• It can charge 6V, 9V and 12V batteries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2.
• Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery.
• Once the battery is fully charged, it will attain certain voltage level (e.g. 13.5-14.2V in the case of a 12V battery), give indication and the charger will switch off automatically. You need not remove the battery from the circuit.
• If the battery is discharged below a limit, it will give deep-discharge indication.
• Quiescent current is less than 5 mA and mostly due to zeners.
• DC source voltage (VCC) ranges from 9V to 24V.
• The charger is short-circuit protected.

R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally. To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its required value.

Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor conducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops.

If the battery terminal voltage drops to, say, 11V in case of a 12V battery, adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts. LED2 will glow to indicate that the battery voltage is low.

Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current provided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 package of IRF540 can handle up to 50W.

Assemble the circuit on a general-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box.

Battery Charger Circuit Diagram

Source: http://www.electronicsforu.com