JPH05336679A - Charging circuit for secondary battery - Google Patents

Abstract

PURPOSE:To provide a charging circuit for secondary battery in which battery life is not damaged by the effect of electric noise. CONSTITUTION:The charging circuit for secondary battery comprises a power supply 1 for charging a secondary battery 3, a transistor Q1 inserted between the charging power supply 1 and the battery 3 in order to control charging current of the battery 3, a comparator 4 for comparing the terminal voltage of the battery 3 with a voltage reference Vref, and circuits 5-8 for controlling the resistance of the transistor Q1 based on the comparison results so that a constant charging current is fed during an interval when the terminal voltage of the battery 3 is lower than the voltage reference Vref and subsequently the charging current is lowered gradually stepwise every time when the terminal voltage reaches the voltage reference Vref.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging circuit for a secondary battery, and more particularly to a charging circuit that regulates the maximum terminal voltage of the secondary battery for charging.

[0002]

2. Description of the Related Art There are various conventional rechargeable battery charging circuits. Particularly, a method of charging a rechargeable battery by defining a maximum terminal voltage of the rechargeable battery within a predetermined range is disclosed in, for example, Japanese Patent Laid-Open No. 2-60073. Have been described. In this conventional charging method, charging is performed with a constant current until the terminal voltage of the battery reaches the maximum voltage, and then the charging current is continuously reduced so that the terminal voltage maintains the maximum voltage, so-called constant voltage charging. It is carried out.

[0003]

However, in the above-mentioned conventional charging method, when electric noise is mixed into the control system in the constant voltage charging state, the terminal voltage of the battery is erroneously detected, resulting in an excessive charging current. There is a possibility that it may flow, which may shorten the battery life.

The present invention has been made in order to solve such conventional problems, and an object thereof is to provide a charging circuit for a secondary battery which does not impair the battery life due to the influence of electrical noise. And

[0005]

In order to solve the above-mentioned problems, a charging circuit of the present invention is provided between a charging power source for charging a secondary battery and the secondary battery and the charging power source. A current control element for controlling a charging current supplied to the secondary battery, a comparison means for comparing a terminal voltage and a set value of the secondary battery, and based on a comparison result of the comparison means,
After the start of charging, the charging current becomes a constant current during the period when the terminal voltage of the secondary battery is less than the set value, and thereafter, the charging current gradually decreases each time the terminal voltage of the secondary battery reaches the set value. And a control means for controlling the current control element.

[0006]

As described above, according to the present invention, constant current charging is performed while the terminal voltage of the secondary battery is less than the set value, and after reaching the set value, the charging current is gradually reduced regardless of the terminal voltage. Therefore, even if electrical noise enters the control system,
Unlike the conventional constant voltage charging, an excessive charging current does not flow.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a charging circuit for a secondary battery according to an embodiment of the present invention. In the figure, one end of the charging power source 1 is connected to the input terminal IN of the control circuit 2, and the other end is grounded. The output terminal OUT and the control terminal C of the control circuit 2 are each a secondary battery (for example, a lithium secondary battery, hereinafter,
The battery 3 is connected to the positive terminal of the battery 3, and the negative terminal of the battery 3 is grounded. As the charging power source 1, a power source that rectifies the output of an AC power source to obtain a direct current, or another battery having a relatively large capacity is used.

When the terminal voltage of the battery 3 applied to the control terminal C is lower than the reference voltage Vref, the control circuit 2 charges the battery 3 with a preset relatively large constant current I1, and the battery 3 Of the reference voltage Vre
When it reaches f, charging is performed with a current I2 smaller than I1.
After this, the terminal voltage of the battery 3 rises again, and the reference voltage V
When it reaches ref, charging is performed with a current smaller than I2, and the same operation is repeated thereafter. That is, the control circuit 2 determines that the terminal voltage of the battery 3 is equal to the reference voltage Vref after the charging is started.
The constant-current charging is performed until the voltage reaches V.sub., But thereafter, the charging current is gradually reduced each time the terminal voltage of the battery 3 reaches the reference voltage Vref.

