JP2000092737A - Device for charging secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device for charting a secondary battery that can safely charge the secondary battery without overcharging by suppressing the heat generation of a charge power supply, and at the same time by miniaturizing the charge power supply. SOLUTION: A device is provided with a voltage measurement circuit l3 that detects a battery voltage Vb of a secondary battery BAT, a switch element 11 that is fitted between the secondary battery and the charge power supply, and a charge control circuit 14 that performs the on/off control of the switch element according to the battery voltage being detected by the voltage measurement circuit and pulsively charges the secondary battery. When the battery voltage exceeds a first setting voltage V1, pulse charge is made. On the other hand, when the battery voltage in the case of off state reaches a second setting voltage V2, the pulse charge is stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery charging apparatus capable of suppressing heat generation of a charging power supply during charging and reducing the size.

[0002]

[Related Background Art] Non-aqueous solvent secondary batteries such as lithium-ion batteries and lead-acid batteries, if the battery voltage becomes too high during charging, battery performance will be significantly degraded or safety will be impaired. And the like. Therefore, in the related art, constant-voltage charging is performed so that the charging voltage (battery voltage) does not exceed a certain voltage. Specifically, when the battery voltage is low, the secondary battery is charged with a constant current (for example, 1 CmA), and the battery voltage of the secondary battery becomes a predetermined voltage value (for example, 4.2 V per battery cell).
When the voltage rises to a certain level, the battery is charged at a constant voltage while reducing the charging current. Then, when the charging current gradually decreases, for example, to about 0.05 CmA, this is regarded as a fully charged state, and charging is stopped, so that an excessive battery voltage is not generated in the secondary battery. I have to.

[0003]

However, when a charging power supply for realizing such constant current / constant voltage charging is constituted by a linear circuit (analog circuit), it is unavoidable that the amount of heat generated increases. It is also difficult to reduce the size. Incidentally, it is conceivable to suppress the heat generation by constructing the charging power supply with a switching circuit. However, from the viewpoint of miniaturization, the effect cannot be expected so much.

The present invention has been made in view of such circumstances, and an object thereof is to suppress the heat generation of a charging power supply used for charging a secondary battery and to reduce the size thereof. In addition, it is an object of the present invention to provide a secondary battery charger capable of safely charging a secondary battery without overcharging the secondary battery.

[0005]

In order to achieve the above object, a secondary battery charging apparatus according to the present invention comprises: a voltage measuring means for detecting a battery voltage as a terminal voltage of the secondary battery;
A switch element interposed between the secondary battery and its charging power supply for controlling charging of the secondary battery, and turning on / off the switch element according to a battery voltage detected by the voltage measuring means. Charge control means for performing pulse charging of the secondary battery by controlling the charging and discharging of the secondary battery by intermittently controlling the charging of the secondary battery by the switch element. It is characterized in that it is designed to be extremely compact.

In a preferred aspect of the present invention, in the charging control means, when the battery voltage Vb of the secondary battery exceeds a first set voltage V1, the switching element is turned on. It is characterized in that on / off control is performed at a constant cycle. That is, at the initial stage of the start of charging of the secondary battery, for example, the switch element is continuously turned on to charge the secondary battery with a constant current, and the battery voltage Vb of the secondary battery increases with the constant current charging. Is characterized in that the switching element is turned on and off at a constant cycle from the time when the voltage exceeds the first set voltage V1, and the secondary battery is pulse-charged.

In a preferred aspect of the present invention, when the secondary battery is pulse-charged as described in claim 3,
When the battery voltage Vb after a predetermined time from the start of the OFF operation of the switch element or the battery voltage Vb immediately before the ON operation of the switch element exceeds the second set voltage V2, the charge control means Is maintained in an off-operation state. Furthermore, as described in claim 4, the battery voltage Vb of the secondary battery
Has a means for turning off the switch element when the voltage reaches a third set voltage V3 which is an allowable maximum charging voltage of the secondary battery.

That is, when the secondary battery is pulse-charged,
When the battery voltage Vb at the time of the OFF operation of the switch element is the second
When the battery voltage Vb exceeds the third set voltage V3 or when the switch element is turned on.
When the threshold value is reached, the switch element is turned off and kept in the off operation state, whereby charging of the secondary battery via the switch element itself is stopped, thereby preventing overcharge of the secondary battery. And

According to a preferred aspect of the present invention, when the secondary battery is configured by connecting a plurality of battery cells in series as described in claim 5, the battery of each battery cell is included in the voltage measuring means. By detecting the voltage and controlling the charging of the secondary battery by detecting the maximum value among them, even if the charging characteristics of a plurality of battery cells vary, the maximum charging voltage among them Is monitored to prevent a specific battery cell from being overcharged.

