GB2278022A - Circuit for detecting charged state of a battery by sensing -DELTA V - Google Patents

Abstract

A first input pin 11 of a comparator 10 works to constantly detect the voltage of a battery 20 being charged, and the detected voltage is transmitted, via voltage reduction resistors R1, R2 (R2 >> R1), to the second input pin 12 of the comparator 10 so as to charge a memory capacitor 30. A fixed-value voltage dropping unit 50 may be in parallel connection to the voltage reduction resistors R1, R2 to keep the voltage difference between the first and second pins 11, 12 at a small value. When a charged battery is charged to saturation and the voltage thereof drops, the voltages of the first and second pins 11, 12 of the comparator 10 drop at the same time, but the voltage at the second pin 12 is affected by the memory capacitor 30 and drops at a lower speed than the first pin 11 causing the comparator output to change state and turn off the charger 40. The unit 50 may be a diode or a zener diode. The battery voltage may be applied to the detector via an amplifier (60),(Fig 3), to improve the detection sensitivity. <IMAGE>

Description

A BATTERY CHARGING SATURATION DETECTING CIRCUIT The present invention relates to a battery charge saturation detecting circuit disposed in a charger which is simple in structure and operates using the well known fact that the voltage of a saturated battery will drop when it is overcharged. This is known as - VprinC-iple whereby the voltage of a charged battery can be precisely detected to check if the same is charged-to saturation.

Generally, there are two types of method used in a prior art battery charge saturation detecting circuit: The first is to set up a voltage at a fixed value, and when a battery is charged to the fixed voltage, the normal charging current of the charger will be converted. into a trickle current for supplement purpose. In other words a very small trickle current is provided to the charged battery to keep the battery fully charged after the large-scale current is stopped. The detecting circuit used in this kind of method is relatively simple, but the result of the detection is often not accurate since charged batteries can vary considerably. Thus, charged batteries can be easily damaged by overcharging or will be inoperable if undercharged because of the inaccurate detection of the prior art detecting circuit.

Another prior art detecting circuit adapted for a charger has been developed to solve the above-mentioned problems. The second type of detecting circuit is operated on the basis that if a charged battery is charged to saturation, further electric charge will cause the voltage of the battery to drop instead of to rise; this is called - A Vprinciple.

In this method, the voltage of the battery must be recorded continuously and compared with the voltage at a preceding moment. If the result is negative, this indicates that the battery has been charged to saturation. This method is dependably accurate, but the circuit employs either a CPU and its program or a special IC. Therefore, the cost of a charger equipped with this kind of detecting circuit is too costly to be popularly accepted by common ccnsluners The present invention provides a battery charge saturation detecting circuit for use in a charger, comprising: a comparing means having a first input pin and a second input pin and an output pin; said first input pin being used to detect the voltage of a charged battery; said second input pin having a voltage just slightly lower than said first input pin; said output pin being connected to the power source of said charger for controlling the supply of power to said charger; a first voltage reduction resistor disposed between said first input pin and said second input pin; a second voltage reduction resistor having a resistance substantially larger than that of said first reduction resistor and used to work in cooperation with said first voltage reduction resistor so as to transmit the voltage of said first input pin which is accordingly reduced to said second input pin, causing the voltage of said second input pin to be just slightly lower than that of said first input pin, i.e. the voltage of said charged battery; a memory capacitor in parallel connection with said second voltage reduction resistor performing a charging and discharging operation via said reduction resistor and the recorded voltage of said memory capacitor being just slightly smaller than that of said charged battery.

The present invention seeks to provide a simple and dependable detecting circuit based on the - hV method for a charger which can precisely detect whether a battery is charged to saturation and make the charger stop when the detection is positive.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a diagram showing a first embodiment of a detecting circuit of the present invention; Fig. 2 is a diagram showing a second embodiment of a detecting circuit of the-present invention; Fig. 3 is a diagram showing a third embodiment of a detecting circuit of the present invention.