The control circuit 2 is specifically constructed as follows. First, the control terminal C of the control circuit 2 is connected to the non-inverting input terminal of the first voltage comparator 4. The reference voltage Vref is applied to the inverting input terminal of the voltage comparator 4. The output terminal of the voltage comparator 4 is connected to the clock terminal CK of the down counter 5. The down counter 5 has, for example, a 2-bit output, outputs the maximum digital value when a start pulse is input to the reset terminal R, and thereafter outputs an output every time a pulse is input from the voltage comparator 4 to the clock terminal CK. The digital value decreases.
However, when the output digital value of the down counter 5 becomes 0, the state does not change thereafter unless a start pulse is applied to the reset terminal R.

The output of the down counter 5 is input to the D / A converter 6. The D / A converter 6 generates an analog voltage output VA corresponding to the output digital value of the down counter 5. Output V of this D / A converter 6
A is connected to the inverting input terminal of the second voltage comparator 7. The output terminal of the voltage comparator 7 is connected to the base of a transistor Q1 which is a current control element via a resistor R1. The emitter of the transistor Q1 is connected to the input terminal IN of the control circuit 2, and the collector is connected to the output terminal OUT of the control circuit 2 via the resistor R2. Resistance R
One ends of resistors R3 and R5 are connected to both ends of 2, respectively, and the other ends of the resistors R3 and R5 are connected to a non-inverting input terminal and an inverting input terminal of the operational amplifier 8, respectively. The non-inverting input terminal of the operational amplifier 8 is further grounded via the resistor R4, and the inverting input terminal is further connected to the output terminal via the resistor R6. The output terminal of the operational amplifier 8 is connected to the non-inverting input terminal of the second voltage comparator 7.

Here, the resistors R2, R3, R4, R5, R
If the resistance values of 6 are r2, r3, r4, r5, and r6, the voltage across the resistor R2 is Vr, the charging current flowing through the battery 3 is I, and the output voltage of the operational amplifier 8 is VB, then I = Vr / R2 (1) VB = Vr × r4 / r3 (2) In this case, the transistor Q1 and the resistors R1 to R
By the action of the feedback control system constituted by 6, the operational amplifier 8 and the voltage comparator 7, VB = VA, that is, the output voltage VB of the operational amplifier 8 becomes equal to the output voltage VA of the D / A converter 6. The output value of the comparator 7 controls the resistance value of the transistor Q1. As a result, the charging current I is

I = VA × r3 / r2 / r4 (3) The charging current I is proportional to the output voltage VA of the D / A converter 6. Since the output of the down counter 5 is 2 bits, the value of VA changes in four steps.

Next, the operation of the charging circuit of FIG. 1 will be described with reference to the waveform diagram of FIG. 2 showing its charging characteristic. In FIG. 2, (a) shows the terminal voltage V1 of the battery 3, (b) shows the charging current I, (c) shows the output VA of the D / A converter 6, and (e) shows the start pulse.

When a start pulse generated at power-on or in conjunction with a switch or the like is input to the reset terminal R of the down counter 5 of the control circuit 2, the output digital value of the down counter 5 becomes maximum, so that D / A converter 6
Output VA becomes the maximum voltage VA1, the charging current I becomes the maximum current I1 corresponding to VA1, and constant current charging is performed with this current I1. Here, I1 is usually set to about 1 CmA.

As the charging with the current I1 progresses, the battery 3
Of the reference voltage Vref (for example, V
When ref = 4.1 V) is reached, the first pulse is generated at the output of the voltage comparator 4. As a result, the output digital value of the down counter 5 decreases by one step, the output VA of the D / A converter 6 becomes VA2, and the charging current I becomes a current IA2 corresponding to VA2, which is one step smaller than I1. Therefore, the terminal voltage V1 of the battery 3 temporarily drops.

When the charging with the current I2 progresses and the terminal voltage V1 of the battery 3 starts to rise and reaches the reference voltage Vref again, a second pulse is generated at the output of the voltage comparator 4. As a result, the output digital value of the down counter 5 is reduced by one step, and the output VA of the D / A converter 6 is reduced.
Becomes VA3, and the charging current I becomes a current IA3 corresponding to VA3, which is one step smaller than I2. Therefore, the voltage V1 of the battery 3 temporarily decreases again, but then starts increasing again.