According to the present invention, in the charging control means, when the load is driven by the charging power source, the charging of the secondary battery via the switching element is stopped. As described in 7, when the load is driven by the charging power source, and when the charging current of the secondary battery through the switching element is equal to or more than a predetermined value, the switching element is prohibited from being turned on, thereby turning off the charging power source. Even when the capacity is limited, load fluctuation of the charging power supply due to charging of the secondary battery is prevented, and stable power is supplied to the load.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A charging device for a secondary battery according to an embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic block diagram showing a secondary battery charging device according to an embodiment of the present invention. BAT is a secondary battery such as a lithium ion battery, and PS is used for charging the secondary battery BAT. Charging power source. The charging power supply PS may be configured as a so-called series control linear circuit that outputs a sufficiently high voltage capable of charging the secondary battery BAT and a limited current, and is controlled by switching control. A so-called switching regulator that outputs a constant voltage may be used. Further, a simple configuration such as a so-called AC adapter that rectifies and smoothes an AC power supply and outputs the rectified AC power may be used.

The secondary battery BAT may be provided as a single battery cell, but a plurality of battery cells are connected in series, and a plurality of battery modules composed of battery cells connected in series are connected. In some cases, they are connected in parallel. Incidentally, in the embodiment shown in FIG. 1, the secondary battery BAT has three battery cells (lithium ion batteries) B.
1, B2 and B3 are connected in series.

In this embodiment, the secondary battery BAT is connected to the charging power supply PS via a connector C and is configured to be charged by the charging power supply PS. That is, the battery pack 10 mainly composed of the secondary battery BAT and the power supply unit 20 mainly composed of the charging power supply PS are mutually connected via the plurality of connection terminals c1, c2, c3, c4 of the connector C. Connected to. In particular, the positive electrode (+) and the negative electrode (-) of the secondary battery BAT on the battery pack 10 side are connected to the positive (+) and negative electrode sides of the charging power supply PS on the power supply section 20 via the connection terminals c1 and c4 of the connector C. (-) To form a charging power supply line. The connection terminals c2 and c3 of the connector C are used as control signal input terminals in the battery pack 10.

Thus, the switching element 11 is interposed in series between the power supply line on the positive electrode (+) side of the battery pack 10, that is, between the positive electrode (+) of the secondary battery BAT and the connection terminal c1 of the connector C. And the secondary battery BA
T is charged via the switch element 11. Incidentally, the switch element 11 is made of, for example, a p-channel FET (field effect transistor), and charges the secondary battery BAT by conduction (ON operation) thereof.
In addition, the charging is prohibited by shutting off (off operation), and is subjected to pulse charging of the rechargeable battery BAT by being turned on / off as described later.

In this example, a switch element (FET) 12 for controlling discharge is interposed in series with the switch element 11. However, the current control direction is the switching element 11
Is set in the opposite direction. The operation of the switch element (FET) 12 is controlled by a protection circuit 16 described later, and is used for controlling discharge inhibition of the secondary battery BAT.

In the battery pack 10, the secondary battery BAT is located between both ends (a positive electrode and a negative electrode) of the secondary battery BAT.
A battery voltage measuring circuit 13 for detecting the battery voltage (terminal voltage) Vb of the first embodiment is provided. The charge control circuit 14 basically controls on / off (conduction / interruption) of the switch element 11 in accordance with the battery voltage Vb detected by the battery voltage measurement circuit 13, and particularly under a predetermined condition. The switch element 11 is controlled to be turned on / off at a predetermined cycle, thereby performing a role of pulse-charging the secondary battery BAT. The operation of the charge control circuit 14 will be described later.

The control output of the charge control circuit 14 is
Switch element 11 via logic circuit (gate circuit) 15
(Gate electrode of FET). Only when the logic circuit 15 receives the output of the protection circuit 16 to be described later and is permitted to charge, and only when the logic gate is opened, the control output of the charge control circuit 14 is applied to the switch element 11 and Element 11 is ON
It is driven off. When the logic circuit 15 is controlled to prohibit charging under the control of the protection circuit 16 and its logic gate is closed, the switch element 11 is forcibly shut off regardless of the control output of the charge control circuit 14. (OFF operation).

