Saturday, July 20, 2013

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

(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.


  1. Thanks for one's marvelous posting! I genuinely enjoyed reading it, you might be a great author.I will be sure to bookmark your blog and definitely will come back from now on.
    Lead Acid Batteries in Asia

  2. hello sir i found this very helpful.but this circuit will work correctly. as my analog is bit weak.

    1. Hi. I built the circuit and worked for me. I even tried to use a solar panel as the DC input source to charge a 12V battery and it charged nicely.

    2. Hi, how to work out output to 13 volts cutout, input constance 12vdc.

  3. Hi SHOng. The circuit is for step down only so if you want to output 13V with 12V DC source then you need a transformer or amplifier circuit.

  4. Can I power on with 12vdc from pc psu to charge a 12v sla battery?

  5. Hi Pedro. Should be ok to use PC PSU in this circuit as long as the input DC source is more than 15 VDC as the circuit has about 1V plus of loss and below 40VDC.