Charger apparatus capable of determining deterioration of second battery

A charger includes an output circuit unit to output a charging current to a secondary battery, a voltage detection unit to detect a voltage of the secondary battery, and a control unit to control the output circuit unit, whereby constant current charging and constant voltage charging are performed. The control unit decreases a constant current value of the charging current by stages during the constant current charging and determines that the secondary battery has deteriorated by using a first voltage drop value of the secondary battery occurring upon first conversion and a second voltage drop value of the secondary battery occurring upon second conversion.

FIELD OF THE INVENTION

The present invention relates to a charger.

BACKGROUND OF THE INVENTION

Conventionally, a secondary battery has been used as a power source for a variety of types of a portable device or the like. As the number of charge and discharge cycles of a secondary battery increases, the discharge capacity thereof decreases. Accordingly, charging times of the secondary battery repeatedly used are limited. For this reason, some chargers that charge secondary batteries are configured to determine life spans of the secondary batteries when charging them.

This type of the charger includes a charging device that includes a battery voltage detection circuit configured to detect a battery voltage of a battery pack provided with a secondary battery and battery voltage gradient calculation means configured to calculate a battery voltage gradient based on the detected battery voltage and a battery voltage before a predetermined time (see, e.g., Japanese Patent Application Publication No. 2007-24541 (JP2007-24541A)).

If the voltage of the battery pack before the initiation of charging is equal to or lower than a predetermined voltage value and a battery voltage gradient within a predetermined time period after the initiation of charging is equal to or greater than a first predetermined value, the charging device disclosed in JP2007-24541A determines that the life span of the secondary battery has reached its end. Further, the charging device includes an LED which displays the status of life span deterioration of the battery pack.

Accordingly, the charging device is capable of determining the life span of the secondary battery and notifying a user of the deterioration status of the secondary battery in a simple manner.

Further, a battery deterioration measuring device that measures deterioration of a battery cell, i.e., a secondary battery, is known for including a voltage measurement unit configured to measure a voltage of battery cell, a current measurement unit configured to measure a charging current of the battery cell, and a control unit configured to measure the deterioration rate of the battery cell (see, e.g., Japanese Patent Application Publication No. 2008-123961 (JP2008-123961A)).

If the value of a charging current being supplied during the constant current charging of the battery cell is a first charging current value, the battery deterioration measuring device disclosed in JP2008-123961A obtains a first cell voltage from the voltage value measured by the voltage measurement unit.

Further, if the value of the charging current is changed to a second charging current value smaller than the first charging current value, the battery deterioration measuring device obtains a second cell voltage from the voltage value measured by the voltage measurement unit. Furthermore, in the battery deterioration measuring device, the control unit calculates an internal resistance of the battery cell based on the first cell voltage and the second cell voltage to determine the deterioration rate of the battery cell.

Therefore, according to JP2008-123961A, the batter deterioration measuring device may accurately determine the deterioration of the secondary battery to charge the secondary battery.

Meanwhile, there is a variety of secondary batteries, including a secondary battery whose internal resistance is high in an initial phase in which charging times of the secondary battery is small, and a secondary battery whose internal resistance is low in the initial phase, depending on the types or differences of secondary batteries. Moreover, the voltage of the secondary battery before the initiation of charging considerably varies depending on the remaining capacity of the secondary battery to be charged.

However, since the charging device described in JP2007-24541A uses the voltage before the initiation of charging to determine the life span, the possibility of erroneous determination may be increases because of the type or the remaining capacity of the secondary battery. In addition, the battery deterioration measuring device described in JP2008-123961 needs to accurately measure the charging current and voltage of the secondary battery and to calculate the internal resistance, and accordingly, there is a possibility of measurement errors.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides a charger capable of more accurately determining the deterioration of the secondary battery regardless of a type of a secondary battery or a remaining capacity of the secondary battery before the initiation of charging.

In accordance with an aspect of the present invention, there is provided a charger including an output circuit unit configured to output a charging current to a secondary battery, a voltage detection unit configured to detect a voltage of the secondary battery, and a control unit configured to control the output circuit unit, whereby constant current charging, in which the charging current is flows at a constant current, and constant voltage charging, in which the charging current flows at constant voltage after the constant current charging, are performed.

The control unit decreases a constant current value of the charging current by stages during the constant current charging and determines that the secondary battery has deteriorated by using a first voltage drop value of the secondary battery that occurs upon first conversion in which the constant current value is converted and a second voltage drop value of the secondary battery that occurs upon second conversion in which a constant current value which is different from that of the first conversion is converted.