Incidentally, the protection circuit 16 detects the battery voltages Vb1, Vb2, Vb3 of the battery cells B1, B2, B3 constituting the secondary battery BAT via the nodes P1, P2, P3, P4 respectively. are doing. The protection circuit 15 basically includes the battery voltages Vb1, Vb of the battery cells B1, B2, B3.
When either of Vb3 and Vb3 rises to the charge prohibition voltage Voc, a charge prohibition signal (L signal) is output to the logic circuit 15 to forcibly cut off the switch element 11 as described above ( Off operation). In other words, when the battery voltages Vb1, Vb2, Vb3 of the respective battery cells B1, B2, B3 do not reach the charging inhibition voltage Voc, the logic circuit 15 is controlled to be cut off by the protection circuit 16. And the charging is permitted.
1 is turned on / off in accordance with a control output provided from the charge control circuit 13. The charging current supplied from the charging power supply PS changes from the switching element 11 which is turned on / off as described above to the switching element 12 for discharging control.
Is supplied to the secondary battery BAT via the

At the same time, when one of the battery voltages Vb1, Vb2, Vb3 of each of the battery cells B1, B2, B3 drops to the discharge prohibition voltage Vud, the protection circuit 15 operates as described above. 12 for a discharge inhibition signal (L
Signal) to forcibly shut off the switch element 12. In other words, the switch element 12 is connected to each of the battery cells B1, B
Only when the battery voltages Vb1, Vb2, and Vb3 of B3 are higher than the discharge prohibition voltage Vud, they are constantly driven (turned on). The discharge current from the secondary battery BAT is on / off controlled via the switch element 12 and discharged via the charge control switch element 11 described above.

On the other hand, in the power supply section 20, a predetermined load RL is connected to the charging power supply PS via a switch 21 in parallel. The load RL receives power supply from the charging power supply PS when the charging power supply PS is connected, and receives power supply from the secondary battery BAT via the connector C when the charging power supply PS is disconnected. It operates by receiving. The switch 21 controls on / off of the power supply to the load RL, and has a function as a power switch for the load RL. In particular, in the power supply section 20, the switch 21
Is provided, and the floating charge detection circuit 22 outputs a charge prohibition signal (H signal) to the secondary battery BAT when the switch 21 is conducting (ON state). Output.

The charge prohibition signal output from the floating charge detection circuit 22 is connected to the connection terminal c2 of the connector C.
Via the charge control circuit 14 in the battery pack 10
As a first external control signal CONT1. The connection terminal c3 of the connector C is connected to the negative (-) side of the power supply unit 20, and a signal applied to the connection terminal c3 is given as a second external control signal CONT2 of the charge control circuit 14.

The charge control circuit 14 will now be described in more detail. The charge control circuit 14 is configured as shown in FIG. 2, for example. That is, the charge control circuit 1
Reference numeral 4 denotes a battery voltage Vb detected by the voltage measuring circuit 13, and the first to third set voltages V1, V2,... Which are set in advance according to the battery characteristics of the secondary battery BAT.
There are provided three comparators 31, 32 and 33 for comparing with V3 (V1 ≦ V2 <V3). These comparators 31, 3
Reference numerals 2 and 33 serve to control the operation mode of the charge control circuit 14 according to the level of the battery voltage Vb. In particular, the first comparator 31 sets the battery voltage Vb higher than the first set voltage V1. In this case, it plays a role of outputting a pulse signal having a predetermined duty ratio and a predetermined duty ratio via the gate circuit 35 at a predetermined cycle generated by the pulse generator (PG) 34. The pulse signal output via the gate circuit 35 is used as a control output for controlling the switching element 11 to be turned on / off at a predetermined cycle via the output gate circuit 36.

The second comparator 32 controls the ON / OFF state of the switch element 11 in accordance with the pulse signal, particularly after a predetermined time has elapsed after the switch element 11 has been turned off, or Immediately before the detection, it is for detecting whether or not the battery voltage Vb is higher than the second set voltage V2. The output of the second comparator 32 for comparing the battery voltage Vb with the second set voltage V2 is supplied to the flip-flop 37.

