Battery pack, method of charging secondary battery and battery charger

A battery pack includes a secondary battery including a plurality of cell blocks, a measuring section, a charge and discharge control switch, a protection circuit, and a memory. The measuring section detects a voltage, a current, and an internal resistance, of the secondary battery. The controller monitors the voltage and the current of the secondary battery and outputs a request signal indicative of a charge condition to charge the secondary battery in accordance with the charge condition which is set. The protection circuit monitors voltages of the plurality of cell blocks. The memory registers an initial internal resistance of the secondary battery. The controller calculates a deterioration coefficient by a ratio of the internal resistance detected by the measuring section to the initial internal resistance registered in the memory, and changes the charge condition in accordance with the deterioration coefficient.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of priority of Japanese patent Application No. 2007-281764 filed in the Japanese Patent Office on Oct. 30, 2007, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present application relates to a battery pack, a method of charging a secondary battery, and a battery charger. More particularly, the application relates to a battery pack, a method of charging a secondary battery, and a battery charger, which control a charge condition in accordance with the deterioration coefficient of a secondary battery.

A combination of constant-current charge and constant-voltage charge (constant-current constant-voltage method) is known as a method of charging a secondary battery. This charging method will be described with reference toFIG. 9, on which the abscissa represents charging time and the ordinate represents cell voltage and charging current. InFIG. 9, the region indicated by arrows a and b (hereinafter referred to as a “region a-b”) is the range of constant-current charge, and the region indicated by arrows c and d (hereinafter referred to as a “region c-d”) is the range of constant-voltage charge. The arrow I indicates charging current, and the arrow V indicates cell voltage. A power supply section for charging performs the operation of constant-current control in the region a-b, and performs the operation of constant-voltage control in the region c-d.

As shown inFIG. 9, during the region a-b, the constant-current charge is performed by a predetermined current value, and then the cell voltage rises. In this example, the charging current is maintained at, for example, 3300 mA. When the charging proceeds, so that the cell voltage reaches a predetermined voltage value, for example, 4100 mV, switching takes place from constant-current charge to constant-voltage charge. During the region c-d, the charging current is gradually lowered, and the cell voltage rises toward the output voltage (e.g. 4200 mV) of the power supply section. Thereafter, the charging is completed when the charging current becomes smaller than a predetermined value (a current value at end-of-charge).

The secondary battery gradually deteriorates with increasing the number of charge and discharge cycles, where each cycle consists of charge, and discharge and pause, as described above. The secondary battery deterioration has been determined by the count of charge and discharge cycles, the actual discharge amount, or charging time. For example, in the method using the count of charge and discharge cycles, the number of charge and discharge cycles is calculated by using charging amount or discharging amount, and a deterioration coefficient is obtained from the number of charge and discharge cycles. When the actual discharging amount is used, a deterioration coefficient is obtained from the actual discharging amount and a design capacity value by using a calculation equation: Deterioration coefficient=Design value capacity/Actual discharging amount.

Japanese Unexamined Patent Application Publication No. H11-174136 discloses the method including the steps of measuring the effective resistance in a battery pack and determining the deterioration thereof by the measured value. When the effective resistance is used, the deterioration can be determined in a short time.

SUMMARY

In the charging by the constant-current constant-voltage method described above, the charging current value during a constant-current charge period, and the charging voltage value are set to fixed values, respectively. Accordingly, when the battery pack deterioration progresses to raise the internal resistance, the rise in voltages is large in proportion to the rise in internal resistance. As a result, the time of the region a-b which is the range of constant-current charge becomes short, and the time of the region c-d which is the range of constant-voltage charge becomes long, as shown inFIG. 10.

When the range of constant-voltage charge increases in length, the time period over which the secondary battery has a high voltage of, for example, 4100 mV and above, becomes long, so that the secondary battery deterioration is accelerated. It is therefore desirable to control the charge condition in accordance with the progress of the secondary battery deterioration.

