# Small battery cut off when fully charged

I am working on a small 1.2 V battery. I want the charging current of 7 mA to stop when the battery voltage reaches 1.5 V. What cut off circuit should I use ?

Basically you have two problems. The first is to provide a charging current of 7mA and the second to cut off charging when the battery terminal reaches 1.5V.

First problem - A 7mA charging current

Tr1, R2, D1, D2 and R3 form a constant current source. Tr3 is turned ON by a small current through R1 and acts as a switch to ground.

D1 and D2 set a constant voltage drop of (2 x 0.6V) 1.2V volts. This biases Tr1 so that 0.6 volts will be dropped across R2. The current that flows through R2 is 0.6/85 = 7mA. This is also the collector (charging) current since the base current is very small.

Second problem - cutting off at 1.5V

R4 and VR1 form a voltage divider. At 1.5 volt output VR1 is adjusted to give about 0.6V (about mid way). At this voltage Tr2 is turned ON and switches the base of Tr3 below 0.6V turning it OFF. With Tr3 OFF no base current flows in Tr1 and it will be turned OFF, stopping the charging current. If the battery is left in circuit a small discharge current through R4 and VR1 will lower its voltage, the circuit will switch back on and top it up again.

Well regarding your problem you have to use lm324 ic. its is a comparator ic and now thing is how to use it, following are the steps for solving your problem. 1. Set your desire voltage on which you want to cut off on -VE(pin no 2) pin of ic. 2. Now the set the battery output voltage on +VE (pin no 3) pin of ic.

Now when your output voltage swings above your desire voltage than comparator gives output 1 on (pin no 1) of lm324 ic and using output use transistor and relay to trigger switch on and off accordingly. if you are still getting prob than let me know i will give you diagrams of circuit.