This is a simple circuit that I use to recharge my sealed lead-acid 12V-7A battery during camping. I have two 12V-10W solar panels (VMP - 18V) in parallel and the maximum current obtained is a little bit more than 1.5A.

The charging current should be 1/10 of the battery capacity so, for example, for a 7A battery the charging current should be limited to 0.7A. The LM317 device is used as a fixed voltage/fixed current regulator. The 4K7 pot., sets the charging voltage, usually 13.5-13.8 for stand-by use, and 14.4-15.0 for cycle use (see indication on your battery). I recharge my battery at 13.8V and I use it with a led light for the night, some phone car charger and I am fully satisfied.


The charging current is limited by RX. Its value can be calculated with the formula RX=0.6V/charge current. In my example, for a charge current of 0.7A, the value of RX will be 0.6V/0.7A = 0.85 Ohm. This resistor should be at least 1W. The 0.6V in the formula is the voltage required for the conduction of the transistor. When the battery is fully charged, the 0.6V across RX drops to 0 and from this moment the battery does not absorb any current even if is still present the charging voltage on the output.