However, a known method of determining the secondary battery deterioration does not calculate the deterioration coefficient properly. For example, in the method using the count of the charge and discharge cycles, the deterioration due to preservation, namely when the secondary battery is preserved for a long time, is not reflected, making it difficult to accurately calculate the deterioration coefficient. Further, in the method of calculating a deterioration coefficient based on the discharging amount and the charging time, an error may occur between the design capacity and the actual secondary battery capacity. This method is also considerably affected by the battery states such as discharging, standing, and charging, as well as temperature conditions. These make it difficult to obtain an accurate deterioration coefficient. Thus, it has been difficult to obtain a proper charge condition depending upon the secondary battery deterioration.

Accordingly, it is desirable to provide a battery pack, a method of charging a secondary battery, and a battery charger, which are capable of suppressing the progress of the secondary battery deterioration by accurately calculating a deterioration coefficient depending upon the secondary battery deterioration, and controlling a charge condition on the basis of the calculated deterioration coefficient.

In an embodiment, there is provided a battery pack which includes a secondary battery including a plurality of cell blocks, a measuring section, a controller, a charge and discharge control switch, a protection circuit, and a memory. The measuring section detects a voltage, a current, and an internal resistance, of the secondary battery. The controller monitors the voltage and the current of the secondary battery and outputs a request signal indicative of a charge condition to charge the secondary battery in accordance with the charge condition which is set. The charge and discharge control switch prevents overcharge and overdischarge. The protection circuit monitors voltages of the plurality of cell blocks. The memory registers an initial internal resistance of the secondary battery. The controller calculates a deterioration coefficient by a ratio of the internal resistance detected by the measuring section to the initial internal resistance registered in the memory, and changes the charge condition in accordance with the deterioration coefficient.

In another embodiment, there is provided a method of charging a secondary battery including the steps of: detecting a voltage, a current, and an internal resistance, of a secondary battery; and monitoring the voltage and the current of the secondary battery and outputting a request signal indicative of a charge condition to charge the secondary battery in accordance with the charge condition which is set. In the step of outputting the request signal, a deterioration coefficient is calculated by a ratio of the internal resistance detected to the initial internal resistance of the secondary battery registered in the memory, and the charge condition is changed depending upon the deterioration coefficient.

In a further embodiment, there is provided a battery charger which includes a secondary battery, a measuring section, a controller, a memory, and a charge controller. The measuring section detects a voltage, a current and an internal resistance, of the secondary battery. The controller monitors the voltage and the current of the secondary battery and outputs a request signal indicative of a charge condition to charge the secondary battery in accordance with the charge condition which is set. The memory registers an initial internal resistance of the secondary battery. The charge controller controls outputs of the charging current and the charging voltage of the secondary battery in accordance with the request signal outputted from the controller. The controller calculates a deterioration coefficient by a ratio of the internal resistance detected by the measuring section to the initial internal resistance registered in the memory, and changes the charge condition depending upon the deterioration coefficient.

Preferably, the charge condition includes a charging current value in constant-current charge, and the charging current value is changed depending upon the deterioration coefficient.

Preferably, the charge condition includes a charging voltage value, and the charging voltage value is changed depending upon the deterioration coefficient.

Preferably, the initial internal resistance is measured within a predetermined time period after manufacturing.

Preferably, the controller calculates the deterioration coefficient by a ratio of a maximum internal resistance among the plurality of cell blocks to the initial internal resistance of a cell block exhibiting the maximum internal resistance.

According to an embodiments, the deterioration coefficient is calculated from the ratio of the initial internal resistance of the secondary battery to the measured current internal resistance, making it possible to calculate a proper deterioration coefficient depending upon the secondary battery deterioration.

Thus, the proper deterioration coefficient is calculated depending upon the secondary battery deterioration, and the charge conditions of the secondary battery are dynamically controlled on the basis of the calculated deterioration coefficient. It is therefore capable of suppressing that the secondary battery is charged in a high voltage state for a long time, thereby suppressing the progress of the secondary battery deterioration.

DETAILED DESCRIPTION

An embodiment will be described below with reference to the accompanying drawings.FIG. 1is a block diagram showing a configuration example of a battery pack20according to an embodiment, and that of a notebook-type personal computer (PC)30connected to the battery back20.

