121restarts the balance operation of the channel (S103). On the other hand, when the balance frequency FBAL is the OFF period Toff(S111: Toff), the control unit121determines whether or not the voltage difference ΔVB when a next cycle starts (at a time when a next ON period Tonstarts) is equal to or less than a predetermined threshold value (an intermediate value between ΔVBonand ΔVBoff, for example) (S108). Herein, when the voltage difference ΔVB is equal to or less than the threshold value (S108: YES), the control unit121completes the balance operation of the channel. On the other hand, when the voltage difference ΔVB is more than the threshold value (S108: NO), a balance operation of the next cycle is started (S103).

An operation of the control unit121transmitting a signal for notifying an abnormality from the communication unit122will be described by usingFIG. 13.FIG. 13is a flowchart illustrating a flow of processing of the control unit121transmitting a signal for notifying an abnormality from the communication unit122.

The control unit121reads out an operation stop time or an operation stop number-of-times of each of the balance circuits112held in a storage region (S201). Then, the control unit121calculates, for each of the balance circuits112, an operation stop time or an operation stop number-of-times per unit time (S202). Then, the control unit121determines whether or not the calculated operation stop time is equal to or more than a predetermined reference value, or whether or not the calculated operation stop number-of-times is equal to or more than a predetermined reference number-of-times (S203). When there is the balance circuit112concerned as a result of determination in S203(S203: YES), the control unit121generates information capable of specifying the balance circuit112or the battery cell311corresponding to the balance circuit112, and causes the communication unit122to transmit the information toward an external monitoring device and the like (S204).

FIG. 14is a diagram schematically illustrating another circuit configuration example of the power storage device according to the first example embodiment. As illustrated inFIG. 14, the first temperature sensor111may be provided for the plurality of balance circuits112. In an example of the present figure, one first temperature sensor111is provided for every one battery module31(for every three balance circuits112). Other configurations are similar to those inFIG. 10.

An example of a balance operation in the power storage device inFIG. 14will be described by usingFIG. 15.

FIG. 15is a diagram exemplifying a specific flow of a balance operation of each channel in the power storage device inFIG. 14. Note that example inFIG. 15exemplifies an operation when a lowest voltage cell is present in another battery module31that are not illustrated inFIG. 14.

First, a voltage difference ΔVB between the battery cell311of CH1and the lowest voltage cell reaches ΔVBonat time t1. Then, the control unit121sets the switch element1122of CH1to an ON state, and starts a balance operation of CH1. Accordingly, the voltage difference ΔVB of CH1begins to decrease. In addition, a first measured temperature begins to rise due to heat generated from the heat-generating element1121of CH1.

Thereafter, a voltage difference ΔVB between the battery cell311of CH2and the lowest voltage cell reaches ΔVBonat time t2. Then, the control unit121sets the switch element1122of CH2to an ON state, and starts a balance operation of CH2. Accordingly, the voltage difference ΔVB of CH2begins to decrease. In addition, a rise value per unit time of the first measured temperature increases from time t2, due to heat generated from the heat-generating elements1121of CH1and CH2.

Thereafter, a voltage difference ΔVB between the battery cell311of CH3and the lowest voltage cell reaches ΔVBonat time t3. Then, the control unit121sets the switch element1122of CH3to an ON state, and starts a balance operation of CH3. Accordingly, the voltage difference ΔVB of CH3begins to decrease. In addition, a rate of rise of the first measured temperature further increases from time t3, due to heat generated from the heat-generating elements1121of all the channels.

Thereafter, the first measured temperature continues to rise, and the first measured temperature reaches an upper reference temperature TRU at time t4. Then, the control unit121sets the switch element1122of a channel (CH1, CH2, and CH3) executing a balance operation among the channels corresponding to the first temperature sensor111to an OFF state, and stops the balance operation of the channel. Since the balance operation is stopped in CH1, CH2, and CH3, heat generation of the heat-generating elements1121of the channels calms down, and the first measured temperature begins to decrease.

