Here I am showing a Lead acid battery charger. It is used to charge a 4V 1.5AH battery. The C-rate of this charger is C/4(1.5/4=0.375A) which means the charging current is about 400ma. This is a constant voltage constant current charger i.e. during initial charging it consumes about 400ma and when approaching full charge the charging current decreases. When the battery is fully charged then the charging current is 40ma. So this charger is having overcharge protection. Also charging (Red Led) and Full Charge (Green Led) indication is provided. The red led will turn off and green led lights up when full charge is attained.

NOTE: Once the battery is fully discharged and when connected to the charger the green led will light up for some time. This is the time required by the IC in this circuit to sense the voltage level of the battery. After some time (about 10-20 seconds) the red led will light up, indicates charging started.

NOTE: This is not a perfect charger for lead-acid battery when compared with branded chargers available in the market. But this charger will protect the battery from overcharging and it will provide charging and full charge indication.

Supplies

Components Required:

1)LM358 IC

2)5mm Led: Green, Red

3)Resistors: 47,47,1K,1.5K All are 1/4W

4)Electrolytic Capacitors:10uf 25v -2 nos

5)Zener Diode:4.2V 1W

6)Transistor: BD139

Step 1: Circuit

The input voltage is 6V, given at J1 terminals and the output is taken from J2 terminals. Check the polarity before connecting the supply. The power supply can be made using a 9V,500ma transformer, and a bridge rectifier. For getting a 6V regulated supply you can use 7806 IC

NOTE: If we use 1A transformer then the initial charging current will be more than 400ma(450ma-500ma).Always try to use a 500ma transformer.

Step 2: PCB

The PCB layout of the circuit is provided here.

Step 3: Finished Board

You can also use this circuit for charging batteries with higher AH capacity. The time required to charge the battery will increase. For 1.5AH battery the approximate time required is 4hours. The charging time will vary depending upon the usage of the battery.