A positive terminal1and a negative terminal2of the battery pack20are connected to a positive terminal31and a negative terminal32of the notebook type PC30, respectively. A clock terminal3aand a communication terminal3bof the battery pack20are connected to a clock terminal33aand a communication terminal33bof the notebook type PC30, respectively. The battery pack20is a so-called smart battery, which is capable of performing communication with the notebook-type PC30through the communication terminal3bto send information about the state of the battery pack20. Upon the receipt of the information, the notebook-type PC30controls the output of a current value or a voltage value depending upon the state of the battery pack20, and charges a secondary battery7by the constant-current constant-voltage charge method. InFIG. 1, for the sake of simplification of the configuration of the notebook type PC30, only the configuration related to charging is shown.

The battery pack20includes mainly the secondary battery7, a temperature detecting element8, a protection circuit9, a fuse10, a measuring section11, a current detecting resistor12, a CPU13, and a charge and discharge control switch4. The positive terminal1of the battery pack20is connected to the positive terminal of the secondary battery7through the charge and discharge control switch4and the fuse10. The negative terminal2is connected to the negative terminal of the secondary battery7through the current detecting resistor12.

A measuring section for detecting a voltage, which is a constituent feature, corresponds to the measuring section11, and a measuring section for detecting a current and an internal resistance corresponds to the CPU13. A controller to output a request signal indicative of a charge condition corresponds to the CPU13. A memory which registers the initial internal resistance corresponds to the memory14.

The secondary battery7is a secondary battery such as a lithium ion battery, and has the configuration of connecting in series a cell block7a, a cell block7b, and a cell block7c, each having, for example, two battery cells arranged in parallel. For example, a secondary battery, whose full charge voltage per battery cell is 4.2 V, can be used as the secondary battery7.

The measuring section11measures each of the voltages of the cell blocks7a,7b, and7cincluded in the battery pack, and supplies the measured values to the CPU13. In the following, the individual voltages of the cell blocks7a,7b, and7c, each having two battery cells are referred to as “cell voltage”. The measuring section11also has a function as a regulator which generates a power supply voltage by stabilizing the voltage of the secondary battery7.

When the cell voltage of any one of these cell blocks7a,7b, and7cbecomes an overcharge detecting voltage, or when any one of the cell voltages is below an overdischarge detecting voltage, the measuring section11prevents overcharging and overdischarging by transmitting a control signal to the charge and discharge control switch4. In the case of the lithium ion battery, the overcharge detecting voltage is set to, for example, 4.2 V±0.5 V, and the overdischarge detecting voltage is set to, for example, 2.4 V±0.1 V.

The charge and discharge control switch4is composed of a charge control FET (field effect transistor)5and a discharge control FET6. A parasitic diode5aexists between the drain and the source of the charge control FET5, and a parasitic diode6aexists between the drain and the source of the charge control FET6. The parasitic diode5ahas a backward polarity with respect to charging current flowing in the direction from the positive terminal1to the secondary battery7, and a forward polarity with respect to discharge current flowing in the direction from the negative terminal2to the secondary battery7. The parasitic diode6ahas a forward polarity with respect to the charging current, and a backward polarity with respect to the discharge current.

Control signals from the measuring section11are supplied to the respective gates of the charge control FET5and the discharge control FET6. In the normal charge and discharge operations, the control signals are set at a low level to turn the charge control FET5and the discharge control FET6on-state. Since the charge control FET5and the discharge control FET6are of P-channel type, both are turned to the on-state by a gate potential that is lower than a source potential by a predetermined value or above.

When the battery voltage becomes the overcharge detecting voltage, the charge control FET5is turned off to control so that no charging current flows. When the battery voltage becomes the overdischarge detecting voltage, the discharge control FET6is turned off to control so that no discharging current flows.

The protection circuit9monitors the voltages of the cell blocks7a,7b, and7c, and melts the fuse10connected to the protection circuit9for the safety of the battery pack20when the cell voltage exceeds a charge inhibiting voltage (e.g. 4.30 V). When the fuse10is melted, the battery pack20becomes permanent failure mode in which neither charging nor discharging is possible.