A memory device may be provided in a battery pack including the secondary battery, and the control unit stores numbers of charging times of the secondary battery and the first and the second voltage drop values of the secondary battery corresponding to the numbers of charging times in the memory device. The control unit compares the first and the second voltage drop values in an initial phase in which the number of charging times of the secondary battery is equal to or smaller than a predetermined value with the first and the second voltage drop values in a phase in which the number of charging times of the secondary battery is greater than that in the initial phase to exclude a deviation in first and second voltage drop values of secondary batteries of respective battery packs in the initial phase.

The charger may further include a notification unit configured to provide a result of the determination to an outside.

The charger in accordance with the embodiment of the present invention is capable of more accurately determining the deterioration of the secondary battery regardless of the type of secondary battery or the remaining capacity of the secondary battery before the initiation of charging.

DETAILED DESCRIPTION OF THE EMBODIMENTS

First Embodiment

Referring toFIGS. 1 to 3, a charger10and a battery pack20connected thereto in accordance with the present embodiment will be described. The same reference numerals will be used to designate the same components throughout the drawings, and redundant descriptions thereof will be omitted.

The charger10in accordance with the present embodiment, as shown inFIG. 1, charges a secondary battery21using an external power source AC such as a commercial power source or the like. The secondary battery21is contained in the battery pack20that can be detachably attached to the housing (not shown) of the charger10. Two small batteries E1and E2that are electrically connected in series are used as the secondary battery21.

The charger10in accordance with the present embodiment includes an output circuit unit3for outputting a charging current to the secondary battery21and a voltage detection unit2for detecting a voltage of the secondary battery21. The charger10further includes a control unit1that controls the output circuit unit3so that the output circuit unit3performs constant current charging to flow the charging current at a constant current and constant voltage charging to flow a charging current at a constant voltage after the constant current charging, as shown inFIGS. 2 and 3.

The control unit1decreases the constant current value of the charging current in a stepwise manner during constant current charging, and determines whether or not the secondary battery21has deteriorated by using a first voltage drop value of the secondary battery21that is obtained upon first conversion in which the constant current value is converted and a second voltage drop value of the secondary battery21that is obtained upon second conversion in which a constant current value different from that of the first conversion is converted. The control unit1increases the accuracy of the determination whether or not the secondary battery21has deteriorated based on the first and second voltage drop values. The charger10in accordance with the present embodiment includes a notification unit6notifying of the determination result.

First, the fundamental operation of the charger10in accordance with this embodiment will be described with reference toFIGS. 4 to 6.

The charger10, as shown inFIG. 4, flows a charging current into the secondary battery21at constant current at time t11at which charging is initiated. Then, the charger switches from constant current charging to constant voltage charging at time t14(refer toFIG. 5) in such a way that the control unit1controls the output circuit unit3based on a signal from a constant voltage controller26(which will be described later) of the battery pack20. When the voltage of the secondary battery21reaches a specific voltage, e.g., the reference voltage Vb inFIG. 5, the constant voltage controller26transmits a corresponding signal to the control unit1to switch from the constant current charging to the constant voltage charging, based on a signal from a battery voltage monitoring unit28which is provided in the battery pack20to monitor the voltage of the secondary battery21. That is, the charger10performs constant current charging during the period between time t11and time t14. When the secondary battery21is charged during constant current charging in the charger10, the voltage of the secondary battery21gradually increases. The voltage of the secondary battery21drops from voltage Va to voltage Va1at time t13at which the charging current value is changed from a constant current value I11to a constant current value I12during constant current charging.

Further, the charger10performs constant voltage charging in which a charging current flows into the secondary battery21at constant voltage during the period between time t14and time t15, which comes after the constant current charging. Furthermore, when the secondary battery21is charged during constant voltage charging in the charger10, the charging current flowing into the secondary battery21gradually decreases. In the charger10, the control unit1determines that the secondary battery21has been fully charged based on a signal from a charging current monitoring unit29provided in the charger10and stops the charging of the secondary battery21at time t15.

In other words, the charger10in accordance with the present embodiment charges the secondary battery21by performing constant current charging and constant voltage charging to which the constant current charging has switched after the completion thereof. Accordingly, the charger10does not allow overcharging of the secondary battery21which may easily occur when only constant current charging is performed, while reducing the time taken to charge the secondary battery21compared to performing charging only by constant voltage charging.

Meanwhile, in the secondary battery21, a representative of which is a lithium ion secondary battery (hereinafter also referred to as a “lithium secondary battery”), when the charging times of the secondary battery21increases, an electrical characteristics of the charge or discharge of the secondary battery21tend to deteriorate due of an increase in internal resistance or the like. For this reason, the electrical characteristics of the secondary battery21in an initial phase in which the charging times of the secondary battery21is small are different from those of the deteriorated secondary battery21in a phase in which the charging times of the secondary battery21is great.