Thereafter, the first measured temperature drops down to a lower reference temperature TRL at time t5. Then, the control unit121restarts a balance operation of a channel of which a balance frequency FBAL is an ON period Tonamong the channels (CH1, CH2, and CH3) corresponding to the first temperature sensor111. In the example inFIG. 15, all the channels are in the ON period Ton. Thus, the control unit121sets the switch elements1122of all the channels to an ON state, and a balance operation is restarted in all the channels. Consequently, the voltage difference of each channel begins to decrease again, and the first measured temperature begins to rise again.

Thereafter, the voltage difference ΔVB between the battery cell311of CH1and the lowest voltage cell reaches ΔVBoffat time t6. Then, the control unit121sets the switch element1122of CH1to an OFF state before time t7at which the ON period Tonof CH1ends, and completes the balance operation of CH1. Since the cell balance operation is executed in only CH2and CH3from time t6, the first measured temperature decreases.

Thereafter, the ON period Tonends in CH2and CH3respectively at time t8and time t9. Then, the control unit121sets the switch element1122of CH2to an OFF state at time t8, and stops the balance operation of CH2. The control unit121also sets the switch element1122of CH3to an OFF state at time t9, and stops the balance operation of CH3.

In addition, as described inFIG. 11, the control unit121completes the balance operation in CH2and CH3when the voltage difference ΔVB, after occurrence of voltage rebound, is equal to or less than a predetermined threshold value (an intermediate value between ΔVBonand ΔVBoff, for example) at a time of entering a next ON period Tonafter a lapse of the OFF period Toff. When the voltage difference ΔVB between the battery cell311of each channel and the lowest voltage cell is more than the predetermined threshold value, the control unit121continues the balance operation of the channel in a next cycle.

The flow of the balance operation of the power storage device inFIG. 15is similar to that in the flowchart illustrated inFIG. 12, except for the following point.

When the determination in S105indicates that the first measured temperature is equal to or more than the upper reference temperature TRU (S105: YES), the control unit121stops the balance operations of all the channels (CH1, CH2, and CH3in the example inFIG. 14) corresponding to the first temperature sensor111(S109). Then, the control unit121holds a state in which the balance operations of the channels are stopped until the first measured temperature becomes equal to or less than the lower reference temperature TRL (S110: NO). When the determination in S110indicates that the first measured temperature drops down to the lower reference temperature TRL (S110: YES), the control unit121determines, for each of the channels stopped in S109, whether a balance frequency is an ON period Tonor an OFF period Toff(S111). Then, the control unit121restarts a balance operation for a channel of which the balance frequency is the ON period Ton(S103), and performs determination in S108for a channel of which the balance frequency is the OFF period Toff.

Another example of a balance operation in the power storage device inFIG. 14will be described by usingFIG. 16.FIG. 16is a diagram exemplifying a specific flow of a balance operation of each channel in the power storage device inFIG. 14. The present example is different from the case ofFIG. 15in that each channel does not have a frequency of a balance operation (a balance frequency FBAL).

A flow from time t1to time t6inFIG. 16is similar to that in the case ofFIG. 15. In the present example, since the balance frequency FBAL is absent, the control unit121continues a balance operation of each channel until a voltage difference ΔVB from a lowest voltage cell becomes ΔVBoff, except for a period of time from when a first measured temperature becomes an upper reference temperature TRU to when the first measured temperature drops to a lower reference temperature TRL (from time t4to time t5). In the example inFIG. 16, the voltage difference ΔVB between the battery cell311of CH2and the lowest voltage cell becomes ΔVBoffat time t7, and the control unit121completes the balance operation of CH2at time t7. Although not illustrated, the control unit121also completes the balance operation of CH3when the voltage difference ΔVB between the battery cell311of CH3and the lowest voltage cell becomes ΔVBoff.