The CPU13uses the current detecting resistor12to measure the magnitude and direction of current. The CPU13also captures the battery temperature measured by the temperature detecting element8formed by a thermistor or the like. The CPU13calculates the internal resistance values of the individual cell blocks by the voltage values supplied from the measuring section11, and the measured current values and temperatures. These measured values are stored in the memory14included in the CPU13. The memory14is composed of, for example, a non-volatile electrically erasable and programmable read only memory (EEPROM) or the like.

The initial internal resistances of the cell blocks7a,7b, and7care registered in the memory14. Each of the initial internal resistance is the internal resistance measured before using the battery pack20, and is obtained by measuring the internal resistance of the cell blocks within a predetermined time period (for example, within three months) after manufacturing the battery pack20. A specific procedure of registering the initial internal resistances will be described later.

Using, as a reference value, the initial internal resistances of the individual cell blocks registered in the memory14, the CPU13calculates the degrading coefficient of the secondary battery7by using these initial internal resistances and the internal resistances of the cell blocks7a,7b, and7csupplied from the measuring section11, based on the following equation (1).
Deterioration coefficient=Initial internal resistance/Measured internal resistance  (1)

In the equation (1), the maximum value among the internal resistances of the cell blocks7a,7b, and7cis used as the measured internal resistance, and the initial internal resistance value of the cell block exhibiting the maximum internal resistance is used as the initial internal resistance. In the cell block having a high internal resistance, the cell voltage is likely to rise. Consequently, it becomes possible to reduce the possibility of overcharging by obtaining the deterioration coefficient by using the cell block value having a high internal resistance.

The CPU13also obtains a suitable charge condition of the secondary battery7on the basis of the obtained deterioration coefficient of the secondary battery7, as will be described later. The charge condition of the secondary battery7includes a charging current value, a charging voltage value and the like, and these values are changed depending upon the deterioration coefficient. The charge condition is also stored in the memory14and updated whenever the suitable charge condition is obtained.

The CPU13monitors the voltage and the current of the secondary battery7, and outputs a request signal indicative of a charge condition to the notebook type PC30through the communication terminal3aand the communication terminal33a, to charge properly the secondary battery7under the charge condition which is set.

The controller34of the notebook type PC30controls the output voltage and the output current of the charging section35in accordance with the charge condition supplied from the CPU13. Specifically, at the beginning of charging, the controller34performs constant-current control to maintain the output current of the charging section35at the charging current value required by the CPU13. Upon receiving notification from the CPU13that the cell voltage has reached a predetermined voltage, the controller34switches from constant-current charge to constant-voltage charge, and maintains the output voltage from the charging section35at a charging voltage value required by the CPU13. Upon receiving notification from the CPU13that the charging current value has dropped to the current value at end-of charge, the controller34terminates the charging of the secondary battery7.

The charging section35is connected through an AC connector36to a commercial power supply, and outputs DC power to the positive terminal31and the negative terminal32by AC-DC conversion. The current and the voltage outputted from the charging section35are stably maintained at a predetermined voltage value and a predetermined current value required by the battery pack20, under the control of the controller34.

The variable control of charge condition depending upon the deterioration coefficient of the secondary battery7will be described specifically below.

(1) First Example

The first example illustrates a case where the charging current value during a constant-current charge period is varied depending on the deterioration coefficient of the secondary battery7.

FIGS. 2 and 3are graphs showing examples of changes in charging current and cell voltage when the charging of constant-current constant-voltage method is performed in the battery back after 500-cycle repetition of charge and discharge, respectively. InFIGS. 2 and 3, the abscissa represents the charging time and the ordinate represents the charging current and cell voltage. When the battery pack is composed of a plurality of cell blocks as shown inFIG. 1, the cell voltages inFIGS. 2 and 3indicate the maximum cell voltage value among these cell blocks. InFIGS. 2 and 3, the arrow I indicates charging current, and the arrow V indicates cell voltage.

FIG. 2is an example in which the charge condition is fixed and a charging is performed under the charge condition in the initial setting. Here, the charge condition is as follows: 3300 mA of the charging current value of constant-current charge period in the initial setting (hereinafter referred to as initial charging current in some cases), and 4200 mV of the charging voltage value in the initial setting (hereinafter referred to as initial charging voltage in some cases).