Thereafter, in the same manner, by repeating the operation of gradually decreasing the charging current each time the terminal voltage V1 of the battery 3 reaches the reference voltage Vref, the charging current finally becomes 0 and the charging operation is performed. finish.

As described above, after the terminal voltage V1 of the battery 3 once reaches the reference voltage Vref, the charging current is simply reduced stepwise, so that electrical noise is mixed in the detection system of the control circuit 2, for example. However, an excessive charging current does not flow, and the life of the battery 3 can be extended. The present invention is not limited to the above embodiments, but can be implemented with various modifications as follows.

(1) In the embodiment, the charging current is changed in four steps including the current value of 0.
Alternatively, the number of steps may be 5 or more. If the number of stages is reduced, the charging circuit can be made inexpensive, and if the number of stages is increased, charging can be performed in a shorter time.

(2) The rate of decrease when the charging current is reduced stepwise may be a geometric series, an arithmetic series, or another method. In terms of geometric progression, down counter and D
The / A converter can be made relatively inexpensive.

(3) In the embodiment, the case of charging one battery is shown, but it is also possible to connect a plurality (n) of batteries in series and charge those batteries at the same time. In that case, the same control can be performed by multiplying the reference voltage Vref by n times. Alternatively, the terminal voltage of each of the n batteries may be measured and the maximum voltage may be input to the control terminal C of the control circuit 2. In the former case, since it is only necessary to change the setting of the reference voltage Vref, the charging circuit can be made inexpensive,
In the latter case, a circuit for measuring the terminal voltage of n batteries is required, but the life of the batteries is not impaired.

(4) In the embodiment, when the control circuit 2 controls the charging current, the resistance value of the transistor Q1 is changed to convert the extra power into heat. The current may be controlled. By doing so, collector loss in the transistor is reduced and heat generation can be reduced.

(5) In the embodiment, the charging control is performed only on the basis of the terminal voltage of the battery. However, timer control may be combined, or temperature control for detecting a chargeable temperature range may be combined.

(6) In the embodiment, a lithium secondary battery is used as the secondary battery, but the charging circuit of the present invention can also be applied to the case where another secondary battery such as a lead storage battery is used. Is.

(7) In the embodiment, the control circuit 2 is composed entirely of hardware. However, for example, the down counter 5, D /
A part or all of the portions corresponding to the A converter 6 and the voltage comparators 4 and 7 may be processed by a program using a microcomputer or the like and implemented by software.

(8) In the embodiment, the minimum value of the charging current is 0.
However, instead of setting the minimum value to 0, a certain minute current may be passed. By doing so, charging can be performed to a state closer to full charge, and the capacity is not reduced due to self-discharge.

(9) In the embodiment, the output of the voltage comparator 4 is directly input to the down counter 5, but it may be input via a waveform shaper. In this way, the operation becomes more reliable. Besides, the present invention can be variously modified and implemented without departing from the scope of the invention.

[0028]

As described above, according to the present invention, when the terminal voltage of the secondary battery is less than the set value, constant current charging is performed with a relatively large current, and thereafter, every time the terminal voltage reaches the set value. By gradually decreasing the charging current, it is possible to prevent an excessive charging current from flowing due to the influence of electrical noise, and to extend the battery life.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram for explaining the operation of the charging circuit of FIG.

[Explanation of symbols]

1 ... Charging power source 2 ... Control circuit 3 ... Secondary battery 4 ... Voltage comparator 5 ... Down counter 6 ... D / A converter 7 ... Voltage comparator 8 ... Operational amplifier

Claims (1)

[Claims] 1. A charging power supply for charging a secondary battery, and a current control element provided between the secondary battery and the charging power supply for controlling a charging current supplied to the secondary battery. , Comparing means for comparing the terminal voltage of the secondary battery and a set value, based on the comparison result of the comparing means, the charging current is during the period when the terminal voltage of the secondary battery is less than the set value after charging is started. A constant current is provided, and thereafter, each time the terminal voltage of the secondary battery reaches a set value, the charging current is gradually reduced in a stepwise manner. Secondary battery charging circuit.