FIG. 17is a flowchart illustrating a flow of processing of the balance operation inFIG. 16. The flowchart inFIG. 17is different from the flowchart inFIG. 12in that processing relating to a balance frequency FBAL is absent.

A flow of processing from S101to S105is similar to that in the flowchart inFIG. 12.

When the first measured temperature is less than the upper reference temperature TRU (S105: NO), processing from S104is repeated. On the other hand, when the first measured temperature is equal to or more than the upper reference temperature TRU (S105: YES), the control unit121stops the balance operations of all the channels (CH1, CH2, and CH3in the example inFIG. 14) corresponding to the first temperature sensor111(S109). Then, the control unit121holds a state in which the balance operations of the channels are stopped until the first measured temperature becomes equal to or less than the lower reference temperature TRL (S110: NO). When the determination in S110indicates that the first measured temperature drops down to the lower reference temperature TRL (S110: YES), the control unit121restarts the balance operation of each channel (S103).

[Operation and Effect of First Example Embodiment]

In the present example embodiment, as above, an operation of the balance circuit112that generates heat through a balance operation is controlled by a measured temperature of the first temperature sensor111corresponding to the balance circuit. Specifically, when a temperature that is measured by the first temperature sensor111corresponding to a certain balance circuit112becomes equal to or more than an upper reference temperature, a balance operation of the balance circuit112is stopped. This prevents the balance circuit112from generating heat to a fixed temperature or more, and enables to prevent decrease in service life of the battery cell311neighboring the balance circuit112as well as decrease in service life and operation reliability of components in a power storage device due to heat. In addition, the present example embodiment maintains a state in which the balance operation of the balance circuit112is stopped until a measured temperature of the first temperature sensor111drops to a lower reference temperature after the operation of the balance circuit112is stopped. This allows the heat-generated balance circuit112to cool down, and enables to prevent decrease in service life of the battery cell311as well as decrease in service life and operation reliability of components in a power storage device due to heat.

Second Example Embodiment

The present example embodiment is similar to the configuration of the first example embodiment, except that a power storage device further includes a second temperature sensor13.

FIG. 18is a diagram conceptually illustrating a processing configuration of a power storage device according to a second example embodiment. As illustrated inFIG. 18, a balance circuit board1further includes a second temperature sensor13. Unlike a first temperature sensor111, the second temperature sensor13is provided for measuring an ambient temperature. Thus, the second temperature sensor13is provided at a position more distant from the balance circuit112than the first temperature sensor111is, so as not to be influenced by heat generation of each of balance circuits112. A range not influenced by heat generated from the balance circuit112depends on a parameter such as a dimension of a balance circuit block11, a thickness and thermal resistance of the balance circuit board1, and layout of the balance circuit112. For example, when the balance circuit board1is a printed circuit board (PCB) having a thickness of 1.6 cm and the balance circuit block11has a dimension of 10 cm square, the second temperature sensor13is provided at a position distant by 10 cm or more from an edge portion of the balance circuit block11, for example. In addition, a range not influenced by heat generated from the balance circuit112can be grasped by carrying out a test operation or simulation of the balance circuit board1and measuring temperature distribution of the balance circuit board1, for example. Accordingly, a position of the second temperature sensor13can be determined by using a result of the measurement.

The balance circuit board1preferably includes an isolation region that is a region for separating a region where the plurality of balance circuits112each including a heat-generating element1121as a main heat source are provided from a region where the second temperature sensor13is provided, and that is a region where no conductive pattern is provided. The conductive pattern is metal and generally has a higher thermal conductivity than that of a base material of the balance circuit board1. Thus, the conductive pattern-free isolation region interposed between the region where the balance circuits112are provided and the region where the second temperature sensor13is provided can prevent heat from transferring to the second temperature sensor13through the balance circuit board1. As a result, the second temperature sensor13can accurately measure an ambient temperature.