The rise in cell voltage is generated by current and internal resistance, as expressed by the equation: Voltage rise=Charging current×Internal resistance. Therefore, the secondary battery7deteriorates due to repetition of charge and discharge cycles and the internal resistance rises, the voltage rise greatly in proportion to the rise in the internal resistance. Therefore, in the example shown inFIG. 2, a high voltage region having not less than 4100 mV accounts for 2.8 hours of the entire charging time of 2.9 hours. This means a 97% of the secondary battery7is subjected to the high voltage state of not less than 4100 mV during the charging, and thus the deterioration of the secondary battery7progresses.

Consequently, an abrupt rise in cell voltage is suppressed by performing variable control of charging current depending upon the deterioration coefficient of the secondary battery7.

The deterioration coefficient is obtained from the above-mentioned equation (1). For example, when the initial internal resistance is 65 mΩ and the internal resistance measured after 500-cycle repetition of charge and discharge is 128 mΩ, the deterioration coefficient is obtained from the following equation:

A new charging current value in the constant-current charge period (hereinafter referred to as a variable charging current in some cases) is calculated from the obtained deterioration coefficient. A variable charging current value is obtained from the following equation (2).
Variable charging current=Initial charging current×Deterioration coefficient  (2)

For example, when the initial charging current is 3.3 A and the deterioration coefficient is 0.5, the variable charging current is obtained from the following equation.

FIG. 3shows an example when charging is performed by the variable charging current thus obtained. As shown inFIG. 3, the abrupt rise in cell voltage can be suppressed by controlling the charging current in the constant-current charge period so as to be 1650 mA. In the example shown inFIG. 3, a high voltage region which has not less than 4100 mV accounts for 2.1 hours in the entire charging time of 3.7 hours. Namely, the rate at which the secondary battery7becomes a high voltage state of not less than 4100 mV during the charging is 57%.

As apparent fromFIGS. 2 and 3, by changing the charging current depending upon the deterioration, the ratio of the time the secondary battery7into the high voltage state can be improved, for example, from 97% to 57%. It is therefore capable of suppressing the progress of deterioration of the secondary battery7.

Hereinbelow, the steps in the procedure for registering an initial internal resistance and the procedure for variable control processing of charging current depending upon the deterioration of the secondary battery7will be described with reference toFIGS. 4 and 5.

FIG. 4is a flow chart schematically illustrating the procedure for registering an initial internal resistance. The procedure shown inFIG. 4is conducted to the battery pack20before use, for example, at the time of shipment from factory. In the procedure shown inFIG. 4, the battery pack20is in the state of connection with a power supply device capable of outputting current and voltage, or the like. The following procedures are performed by the CPU13unless otherwise noted.

First, in step S1, the measuring section11measures each of the voltages of the cell blocks7a,7b, and7c. The measured values are supplied to the CPU13.

Next, in step S2, the magnitude of the current of the battery pack20is measured. In Step S3, each of the battery temperatures of the cell blocks7a,7b, and7care measured.

Subsequently, in step S4, the initial internal resistances of these cell blocks are calculated by using the voltages supplied from the measuring section11and the measured currents and battery temperatures.

Then, in step S5, it is judged whether the initial internal resistance value variation of each cell block obtained in step S4is within a defined range. If judged that the initial internal resistance value variation is larger than the defined range, the procedure proceeds to step S6.

In step S6, the fact that the initial internal resistance variation of each cell block is larger than the defined range is notified by, for example, sending a reject message to the power supply device to which the battery pack20is connected. The battery pack20notified of the reject message is not shipped and removed as not satisfying the product acceptability criterion.

On the other hand, if judged that the initial internal resistance value variation is within the defined range, the procedure proceeds to step S7. In step S7, the initial internal resistance calculated in step S4is registered as a reference resistance value in the memory14. Thus, the registration procedure of the initial internal resistance is terminated.

Next, the steps in the variable control procedure of charging current depending upon the deterioration of the secondary battery7will be described with reference toFIG. 5. In the procedure shown inFIG. 5, the battery pack20is connected to, for example, the notebook-type PC30and is in a state of chargeable by constant-current constant-voltage method. The following procedures are performed by the CPU13unless otherwise noted.

