Schematic Diagram

Oops, that LED is backwards!!

Parts List

1 each - Wall transformer, 16VAC output, 40VA (Mouser part # 507-XP1640)

1 each - 7815 15 volt regulator (TO-220) 1.5 Amp (Mouser part # 511-L7815CV)

1 each - Heatsink (TO-220) for 7815 (Mouser part # 532-577102B00)

4 each - Diode, Rectifier 1N4001 (Mouser part # 625-1N4001)

1 each - LED, standard

1 each - Transistor PNP 2N3638

2 each - Resistor, 25 ohm (10 watt) (Mouser part # 280-CR10-25)

1 each - Resistor, 1K (1/4 watt)

1 each - Electrolytic Capacitor, 470uF 35 volt

 

Operation

For 12 Volt Lead Acid (SLA) type batteries only. Do not use on NiCad or NiMH.

Not a trickle charger. Disconnect battery from charger after LED turns off.

Charge LED will be on while battery is charging. Once battery is fully charged, it's internal resistance will increase and prevent current from flowing through the limit resistors. This will remove forward bias on transistor and the LED will turn off.

I have used this circuit very successfully on 2.9Ah to 7Ah batteries. The LED is a great convenience to prevent overcharging the battery. Also very inexpensive to build.

WARNING!!!  Do not leave the battery connected to the charger while the charger is unplugged. This will damage the circuit!