DETAILED DESCRItTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, the first embodiment of a detecting circuit of the present invention still has a problem to overcome wherein the memory capacitor 30 can not record the accurate voltage of the battery and the voltage V1 is not easily brought lower than V2 so as to - make the comparator 10 to transform in its state.

Referring to Fig. 2, the secod embodiment of a detecting circuit of the present invention comprises a comparator 10; first and second voltage reduction resistors R1 and R2; a memory capacitor 30; a fixed-value voltage drop unit 50, a grounding R3.

The comparator 10 has a first pin 11 used to detect the voltage of a battery 20 and a second pin 12 having a slightly lower voltage than the first pin 11, and a third output pin 13 coupled to a power supply 40 of the charger and used to control the on/off of the same.

The first voltage reduction resistor R1 is disposed between the first- pin 11 and the second pin 12.

The second voltage reduction resistor R2 is of much larger resistance than the resistor R2 and is connected to the second pin 12 and wrkswith resistor R1 so as to transmit the reduced voltage of the first pin 11 of the comparator 10 to the second pin 12, making the voltage of the second pin 12 slightly lower than that of the first pin 11 of the comparator 10,i. e., the voltage of the battery 20.

The memory capacitor 30 in parallel with the second voltage reduction resistor R2, make the same charge and discharge via the resistor R2 with the voltage thereof constantly slightly lower than that of the battery 20.

The fixed value voltage dropping unit 50 is in parallel with the first and second voltage reduction resistors R1, R2 for maintaining the voltage difference between the first pin 11 and the second pin 12 of the comparator 10 at a small value.

The grounding resistor R3 is connected at one end to the fixed-value voltage dropping unit 50 and the other end thereof is grounded directly.

The fixed-value voltage dropping unit 50 is in parallel with the first and second voltage reduction resistors R1, R2 as shown in Fig.l and then is grounded via the resistor R3. The fixed - value voltage dropping unit 50 is a diode such as a Zener diode and the like. In the present application, a diode is used, so that no matter what the value of the voltage V1 of the battery 20 is (as long as it is larger than the bias VD of the diode ), the voltage difference between the ends of the voltage dropping unit 50 varies only slightly - and is maintained at about VD whereby the voltage Vd between the first pin and the second pin is kept at a fixed value: Vd = Vi - V2 = ( R1 x VD ) . ( R1 + R2 ) and when the value of R2 is much larger than that of Ri, the voltage Vd is nearly a constant and can be very small too whereby as a so called - A V situation takes place, the voltage V1 of the battery 20 can be lowered with ease to a value smaller than the voltage Vl of the memory capacitor 30, and the voltage state of the output pin 13 of the comparator 10 is converted accordingly, terminating the charge operation on the battery 20 by cutting off the power source 40.

Since the value of Vd is fixed and very small, the recorded voltage of the memory capacitor 30 at any instant is very close to the voltage of the battery 20 and the final recorded saturation voltage is almost equal to the saturation voltage of the battery.

It can be seen in Fig. 2 that as long as the values of the second voltage reduction resistor R2 and the memory capacitor 30 are properly controlled for making the voltage V2 of the memory capacitor 30 drop at a slower speed than that of the voltage V1 of the battery, and the resistor R1 is of much smaller resistance than the resistor R2 ( R1 is in the range of thousands of ohms, and R2 in the range of millions of ohms ), then the present detecting circuit can accurately detect the saturation of the charged battery and further terminate the charging cooperation instantly.

Furthermore, in some kinds of battery, the voltage drop at the saturation state is not obvibus so that the detecting circuit shown in Fig. 2 becomes incapable.

Therefore, an amplifying device 60 disposed between the battery charging terminal and the first input pin of the comparator 10 is adopted to improve the sensitivity of detection, as shown in Fig. 3.

The detected drop of voltage is first amplified and then transmitted to the comparator 10, so even a nonobvious drop of voltage in the battery can be obviously detected by the comparator 10 after the voltage drop signal at the first input pin 11 is amplified.