The second temperature sensor13may also be provided outside the balance circuit board1, instead of on the balance circuit board1. For example, the second temperature sensor13may be provided on a housing face of the power storage device, or may be provided in midair away from the balance circuit board1by a fixed distance or more, by means of a wire, a binding material, and the like. The second temperature sensor13may use a thermistor resistor, a semiconductor temperature sensor, an RTD, and the like, similarly to the first temperature sensor111.

In a control unit121according to the present example embodiment, the control unit121controls a balance operation of each of the balance circuits112, by using a difference temperature between a first measured temperature measured by the first temperature sensor111and a temperature measured by the second temperature sensor13(hereinafter, written as a second measured temperature), instead of using the first measured temperature. Since the difference temperature is a parameter different from the first measured temperature, the control unit121holds an upper reference temperature and a lower reference temperature (a second upper reference temperature and a second lower reference temperature) for the difference temperature, separately from an upper reference temperature and a lower reference temperature (a first upper reference temperature and a first lower reference temperature) that are set for the first measured temperature. The control unit121calculates the difference temperature between the first measured temperature and the second measured temperature. When the difference temperature is equal to or more than the second upper reference temperature, the control unit121stops the operation of the balance circuit112corresponding to the first temperature sensor111. When the balance circuit112is stopped, heat generation of the balance circuit112calms down, and the first measured temperature begins to decrease. Then, the control unit121restarts the operation of the balance circuit112corresponding to the first temperature sensor111when the difference temperature between the first measured temperature measured by the first temperature sensor111and the second measured temperature measured by the second temperature sensor13becomes equal to or less than the second lower reference temperature that is lower than the second upper reference temperature.

Depending on an ambient temperature, the first measured temperature (an absolute temperature) measured by the first temperature sensor111may become equal to or more than the first upper reference temperature even when the difference temperature (in other words, a relative temperature relative to the ambient temperature) does not reach the second upper reference temperature. In this case, the balance circuit112has a possibility of continuing the balance operation while being in a state at the first upper reference temperature or more (in other words, a high temperature state that may possibly affect the neighboring battery cell311and other components adversely). In order to avoid continuing the operation of the balance circuit112in such a state, the control unit121may stop the operation of the balance circuit corresponding to the first temperature sensor111when the first measured temperature of the first temperature sensor111is equal to or more than the first upper reference temperature, regardless of the difference temperature. In this case, similarly to the first example embodiment, the control unit121maintains a state in which the corresponding balance circuit112is stopped until the first measured temperature becomes equal to or less than the first lower reference temperature.

Similarly to the first example embodiment, the control unit121according to the present example embodiment also stores programs for implementing the above-described functions of the present example embodiment in, for example, a not-illustrated storage region such as a ROM and a RAM. Further, the control unit121according to the present example embodiment implements the functions of the present example embodiment by executing the programs stored in the storage region by use of a not-illustrated CPU.

FIG. 19is a diagram conceptually illustrating a circuit configuration example of the power storage device according to the second example embodiment.

In the example inFIG. 19, the plurality of balance circuits112, the first temperature sensor111corresponding to each balance circuit112, and the second temperature sensor13are provided on the balance circuit board1. Then, the balance circuit board1includes, in an isolation region14between the balance circuit112and the second temperature sensor13, a through-hole region141that penetrates through the balance circuit board1. Since the through-hole region141includes no conductive pattern, the through-hole region141serves a role of preventing heat from transferring from the balance circuit to the second temperature sensor13through the balance circuit board1. Accordingly, even in such a case of being unable to assure a sufficient distance for preventing reception of heat generated from the balance circuit112, the through-hole region141reduces heat transferring from the balance circuit112, and the second temperature sensor13can accurately measure an ambient temperature.

FIG. 20is a flowchart illustrating a flow of a balance operation in the power storage device inFIG. 19. The flowchart inFIG. 20is based on the flowchart inFIG. 12, and is similar to the flowchart inFIG. 15except for processing of S301and S302. In the following, the processing of S301and S302will be mainly described.