As the number of the charging times increases, e.g., as shown inFIG. 6, the electrical characteristics of the secondary battery21during constant current charging are changed from the electrical characteristics of the secondary battery21in the initial phase (e.g., the dashed dotted line inFIG. 6) to the electrical characteristics of the deteriorated secondary battery21(e.g., the solid line inFIG. 6).

In the secondary battery21in the initial phase, the battery voltage of the secondary battery21becomes voltage Va at time t13during the constant current charging with a constant current value I11. The control unit1in the charger10controls the output circuit unit3, so that the constant current value is decreased from the constant current value I11to the constant current value I12at time t13at which the battery voltage of the secondary battery21has become voltage Va, as shown inFIG. 4. The secondary battery21in the initial phase exhibits the characteristic in which a voltage drop occurs by a voltage drop value (Va−Va1) when the constant current value of the charging current changes from the constant current value I11to the constant current value I12.

In contrast, in the deteriorated secondary battery21, the battery voltage of the secondary battery21becomes voltage Va at time t12during the constant current charging with the constant current value I11. The control unit1in the charger10controls the output circuit unit3, so that the constant current value is decreased from constant current value I11to constant current value I12at time t12at which the battery voltage of the secondary battery21has become voltage Va. The deteriorated secondary battery21exhibits the characteristic in which a voltage drop occurs by a voltage drop value (Va−Va2), when the constant current value of the charging current changes from the constant current value I11to the constant current value I12. For this reason, the voltage drop value of the deteriorated secondary battery21tends to increase compared to the voltage drop value of the secondary battery21in the initial phase as shown inFIG. 6.

In the charger10in accordance with the present embodiment, voltage Va is set as a predetermined first voltage in advance. Further, the control unit1calculates the value of a voltage drop from voltage Va to voltage Va2at time t12, when voltage Va is detected. Furthermore, the charger10in accordance with the present embodiment is not limited to the configuration in which the control unit1calculates the voltage drop value based on a preset voltage. That is, in the charger10in accordance with the present embodiment, the battery voltage of the secondary battery21is measured at every preset time and the voltage drop value is calculated at the preset time.

In the charger10, the control unit1may determine that the secondary battery21has deteriorated if the value of the voltage drop occurring when the constant current value is changed during the constant current charging is equal to or larger than a preset value. Since the charger10determines that the secondary battery21has deteriorated by using the value of the voltage drop occurring when the current value is changed during the constant current charging, it can determine whether or not the secondary battery21has deteriorated regardless of the type of secondary battery21or the remaining capacity of the secondary battery21before the initiation of charging. In other words, the charger10functions as a battery deterioration determination device that determines whether or not the secondary battery21has deteriorated by using the characteristic of the voltage drop value during the constant current charging of the secondary battery21.

In this case, in the charger10, if the voltage drop value attributable to the deterioration of the secondary battery21is very small in the initial phase, there is concern about decrease in the accuracy of the determination of whether or not the secondary battery21has deteriorated due to a detection error of the voltage detection unit2. The charger10in accordance with the present embodiment decreases the constant current value of charging current during constant current charging in a stepwise manner, and performs calculation using a plurality of values of voltage drops occurring when the constant current values are changed, thereby increasing the accuracy of the determination of whether or not the secondary battery21has deteriorated. The charger10in accordance with the present embodiment can increase the accuracy of the determination of whether the secondary battery21has deteriorated by using an average of two values of voltage drops, i.e., a first voltage drop value and a second voltage drop value.

The configuration of the charger10in accordance with the present embodiment will be described in detail below.

The battery pack20is mounted to the housing of the charger10, so that the charger10is electrically connected to the battery pack20, and the secondary battery21can be charged by converting a power from the external power source AC and supplying the converted power to the secondary battery21. The charger10includes a rectifier circuit unit4which is electrically connected to the external power source AC. The rectifier circuit unit4, although now shown, includes, e.g., a diode bridge circuit, to which the external power source AC is connected via a fuse. Further, a rectifier circuit unit4may be configured such that a smoothing capacitance is connected between output terminals of a diode bridge circuit and the smoothing capacitance smoothes a DC voltage rectified by the diode bridge circuit.

In the charger10, an output circuit unit3that outputs a charging current to the secondary battery21is electrically connected to the output terminals of the rectifier circuit unit4. Further, in the charger10, an output terminal of the output circuit unit3is connected to a primary side of a first isolation transformer7for the charging of the secondary battery. In the charger10, a secondary side of the first isolation transformer7is connected to a pair of terminals13. The charger10is configured such that the terminals13of the charger10are electrically connected to terminals23electrically connected to the secondary battery21contained in the battery pack20.

