Homemade 12V Sealed-Lead-Acid Battery Charger

We need 14.2V to proper charge a 12V sealed-lead-acid (SLA) batteries (like batteries in your car, computer UPS power supply, emergency light, etc). Too high a voltage will damage the batteries and too low the batteries will never get charged.

I use an old Acer laptop charger as the DC input source which gives about 19V. The homemade charger reduces the 19V to a stable 14.2V.

The charger composes LM317 voltage regulator, NPN transistor, capacitors and resistors. By setting the resistor R3 (refer to circuit below) to 830 ohm I was able to get 14.2V of output. If you want to charge a 6V SLA battery you can try to set R3 to ~ 120 ohm to output 7.1V.

The output voltage can be calculated using the formula: 1.2 x (R1 + R2 + R3) / R1

REMINDERS:

(1) LM317 gets warm during charging so it needs to be mounted on heat sink.
(2) Never set the output more than 14.2V to charge 12V SLA battery.

(3) The charger is meant for SLA batteries only.

(4) Use cautiously and at your own risk.