First, in step S11, the measuring section11measures each of the voltages of the cell blocks7a,7b, and7c. The measured values are supplied to the CPU13.

Next, in step S12, the magnitude of current of the battery pack20is measured. In step S13, the battery temperatures of each of the cell blocks7a,7b, and7care measured.

Subsequently, in step S14, the current internal resistances of the cell blocks are calculated by using the voltages supplied from the measuring section11, and the measured current and battery temperatures.

Then, in step S15, the maximum internal resistance among the internal resistances of the individual cell blocks obtained in step S14is obtained.

Then, in step S16, the deterioration coefficient of the secondary battery7is obtained from the ratio of the maximum internal resistance of the cell blocks obtained in step S14to the initial internal resistance of the cell block exhibiting the maximum internal resistance. The above-mentioned equation (1) is used to calculate the deterioration coefficient.

Then, in step S17, a variable charging current value is calculated from the deterioration coefficient obtained in step S16and the initial charging current. The above-mentioned equation (2) is used to calculate the variable charging current value.

In step S18, a request for a variable charging current is performed by outputting the variable charging current value to the controller34of the notebook-type PC30. In accordance with the request for variable charging current from the CPU13, the controller34of the notebook-type PC30controls to perform the constant-current charge of the secondary battery7while maintaining the output current from the charging section35at the variable charging current value.

(2) Second Example

The second example illustrates the case where the charging voltage is varied depending on the deterioration coefficient of the secondary battery7.

FIGS. 6 and 7show examples of changes in charging current and cell voltage when the charging of constant-current constant-voltage method is performed in the battery back after 500-cycle repetition of charge and discharge, respectively. InFIGS. 6 and 7, the abscissa represents charging time and the ordinate represents charging current and cell voltage. When the battery pack is composed of a plurality of cell blocks as shown inFIG. 1, the cell voltages inFIGS. 6 and 7indicate the maximum cell voltage value among these cell blocks. InFIGS. 6 and 7, the arrow I indicates charging current, and the arrow V indicates cell voltage.

FIG. 6is an example when a charge condition is fixed and the charging is performed under the charge condition in the initial setting. Here, the initial charging current is set to 2400 mA, and the initial charging voltage is set to 4200 mV.

As shown inFIG. 6, when the secondary battery7deteriorates due to the repetition of charge and discharge cycles and then the internal resistance rises, the abrupt voltage rise of the secondary battery7occurs. Consequently, the cell voltage goes into a high voltage region exceeding 4100 mV immediately after starting charging, so that the deterioration of the secondary battery7progresses.

Consequently, the charging of the secondary battery7in the high charging region is suppressed by performing variable control of charging voltage depending upon the deterioration coefficient of the secondary battery7.

As in the case of the first example, the deterioration coefficient is obtained from the equation (1). For example, when the initial internal resistance is 65 mΩ and the internal resistance measured after 500-cycle repetition of charge and discharge is 85 mΩ, the deterioration coefficient is obtained from the following equation:

A new charging voltage value (hereinafter referred to as a variable charging voltage in some cases) is calculated from the obtained deterioration coefficient. For example, assuming that the upper limit value of the variation range of variable charging voltage is 4.2 V which is the initial charging voltage value, and the lower limit thereof is 4.0 V, the variable charging voltage value is obtained from the following equation (3).
Variable charging voltage=4.0 V+(0.2 V×(1−Deterioration coefficient))  (3)

For example, assuming that the deterioration coefficient is 0.76, the variable charging voltage is obtained from the following equation.

FIG. 7shows an example when charging is performed by the variable charging voltage thus obtained. As shown inFIG. 7, by controlling the charging voltage to be at 4050 mV, the cell voltage prevents from entering a vigorous deterioration region of not less than 4100 mV.

As apparent fromFIGS. 6 and 7, the charging of the secondary battery7in the high charging voltage region can be suppressed by changing the charging voltage depending upon deterioration. It is therefore capable of suppressing the progress of deterioration of the secondary battery7.

Next, the steps in procedure of variable control of charging voltage depending upon deterioration will be described with reference toFIG. 8. The procedure for obtaining the initial internal resistance is identical to that described with reference toFIG. 4, and the description thereof is therefore omitted.