Further, although the charger10is configured such that the terminals13of the charger10are electrically and mechanically connected to the terminals23of the battery pack20in a direct manner, the charger10is not limited thereto. The charger10and the battery pack20may be electrically connected without a mechanical contact using an isolation transformer (not shown) or the like. That is, the charger10has a configuration capable of non-contact charging.

The charger10rectifies the AC voltage from the external power source AC by using the rectifier circuit unit4, converts the rectified AC voltage into a predetermined voltage via the output circuit unit3and the first isolation transformer7, and then outputs the converted voltage from the terminal13to the battery pack20. Accordingly, the charger10may supply a charging current to the battery pack20of the secondary battery21as a DC constant voltage power source.

The output circuit unit3functions as an output control unit that controls the supply and cutoff of a power to the first isolation transformer7or controls the amount of current flowing through the first isolation transformer7. The output circuit unit3is connected to the control unit1via a photocoupler9. The control unit1is electrically connected to the voltage detection unit2that is provided on the secondary side of the first isolation transformer7.

The control unit1may include a CPU (Central Processing Unit) such as a microprocessor or the like capable of executing predetermined calculation processing. Further, the control unit1includes a memory unit that stores a predetermined value to be compared with a voltage drop value in order to determine whether or not the secondary battery21has deteriorated.

Further, the control unit1includes, as a storage unit, a ROM (Read Only Memory) which is a nonvolatile semiconductor device storing a predetermined control program therein. The control unit1includes an AD converter which converts the signal of a voltage detected by the voltage detection unit2into a digital value. The control unit1outputs a control signal to the output circuit unit3via the photocoupler9by executing the control program stored in the ROM, thereby controlling the operation of charging the secondary battery21.

The voltage detection unit2is, e.g., a circuit that detects a voltage between the terminals13to which the secondary battery21is electrically connected and is configured in that the circuit in which, two resistors have been connected in series, is connected between the terminals13in parallel. The voltage detection unit2outputs a voltage resulting from a voltage drop of the resistors to the control unit1as a voltage of the secondary battery21.

The output circuit unit3includes a switching element formed with, e.g., an MOS transistor, and the switching element is controlled to be turned on/off in response to a PWM signal transmitted from the control unit1as a control signal. That is, the switching frequency and “ON” time of the switching element is controlled based on PWM signal. In the charger10, the control unit1outputs a control signal based on the detection result of the voltage detection unit2, and the switching element of the output circuit unit3is operated in response to the PWM signal, i.e., the control signal, so that a desired charging current is outputted to the battery pack20.

Further, a power supply unit5for the control unit1is electrically connected to the output terminals of the above-described rectifier circuit unit4. The AC voltage of the external power source AC is rectified by the rectifier circuit unit4, and then applied to the power supply unit5. The power supply unit5adjusts a current or a voltage to be supplied to the control unit1so that it functions as a power source to operate the control unit1. The output terminals of the power supply unit5are electrically connected to the primary side of a second isolation transformer8for the control unit1.

A peripheral circuit unit11that includes a cooling fan (not shown) configured to cool the charger10or a circuit block that controls the operation of the cooling fan (not shown) is connected to the secondary side of the second isolation transformer8. The peripheral circuit unit11is electrically connected to the control unit1so that the circuit block, which controls the operation of the cooling fan, can be controlled. Further, at the secondary side of the second isolation transformer8, a voltage conversion circuit unit12is connected to an output terminal of the second isolation transformer8. The voltage conversion circuit unit12is electrically connected to the control unit1to stabilize a voltage inputted to the control unit1. The voltage conversion circuit unit12is, e.g., a series regulator configuring a DC power source of a constant voltage, and may be formed of a three-terminal regulator or the like.

That is, a power is supplied to the control unit1from the power supply unit5electrically connected to the output terminal of the rectifier circuit unit4via the second isolation transformer8and the voltage conversion circuit unit12. Further, at the secondary side of the second isolation transformer8, a notification unit6that is connected to the control unit1to provide notification of the determination result obtained by the control unit1to the outside is provided.

Further, the notification unit6may include, e.g., a display unit, such as an LED (Light Emitting Diode) or an organic EL (ElectroLuminescence) device, a speaker or a buzzer. When the notification unit6includes an LED therein, the notification unit6appropriately contains a limiting resistor that limits current flowing through the LED. The notification operation of the notification unit6is controlled by the control unit1. The notification unit may provide notification of the results of the determination made that the secondary battery21has deteriorated to the outside in response to a signal from the control unit1.