The flip-flop 37 delays the output of the gate circuit 35 by a predetermined time via a delay line 38, and uses the trailing edge of the inverted signal via an inverter 39 as a clock CK to input its input D. This causes the output of the second comparator 32 at the above timing to be set in the flip-flop 37. Specifically, when the battery voltage Vb is higher than the second set voltage V2 after a predetermined time has elapsed after the switch element 11 has been turned off or immediately before the switch element 11 is turned on, the flip-flop 37 is set to H
The level signal is set. This flip-flop 3
The output Q of 7 is output to the output gate circuit 36 via an inverter 40 as a signal for controlling the operation of the switch element 11. In particular, the flip-flop 37
When the level signal is set, the switch element 11 is forcibly turned off via the output gate circuit 36. Therefore, the output of the flip-flop 37 is kept at the L level until the battery voltage Vb at the timing reaches the second set voltage V2, and in this state, the switch element 11 is forcibly turned off. Never.

On the other hand, the third comparator 33 determines whether or not the battery voltage Vb has reached the maximum allowable charging voltage (third set voltage) V3 of the secondary battery BAT. Then, when the voltage reaches the third set voltage V3, an H level charge inhibition signal is output. This charge inhibition signal (H signal)
Is supplied to the output gate circuit 36 via an inverter 41, whereby the switch element 11 is forcibly turned off.

The charging control circuit 14 is further provided with a first inverter 42 for inputting a charging inhibition signal provided from the connection terminal c2 of the connector C as an external control signal CONT1 and a connection inhibition signal provided from the connection terminal c3 of the connector C. And a second inverter 43 for inputting an external control signal CONT2 to be applied. The first inverter 42 has an external control signal
When the charge prohibition signal (H signal) is not applied from the floating charge detection circuit 22 as the CONT1, that is, on the condition that the external control signal is kept at the L level, the output is set to the H level, and this is set to the output gate. Circuit 3
6 has a role of permitting the operation of the charge control circuit 14. The input terminal of the second inverter 43 is pulled up via a resistor 44, and is connected to a connection terminal c3.
When the L-level external control signal CONT2 is supplied to the power supply line on the negative side through the, the output thereof is set to the H level. The output of the H level is applied to the output gate circuit 36, thereby serving to permit the operation of the charge control circuit 14.

The above-described output gate circuit 36 is gate-controlled by receiving the output of each of the inverters 40, 41, 42, 43 described above.
The output from the gate circuit 35 is output as a drive control signal for the switch element 11 on condition that the outputs of the switches 3 are kept at the H level. When the output of the first comparator 31 is at the L level, that is, when the battery voltage Vb of the secondary battery BAT has not risen to the first set voltage V1, the gate circuit 35 generates a pulse. Irrespective of the output (pulse signal) of the detector 34, its output is maintained at the H level. As a result, in this state, a signal (control output) for continuously turning on the H-level switch element 11 is output via the output gate circuit 36, and the secondary battery B is output via the switch element 11.
The AT is charged continuously.

When the battery voltage Vb increases with the continuous charging of the secondary battery BAT, and the battery voltage Vb exceeds the above-described first set voltage V1, the first comparator 3
1 changes to the H level, the output of the gate circuit 35 inverts at a constant period in response to the pulse signal given from the pulse generator 34, thereby switching the output signal through the output gate circuit 36 as described above. A signal for turning on / off the element 11 is output.

Thus, under the charge control circuit 14 configured as described above, the switching element 1 is switched in accordance with the battery voltage Vb.
According to the charging device in which the operation of the battery 1 is controlled and the charging of the secondary battery BAT is controlled, the charging proceeds, for example, as shown in FIG. That is, when charging the rechargeable battery BAT, first, as a battery test for checking the function of the rechargeable battery BAT, a small current I1 is supplied to the rechargeable battery BAT to perform initial charge, and the battery voltage Vb is increased by the initial charge. Confirm whether to do. The battery test by the initial charge is performed for a predetermined short time, and it is determined whether the battery voltage Vb reaches the initial voltage Vo. At this time, if a rise in the battery voltage Vb is not expected, it is determined that the battery is defective and the subsequent charging process is not performed. The circuit function for the battery test is not directly shown in FIG. 1 because it has no direct relation to the technical idea of the present invention. Also, without performing such a battery test, the secondary battery BAT
Is considered to function normally, or after a battery test is performed using a test device provided separately, the secondary battery B
It is of course possible to directly start the following charging process of the AT.