FIG. 8is a flow chart schematically illustrating the steps in the variable control procedure of charging voltage depending upon the deterioration of the secondary battery7. In the procedure shown inFIG. 8, the battery pack20is connected to, for example, the notebook-type PC30and is in a state of chargeable by constant-current constant-voltage charge method. The following procedures are performed by the CPU13unless otherwise noted.

First, in step S21, the measuring section11measures each of the voltages of the cell blocks7a,7b, and7c. The measured values are supplied to the CPU13.

Next, in step S22, the magnitude of the current of the battery pack20is measured. In Step S23, each of the battery temperatures of these cell blocks7a,7b, and7care measured.

Subsequently, in step S24, the current internal resistances of the cell blocks are calculated by using the voltages supplied from the measuring section11and the measured currents and battery temperatures.

Then, in step S25, the maximum internal resistance among the internal resistances of these cell blocks obtained in step S24is obtained.

Then, in step S26, the deterioration coefficient of the secondary battery7is obtained from the maximum internal resistance value of the cell blocks obtained in step S24, and the initial internal resistance value of the cell block exhibiting the maximum internal resistance value. The equation (1) is used to calculate the deterioration coefficient.

Then, in step S27, a variable charging voltage value is calculated from the deterioration coefficient obtained in step S26and the initial charging voltage value. The equation (3) is used to calculate the variable charging voltage value.

Then, in step S28, a request for a variable charging voltage is performed by supplying the variable charging voltage value to the controller33of the notebook-type PC30connected to the battery pack20. In accordance with the request for variable charging voltage from the CPU13, the controller34of the notebook-type PC30controls to charge the secondary battery7while maintaining the output current from the charging section35at the variable charging voltage value.

As described above, in embodiments, the deterioration coefficient of the secondary battery7is calculated by using the initial internal resistances of the individual cell blocks as a reference value. This makes it possible to reflect the deterioration due to presentation, for example, when the secondary battery7is preserved for a long time, and also to calculate the deterioration coefficient at a desired timing. It is also capable of reducing the error of deterioration coefficient calculation caused by the difference in use conditions of the secondary battery7. This enables the deterioration coefficient of the secondary battery7to be calculated properly and at high frequency.

The proper calculation of the deterioration coefficient of the secondary battery7provides a proper charge condition. The secondary battery7is subjected to proper constant-current constant-voltage charge under the obtained charge condition. It is therefore avoidable that the secondary battery7is charged in a high voltage state for a long time, thereby suppressing the progress of deterioration of the secondary battery7.

Further, the deterioration coefficient of the secondary battery7is calculated by using the internal resistance of the cell having the highest internal resistance among the plurality of cell blocks. Therefore, even if variations occur in the internal resistances of the cell blocks, the secondary battery7of the cell block having the high internal resistance can prevent overcharging. Accordingly, the stability of the battery pack20can be further enhanced.

Although there has been shown herein and described embodiment, modifications may be made based on the technical concept of the application without limiting the above embodiments.

For example, the numerical values in the above embodiments are cited merely by way of example without limitation, and different numerical values may be used if needed.

The individual configurations of the above embodiment may be combined together, without departing from the gist.

Although, in the above embodiments, the deterioration coefficient is calculated from the following equation: Deterioration coefficient=Initial internal resistance value/Measured internal resistance value, the deterioration coefficient may be calculated from the following equation: Deterioration coefficient=(Measured internal resistance value−Initial internal resistance value)/Initial internal resistance value. In this case, using the obtained deterioration coefficient, the increasing rate of internal resistance is obtained by the following equation: Increasing rate of internal resistance=1−Deterioration coefficient. Then, the obtained rising rate of internal resistance and the initial charging current or the initial charging voltage can be used to calculate a variable charging current or a variable charging voltage.

Although, in the above embodiments, the lithium ion battery is used as the secondary battery7, the secondary battery7is not limited thereto, and it is applicable to various types of batteries such as Ni—Cd (nickel-cadmium) battery, Ni-MH (nickel metal hydride) battery.

Although, in the above embodiments, the two parallel secondary batteries are series-arranged in the three blocks, no special limitation is imposed on the number and the configuration of the secondary battery included in the battery pack.