Further, the charger10includes the charging current monitoring unit29for monitoring the charging current to the secondary battery21. The control unit1monitors, by using the charging current monitoring unit29, the charging current of the secondary battery21to determine whether or not the charging current is reduced to a specific threshold value (minimum physical amount for charging) or less during a constant voltage charging. When the charging current is reduced to the specific threshold value or less, the control unit1stops the charging of the secondary battery21.

The battery pack20connected to the charger10in accordance with the present embodiment will be described in further detail below.

The two small batteries E1and E2electrically connected in series, as shown inFIG. 1, are contained as the secondary battery21in the battery pack20which is charged by the charger10in accordance with the present embodiment. Further, the secondary battery21is not limited to the two small batteries E1and E2electrically connected in series. The secondary battery21may be two small batteries E1and E2which are electrically connected in parallel, or a single small battery. Furthermore, the secondary battery21may be three or more small batteries which are appropriately electrically connected in series, in parallel, or in series-parallel. The secondary battery21charged by the charger10in accordance with the present embodiment may be, e.g., a nickel-hydrogen secondary battery or a lithium secondary battery.

Although the use of the secondary battery21charged by the charger10is not particularly limited, the secondary battery21may be put in, e.g., an electric power tool (not shown) as a power source therefor. When the secondary battery21is installed to be used as the power source for an electric power tool, it is preferable to use a battery pack20including a housing (not shown) which is configured to be put in the electric power tool and accommodate the secondary battery21therein.

A memory device27on which data is rewritable from the outside is provided in the battery pack20. The memory device27may store the number of the charging times of the secondary battery21and the voltage drop values corresponding to the number of the charging times of the secondary battery21. Under the control of the control unit1, the memory device27may store the number of charging times of the secondary battery21and the voltage drop values of the secondary battery21corresponding thereto so that the voltage drop values are associated with the number of charging times of the secondary battery21.

The memory device27may not only store the number of charging times of the secondary battery21and the voltage drop values corresponding thereto, but may also store a battery voltage in the case in which the secondary battery21has been fully charged, a discharge completion voltage that is used to prevent overdischarging of the secondary battery21, a threshold value of the charging current that is used to prevent overcharging, the reference voltage Vb, a control voltage for constant voltage and the like. Further, the memory device27may be configured with, e.g., a nonvolatile semiconductor device and the like such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) or the like.

Further, the battery pack20is provided with a constant voltage controller26that keeps the voltage of the secondary battery21to be constant during the constant voltage charging of the secondary battery21. The constant voltage controller26may be configured with, e.g., a control IC or the like. In the charger10in accordance with the present embodiment, during the charging of the secondary battery21, a terminal24of the battery pack20that is electrically connected to the constant voltage controller26is connected to a terminal14of the charger10that is electrically connected to the control unit1. Next, the operation of the charger10in accordance with the present embodiment will be described in detail below.

The charger10in accordance with the present embodiment initiates charging when the terminals23of the battery pack20which are connected to the secondary battery are electrically connected to the terminals13of the charger10. First, the charger10performs constant current charging in which the control unit1controls the output circuit unit3so that the output circuit unit3allows a charging current to flow into the secondary battery21at a constant current. The charger10measures a voltage applied to the secondary battery21by using the voltage detection unit2during the constant current charging. The charger10in accordance with the present embodiment decreases the constant current value of the charging current in a stepwise manner during constant current charging.

The charger10in accordance with the present embodiment decreases the constant current value of the charging current in a stepwise manner as the charging of the secondary battery21proceeds during the constant current charging, and thus, mitigates the deterioration of the secondary battery21by preventing overcharge or the like. Further, the charger10in accordance with the present embodiment can more accurately determine whether the secondary battery21has deteriorated.

The charger10in accordance with the present embodiment, the control unit1controls the output circuit unit3so that the charging of the secondary battery21is initiated and constant current charging is performed to flow a charging current at a constant current value I21during the period from time t21to time t22, i.e., the period immediately after the initiation of charging. Further, the charger10decreases the current value of the charging current from the constant current value I21to a constant current value I22at time t22, and then performs constant current charging during the period from time t22to time t23. In the same manner, the charger10decreases the current value of the charging current from the constant current value I22to a constant current value I23at time t23, and then performs constant current charging during the period from time t23to time t24. Moreover, the charger10decreases the current value of the charging current from the constant current value I23to a constant current value I24at time t24. Then, the charger10switches from constant current charging to constant voltage charging at time t25in such a manner that the control unit1controls the output circuit unit3based on a signal from the constant voltage controller26of the battery pack20. The charger10performs constant voltage charging during the period from time t25to time t26.