When the voltage difference ΔVB from the lowest voltage cell is not equal to or less than ΔVBoff(S104: YES) after starting a balance operation (S103), the control unit121determines whether or not a difference temperature that is calculated by using a first measured temperature measured by the first temperature sensor111and a second measured temperature measured by the second temperature sensor13is equal to or more than a second upper reference temperature TRU′, or whether or not the first measured temperature is equal to or more than a first upper reference temperature TRU (S301). When the difference temperature is equal to or more than the second upper reference temperature TRU′, or when the first measured temperature is equal to or more than the first upper reference temperature TRU (S301: YES), the control unit121stops a balance operation of a channel corresponding to the first temperature sensor111(S109). When a balance operation is stopped because the difference temperature becomes equal to or more than the second upper reference temperature TRU′, the control unit121maintains a state in which the balance operation is stopped until the difference temperature becomes equal to or less than a second lower reference temperature TRL′ (S302: NO). Alternatively, when a balance operation is stopped because the first measured temperature becomes equal to or more than the first upper reference temperature TRU, the control unit121maintains a state in which the balance operation is stopped until the first measured temperature becomes equal to or less than a first lower reference temperature TRL (S302: NO). When the first measured temperature decreases because the balance operation is stopped and the difference temperature becomes equal to or less than the second lower reference temperature TRL′, or when the first measured temperature becomes equal to or less than the first lower reference temperature TRL (S302: YES), the control unit121restarts the balance operation (S103) when a balance frequency of the channel is an ON period Ton(S111: Ton).

[Operation and Effect of Second Example Embodiment]

In the present example embodiment, as above, an operation of each of the balance circuits112is controlled by a difference temperature that is obtained by subtracting an ambient temperature measured by the second temperature sensor13from a temperature measured by the first temperature sensor111. This makes it possible to control each of the balance circuits112by using heat generated in each balance circuit112as a parameter.

As above, the example embodiments of the present invention have been described with reference to the drawings. However, these are examples of the present invention, and various configurations other than the above may be also employed.

For example, the control unit121may change the first upper reference temperature or the second upper reference temperature in accordance with magnitude of a variation in voltage between a plurality of battery cells. For example, the control unit121may be configured to add, to the first upper reference temperature or the second upper reference temperature held in advance, a correction value that takes a larger value for the larger voltage difference ΔVB between each battery cell311and the lowest voltage cell. Accordingly, when the voltage difference ΔVB is large, in other words, when a balance between a certain battery cell311and the lowest voltage cell is largely disrupted, the first upper reference temperature or the second upper reference temperature is increased. Therefore, a balance operation of the balance circuit112corresponding to the battery cell311becomes less likely to be stopped. As a result, when a balance between the battery cells is largely disrupted, it becomes possible to equalize voltages of the plurality of battery cells311in a shorter time.

In addition, since the plurality of battery cells311are connected in series, it is possible that a balance operation rather lower discharge energy especially in the case of a passive balance type. In view of this, in the case where a balance operation is performed when charging the battery cell311, the control unit121may use a first upper reference temperature or a second upper reference temperature that is higher than the temperature when discharging the battery cell311. In this case, the control unit121holds in advance, in a not-illustrated storage region, a first upper reference temperature or a second upper reference temperature at the time of charging, and a first upper reference temperature or a second upper reference temperature at the time of discharging that is lower than the first upper reference temperature or the second upper reference temperature at the time of charging, for example. Such a configuration reduces time for performing a balance operation when charging, and can prevent discharge energy of a power storage device from decreasing.

In addition, in the plurality of flowcharts used in the above description, a plurality of steps (processes) are described in order, but execution order of the steps to be executed in each of the example embodiments is not limited to the order of the description. In each of the example embodiments, the order of the illustrated steps may be changed as far as the change does not detract from contents. The above-described example embodiments may also be combined as far as contents do not conflict with each other.

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-105596, filed on May 25, 2015, the disclosure of which is incorporated herein in its entirety.