The charging of the rechargeable battery BAT is performed on condition that power is not supplied from the charging power source PS to the load RL, that is, the switch 21 is turned off, and the floating charge detecting circuit 22 inhibits charging. The operation is started on condition that no signal (H signal) is output and that the external control signal CONT2 is kept at L level. At the time of such initial charging, the battery voltage Vb is sufficiently lower than the first set voltage V1, and therefore, as described above, the charge control circuit 14 continuously outputs an H-level control signal to output the switch element. 11 is turned on. Continuous ON operation (conduction) of this switch element 11
Thus, a predetermined charging current I2 is supplied from the charging power source PS to the secondary battery BAT, and the secondary battery BAT is continuously charged at a constant current. By this constant current charging, FIG.
The battery voltage Vb is gradually increased as shown in FIG.

When the battery voltage Vb rises with the above-mentioned constant current charging and, for example, when the battery voltage Vb exceeds the above-mentioned first set voltage V1 at the time t2 after the start of charging, the first battery voltage Vb rises. This state is detected by the comparator 31 and the gate circuit 35 is controlled. As a result, the charge control circuit 14 generates a control output that is inverted at a predetermined cycle based on the output of the pulse oscillator 34, and the switch element 11
Are turned on and off at a predetermined cycle. When the switch element 11 is turned on and off, the rechargeable battery BAT is
As shown in the figure, the battery is intermittently charged, that is, pulse-charged.

At the time of this pulse charging, the secondary battery B
The battery voltage Vb of the AT is increased in accordance with the supply (charging) of the charging current, and when the charging is stopped, the battery voltage Vb is reduced to an open voltage according to the charging state at that time. However, as the pulse charging continues, the battery voltage Vb gradually increases as a whole. Moreover, at this time, the charging power source PS is
Since the charging current is only supplied intermittently to the AT,
It is possible to stably supply a current without generating much heat.

In the pulse charging process, especially when the charging of the secondary battery BAT is suspended by turning off the switch element 11, the battery voltage Vb of the secondary battery BAT is increased as described above. It is determined whether the voltage exceeds the second set voltage V2. When the battery voltage Vb exceeds the second set voltage V2, a charge stop signal is issued from the flip-flop 37, and the output of the charge control circuit 14 is set to L level via the output gate circuit 36. The switch element 11 is set to the off state by the output from the charge control circuit 14, and the off state is maintained to stop the pulse charging of the secondary battery BAT.

That is, when the battery voltage Vb at the time of the off control in the pulse charge exceeds the second set voltage V2, it is regarded that the secondary battery BAT has reached the fully charged state, and the switch element 11 is turned off. Then, the pulse charging of the secondary battery BAT is stopped. When the battery voltage Vb reaches the maximum allowable charging voltage V3 of the secondary battery BAT during the above-described pulse charging, the switch element is controlled under the control of the third comparator 33 at this time. 1
1 is forcibly turned off, and the pulse charging is stopped and controlled. By such a pulse charging stop control, the secondary battery BAT is efficiently charged to a fully charged state without being overcharged.

When the above-mentioned floating charge detection circuit 22 outputs a charge prohibition signal (H signal) at the time of the above-described pulse charge or the like, the power supply to the load RL via the switch 21 is started. Then, the control operation of the charge control circuit 14 described above is stopped and controlled. By this stop control, charging of the secondary battery BAT is stopped, and power supply from the charging power supply PS is executed only to the load RL. Therefore, even if the power supply capacity of the charging power supply PS is small, power is supplied stably only to the load RL regardless of the presence of the secondary battery BAT. Further, when power is supplied to the load RL while the secondary battery BAT is pulse-charged, a load fluctuation occurs in the charging power supply PS with the pulse charging of the secondary battery BAT, and the load fluctuation harms the load RL. Sometimes. In this regard, by suspending the charging of the secondary battery BAT when supplying power to the load RL as described above, it is possible to suppress load fluctuations of the charging power supply PS and to stabilize its operation. Then, the rechargeable battery BAT is stably performed only in a state where power is not supplied to the load RL, that is, using the operation suspension period of the load RL.