At respective times t22to t24inFIG. 2, decreasing values ΔI1, ΔI2and ΔI3of the constant currents, which decrease by stages, may be same or different from each other.

That is, the charger10performs constant current charging while decreasing the charging current in a stepwise manner during the period from time t21to time t25, and switches to constant voltage charging by controlling the output circuit unit3by the control unit1. In the charger10, the control unit1determines that the secondary battery has been fully charged based on the signal from the charging current monitoring unit29and stops the charging of the secondary battery21at time t26.

The charger10in accordance with the present embodiment uses the value of voltage drop of the secondary battery21, which is detected by the voltage detection unit2immediately before changing the constant current value to determine whether or not the secondary battery21has deteriorated during constant current charging in which the constant current value of the charging current is decreased in a stepwise manner. The charger10measures the voltage of the secondary battery21by using the voltage detection unit2during the constant current charging.

Further, the control unit1determines whether or not the voltage detected by the voltage detection unit2has reached a predetermined first voltage Ve stored in the memory unit of the control unit1in advance. The control unit1maintains the constant current charging at the constant current value I21until the voltage of the secondary battery21detected by the voltage detection unit reaches the preset voltage Ve (at t22). If it is determined that the voltage detected by the voltage detection unit2is equal to the voltage Ve, the control unit1decreases the constant current value of the charging current from the constant current value I21to the constant current value I22and then performs constant current charging. Herein, the voltage of the secondary battery21is decreased from the voltage Ve to a voltage Ve1by decreasing the constant current value of the charging current.

The charger10detects the voltage Ve immediately before a voltage drop of the secondary battery21and the voltage Ve1, i.e., the minimum voltage value after the voltage drop, and stores them in the memory unit of the control unit1.

Next, the charger10continues the constant current charging and determines whether or not the voltage detected by the voltage detection unit2has reached a predetermined voltage Vf, which was stored in the memory unit of the control unit1in advance, during constant current charging. The control unit1maintains the constant current charging at the constant current value I22until the voltage of the secondary battery21detected by the voltage detection unit reaches the preset voltage Vf (at t23). If it is determined that the voltage detected by the voltage detection unit2is equal to the voltage Vf, the control unit1decreases the constant current value of the charging current from the constant current value I22to the constant current value I23, and then performs constant current charging.

Herein, the voltage of the secondary battery21is decreased from the voltage Vf to a voltage Vf1by decreasing the constant current value of the charging current. The charger10detects the voltage Vf immediately before the voltage drop of the secondary battery21and a voltage Vf1, i.e., the minimum voltage value after the voltage drop and stores them in the memory unit of the control unit1.

In the same manner, the charger10maintains the constant current charging at the constant current value I23until the voltage detected by the voltage detection unit2reaches a voltage Vg (at t24), which was set in the control unit1in advance, during the constant current charging. If it is determined that the voltage detected by the voltage detection unit2is equal to voltage Vg, the control unit1decreases the current value of the charging current from the constant current value I23to a constant current value I24and then performs constant current charging. Thereafter, the control unit1switches from the constant current charging to constant voltage charging at time t25based on a signal from the constant voltage controller26provided in the battery pack20. Further, the charger10determines that the secondary battery21has been fully charged based on the signal from the charging current monitoring unit29, and the control unit1stops the charging of the secondary battery21at time t26.

Next, control unit1calculates, by using a control program, a first voltage drop value (Ve−Ve1) of the secondary battery21that occurs upon first conversion in which a constant current value is converted and stores it in the memory unit of the control unit1. In the same manner, the control unit1calculates, by the control program, the second voltage drop value (Vf−Vf1) of the secondary battery that occurs upon second conversion in which a constant current value that is different from the constant current value converted in the first conversion is converted, and stores it in the memory unit of the control unit1.

Further, the charger10outputs the first voltage drop value and the second voltage drop value, as the voltage drop values, from the control unit1to the memory device27of the battery pack20through the terminals15and25.

The control unit1stores the number of charging times of the secondary battery21and the voltage drop value corresponding to the number of charging times of the secondary battery21. The control unit1may store, e.g., the number of charging times, which is obtained by adding one to the number of charging times, which was stored in the memory device27just before, in the memory device27whenever charging is performed.

Thereafter, the control unit1of the charger10in accordance with the present embodiment calculates, e.g., the ratio (Ve−Ve1)/(Vf−Vf1) of the first voltage drop value (Ve−Ve1) indicated between the triangular symbols ofFIG. 3to the second voltage drop value (Vf−Vf1) indicated between the circular symbols ofFIG. 3.

Then, the control unit1of the charger10compares a predetermined value stored in the memory unit of the control unit1in advance with a ratio between the values of voltage drops (Ve−Ve1)/(Vf−Vf1) calculated by the control unit1. In a case, e.g., where ΔI1(i.e., I22−I21) is greater than ΔI2(i.e., I23−I22), the charger10in accordance with the embodiment determines that the secondary battery21has deteriorated if the calculated ratio between the values of voltage drops (Ve−Ve1)/(Vf−Vf1) is equal to or larger than the predetermined value stored in advance. If the charger10in accordance with the present embodiment determines that the secondary battery21has deteriorated, the control unit1controls the notification unit6so that the notification unit6provides notification of the determination result to the outside.

In the charger10in accordance with the present embodiment, the control unit1calculates a plurality of voltage drop values and a ratio between voltage drop values, and determines whether or not the secondary battery21has deteriorated based on the ratio between the voltage drop values. Since the charger10in accordance with the present embodiment determines whether or not the secondary battery21has deteriorated by using the ratio between the voltage drop values, the size of the deviation in the voltage drop values of secondary batteries21of respective battery packs is reduced, and thus the accuracy of the determining whether the secondary battery21has deteriorated can be increased.

Further, although the charger10in accordance with the present embodiment defines “Ve−Ve1” as the first voltage drop value and “Vf−Vf1” as the second voltage drop value, the first and second voltage drop values are not limited thereto. Accordingly, the charger10may define, e.g., “Vf−Vf1” as the first voltage drop value and voltage drop value “Vg−Vg1”, which is indicated between the square symbols inFIG. 3and is calculated in the same manner as the first voltage drop value, as the second voltage drop value. The charger10may also define “Ve−Ve1” as the first voltage drop value, “Vf−Vf1” as the second voltage drop value, and “Vg−Vg1” as the third voltage drop value. In this case, the charger10may determine that the secondary battery21has deteriorated, e.g., if the average of the first voltage drop value (Ve−Ve1), the second voltage drop value (Vf−Vf1) and the third voltage drop value (Vg−Vg1) is equal to or larger than the predetermined value which has been previously stored in the memory unit of the control unit1.

Further, the ratio between the second the second voltage drop value (Vf−Vf1) and the third voltage drop value (Vg−Vg1) may be compared with a predetermined value to determine the deterioration of the secondary battery21as well as the ratio between first voltage drop value (Ve−Ve1) and the second voltage drop value (Vf−Vf1). Furthermore, the average of two drop voltage values, e.g., first voltage drop value (Ve−Ve1) and the second voltage drop value (Vf−Vf1), may be compared with a predetermined value to determine the deterioration of the secondary battery21.

If it determines that the secondary battery21has deteriorated, the charger10may control the control unit1so that the control unit1stops the charging of the secondary battery21or continues.

If the charger10continues the charging of the secondary battery21, it performs constant voltage charging in which the charging current flows into the secondary battery21at constant voltage during the period from time t25, i.e., just after the constant current charging, to time t26. During the constant voltage charging, the control unit controls the output circuit unit3to apply a constant voltage, and thus, the charging current is decreased. The charger10determines that the secondary battery21has been fully charged based on the signal from the charging current monitoring unit29and stops the charging of the secondary battery21.

In the present embodiment, the memory device27may further store information on the type of the secondary battery21and transmits the information to the control unit1. The control unit1may set the voltages Ve, Vf and Vg, the decreasing values ΔI1, ΔI2and ΔI3of the constant currents and the like according to the received information on the type of the secondary battery21.

Accordingly, the charger10in accordance with the present embodiment can more accurately determine whether or not the secondary battery21has deteriorated regardless of the type of secondary battery21or the remaining capacity of the secondary battery21before the initiation of charging.

Further, the battery capacity of the secondary battery21which has been determined to have deteriorated is lower than that of the secondary battery21in an initial phase. For this reason, the charger10in accordance with the present embodiment provides notification of whether or not the secondary battery21has a battery capacity that can operate an electric power tool, e.g., for a specific time period, to a user by providing notification of the deterioration of the secondary battery21by the notification unit6. That is, since the charger10is provided with the notification unit6, the charger10may provide notification of the deterioration of the secondary battery21to a user or the like, and a user may predict the working time by using an electric power tool including the battery pack20.

Second Embodiment

Although the charger10in accordance with the present embodiment uses the voltage drop values during the constant current charging in the first embodiment without change, there is difference in that the charger10in accordance with the present embodiment determines whether or not the secondary battery21has deteriorated by excluding the deviation in the voltage drop values of the secondary batteries21of respective battery packs20in the initial phase in which the number of charging times of the secondary battery is smaller than a specific number.

Further, the charger10in accordance with the present embodiment and a battery pack20connected to the charger10have the same circuit configuration as the charger10and battery pack20in accordance with the first embodiment shown inFIG. 1. The charger10in accordance with the present embodiment is different from the charger10of the first embodiment in a control program of the control unit1that performs the calculation of a voltage drop value or the like. Herein, the same reference numerals are used to designate elements that are the same as those of the first embodiment, and descriptions thereof will be appropriately omitted.

A memory device27is provided in the battery pack20that has a secondary battery21and is charged by the charger10in accordance with the present embodiment. The control unit1stores, in the memory device27, the number of charging times of the secondary battery21and the voltage drop value of the secondary battery21to be corresponded to the number of charging times. The control unit1of the charger10compares the voltage drop value in the initial phase (e.g., average voltage drop value in the initial phase) where the number of charging times is smaller than a specific number (e.g., 5), with voltage drop value in a phase in which the number of charging times of the secondary battery21is greater than that in the initial phase. Accordingly, the charger10in accordance with the present embodiment may exclude the deviation in the voltage drop values of the secondary batteries21of respective battery packs20in the initial phase.

The operation of the charger10in accordance with the present embodiment will be described below.

In the charger10in accordance with the present embodiment, the terminal25of the battery pack20that is electrically connected to the memory device27is connected to the terminal15of the charger10that is electrically connected to the control unit1during the charging of the secondary battery21.

The battery pack20which is connected to the charger10stores a voltage drop value corresponding to the charging of the battery pack20that is performed before the shipment from a factory as the first charging. Further, the charger in accordance with the present embodiment stores the numbers of charging times of the secondary battery21and voltage drop value corresponding to each number of charging times in the memory device27of the connected battery pack20, as illustrated, e.g., in Table 1:

The charger10in accordance with the present embodiment may apply, e.g., a correction coefficient according to the difference between a predetermined reference value and a voltage drop value in an initial phase, to the voltage drop value calculated in the same manner as in the first embodiment so that the deviation in the voltage drop values of the secondary batteries21of respective battery packs20in the initial phase can be reduced. Further, the charger10may add, e.g., the difference between the predetermined reference value (VR) and the voltage drop value in an initial phase (Va1i) to a voltage drop value after the initial phase (Va2i) (i.e., Va2i+(VR−Va1i)) so that the deviation in the voltage drop values in the initial phase of the secondary batteries21of respective battery packs20can be reduced.

The charger10determines that the secondary battery21has deteriorated if the voltage drop value from which the deviation in the voltage drop values of the secondary batteries21of respective battery packs20in the initial phase has been excluded is equal to or greater than a preset value.

Moreover, the charger10may adjust the predetermined value (Th), which is used to determine the deterioration of the secondary battery21, with respect to each battery pack20by considering the difference between the predetermined reference value (VR) and the voltage drop value in an initial phase (Va1i) (i.e., Th=Th−(VR−Va1i)) so that the deviation in the voltage drop values of the secondary batteries21of respective battery packs20in the initial phase can be reduced.

In the above, the case, in which the correction coefficient is applied to one voltage drop value to reduce deviation in the voltage drop values in the initial phase and the resultant value is compared with the preset value to determine whether or not the secondary battery has been deteriorated, is described, it is not limited thereto. That is, a plurality of voltage drop values after the initial phase (e.g., the first and the second voltage drop values in the first embodiment) are compared with the voltage drop values in the initial phase (e.g., the first and the second voltage drop values in the initial phase) to exclude the deviation in the voltage drop values of the secondary battery21of respective battery packs20in the initial phase. Then, the ratio between or the average of the plurality of the voltage drop values are used to determine the deterioration of the secondary battery21.

The control unit1of the charger10in accordance with the present embodiment determines whether the secondary battery21has deteriorated by comparing the a voltage drop value in an initial phase in which the number of charging times of the secondary battery21is equal to or smaller than a predetermined value stored in the memory device27, with the value of a voltage drop in a phase in which the number of charging times of the secondary battery21is greater than that in the initial phase.

With this, in the charger10in accordance with the present invention, the memory device27is provided in each battery pack20and the voltage drop value corresponding to the number of charging times of the secondary battery21is detected, thereby excluding the deviation between secondary batteries21, e.g., the difference between a secondary battery21whose internal resistance is higher than that in an initial phase and a secondary battery21whose internal resistance is lower than that in the initial phase. That is, the charger10in accordance with the present embodiment can exclude the deviation between the voltage drop values of the secondary batteries21of respective battery packs20in an initial phase, and can further increase the accuracy of the determination of whether or not the secondary battery21has deteriorated.