For instance, if the gain of the amplifying device 60 is N, and the small voltage drop of a battery voltage VB at saturation is Va, then the detected voltage drop by the comparator 10 is: N x VB - N x (VB - Va ) = N Va . The voltage drop Va is amplified N times so that it can be easily detected.

The working voltage of the differential amplifier 61 of the amplifying device 60 is subjected to limitation so that the output amplifying rate is accordingly limited.

Therefore, the input terminal of the differential amplifier 61 can be connected in series to a fixed value voltage dropping unit 62 ( a Zener diode in the present case ) so as to get the battery voltage VB drop at a fixed value Vb before being delivered to a differential amplifier 61.

In such a case when the detected voltage drop in the battery voltage VB is Va, the comparator 10- obtains an amplified voltage V1 equal to Vi = N ( VB - Vb ) - ( ( VB - Va) - Vb ) N = N Va which still remains as N Va unchanged; however, to the amplifier 60, the output thereof will not be limited by its associated differential amplifier 61.

It can be clearly seen that the present invention is simple in structure and cheaP in production and is dependably accurate in cutting-off a charger so as to prevent battery damage by overcharging.

Claims (8)

CLAIMS:

1. A battery charge saturation detecting circuit for use in a charger, comprising: a comparing means having a first input pin and a second input pin and an output pin; said first input pin being used to detect the voltage of a charged battery; said second input pin having a voltage just slightly lower than said first input pin; said output pin being connected to the power source of said charger for controlling the supply of power to said charger; a first voltage reduction resistor disposed between said first input pin and said second input pin; a second voltage reduction resistor having a resistance substantially larger than that of said first reduction resistor and used to work in cooperation with said first voltage reduction resistor so as to transmit the voltage of said first input pin which is accordingly reduced to said second input pin, causing the voltage of said second input pin to be just slightly lower than that of said first input pin, i.e. the voltage of said charged battery; a memory capacitor in parallel connection with said second voltage reduction resistor performing a charging and discharging operation via said reduction resistor and the recorded voltage of said memory capacitor being just slightly smaller than that of said charged battery.

2. A battery charge saturation detecting circuit as claimed in claim 1 wherein a fixed value voltage dropping means is connected in parallel with said first and second voltage reduction resistors so as to keep the voltage difference between said first and second input pins at. a -jMålI fixed value; and a grounding resistor having one end connected to said fixed value voltage dropping means and the opposite end grounded.

3. A battery charge saturation detecting circuit as claimed in claim 1 or claim -2 wherein. an amplifying means is disposed between the charging terminal of said battery and said first input pin of said comparing means so as to amplify a voltage drop in a battery which is charged to saturation, facilitating the detection thereof by said comparing means.

4. A battery charge saturation detecting circuit as claimed in claim 3 wherein said amplifying means has a differential amplifier and a fixed value voltage dropping means having one end connected to said charging terminal and the opposite end connected to said differential amplifier whereby the detected voltage of said battery first has a fixed voltage removed, then is amplified by said differential amplifier and then is transmitted to said comparing means enabling the output of said differential amplifier to be free from the Limitation of its owagworking voltage.

5r A battery charge saturation detecting circuit as claimed in any one of the preceding claims wherein said memory capacitor is used to obtain a-slcw voltage drop so that the speed of the voltage drop of a saturated battery is faster than that of said voltage drop of said memory capacitor.

6. A batterv charge saturation detecting circuit as claimed in claim 1 or claim 2 wherein said fixed value. voltage dropping means is a diode or a Zener diode whch can provide a small voltage difference between said first and second input pins of said comparing means so as to make said first and second input pins have a small voltage difference, enabling the voltage of a saturated battery to be easily dropped below the voltage of said memory capacitor.

7. A battery charge saturation detecting circuit as claimed in any of the preceding claims wherein the resistance of said first voltage reduction resistor is preferrably set in the range of thousands of ohms and the resistance of said second voltage reduction resistor is preferrably set in the range of millions of ohms so as to make the voltage difference between said first and second input pins a fixed miniature value.

8. A battery charge saturation detecting circuit substantially as hereinbefore described wth reference to and as shown in the accompanying drawings.