Therefore, according to the charging device configured as described above, the operation of the switch element 11 is controlled on / off in accordance with the charging voltage Vb of the secondary battery BAT, and the battery voltage Vb changes to the first set voltage V1. After exceeding, the secondary battery BAT
Is charged in a pulsed manner, so that the charging power source PS is not overloaded and the heat generation thereof is suppressed to reduce the rechargeable battery B
The AT can be charged efficiently. In addition, the switch element 11 is turned off in accordance with the battery voltage Vb at the time of pulse charging, and the pulse charging is stopped.
The AT can be charged safely without overcharging. Further, the effects of reducing the load on the charging power source PS, suppressing its heat generation, and reducing the overall size can be obtained.

In controlling the charging of the secondary battery BAT under the control of the charging control circuit 14, the above-described protection circuit 16 may perform the charging control focusing on the battery voltage of each battery cell. Needless to say. Such a function of the protection circuit 16 effectively prevents such a problem that only a specific battery cell reaches full charge quickly and only the battery cell is overcharged. Further, when actually constructing the charging device, it is desirable to have a function of detecting a drop in the battery voltage Vb and canceling the above-described prohibition of charging by the flip-flop 37.

Although a non-aqueous solvent-based secondary battery has been described as an example, the present invention is similarly applicable to a charging device for an alkaline secondary battery such as a lead storage battery, a nickel hydride secondary battery, and a nickel cadmium storage battery. Can be. Further, the switching element 11 is not on the positive electrode side of the secondary battery BAT,
It is also possible to interpose on the negative electrode side. In the embodiment, the example in which the charge control circuit 14 is configured by hardware as shown in FIG. 2 has been described. However, a part of the configuration may be processed by software using a microprocessor or the like. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0039]

As described above, according to the present invention, since the rechargeable battery is pulse-charged in accordance with the battery voltage of the rechargeable battery, the heat generated by the charging power source can be reduced, and the size of the rechargeable battery can be reduced. Can be achieved. In addition, effects such as easy and safe control of charging in the battery pack can be obtained.

Further, during the load operation, the above-described pulse charging is stopped, so that fluctuations in the voltage supplied to the load can be suppressed, and effects such as not adversely affecting the basic operation of the load can be obtained. Can be

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a charging device for a secondary battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration example of a charging control circuit in the charging device shown in FIG.

FIG. 3 is an operation timing chart showing a charging operation of a secondary battery by the charging device shown in FIG. 1;

[Explanation of symbols]

 BAT Secondary battery PS Charging power supply RL Load 10 Battery pack 20 Power supply unit 11 Switch element 13 Battery voltage measurement circuit 14 Charge control circuit 15 Logic circuit 16 Protection circuit 21 Switch 22 Floating charge detection circuit 31, 32, 33 Comparator 34 Pulse oscillator 35 gate circuit 36 output gate circuit 37 flip-flop 39, 40, 41, 42, 43 inverter C connector B1, B2, B3 battery cell

Claims (7)

[Claims] 1. A voltage measuring means for detecting a battery voltage of a secondary battery, a switch element interposed between the secondary battery and a charging power supply thereof, and a battery voltage detected by the voltage measuring means Charge control means for pulse-charging the secondary battery by controlling ON / OFF of the switch element in response to the control signal. 2. The charge control unit according to claim 1, wherein when the battery voltage of the secondary battery exceeds a first set voltage, the switch element is turned on and off at a constant cycle. 3. The charging device for a secondary battery according to claim 1. 3. The charge control unit according to claim 2, wherein, during pulse charging of the secondary battery, the battery voltage after a predetermined time from the start of the OFF operation of the switch element or the battery voltage immediately before the ON operation of the switch element is set to a second voltage. 3. The rechargeable battery charging device according to claim 1, further comprising a unit that keeps the switch element in an off-operation state when the voltage exceeds a set voltage. 4. The charging control means according to claim 1, wherein said charging control means has means for turning off said switch element when a battery voltage of said secondary battery reaches a third set voltage. A charging device for a secondary battery according to any one of the above. 5. The secondary battery has a plurality of battery cells connected in series, and the voltage measuring means detects a maximum value of a battery voltage of each battery cell and charges the secondary battery. The charging device for a secondary battery according to claim 1, further comprising a unit that controls the charging of the secondary battery. 6. The secondary battery according to claim 1, wherein the charging control unit includes a unit that stops charging of the secondary battery via the switch element when the load is driven by the charging power supply. Charging device. 7. The charging control unit prohibits an on operation of the switching element when the load is driven by the charging power supply and when a charging current of the secondary battery through the switching element is equal to or more than a predetermined value. The charging device for a secondary battery according to claim 6, wherein: