Can I limit the current from my SLA charger?

I've got a couple of SLA 12v batteries that I charge using a 700ma wall wart. It only puts around 400ma into the batteries. I would like to be able to limit the current even further, because I'm usually not in a hurry to get them charged and I usually overcharge them. Is it possible to limit the current without lowering the voltage? If so how?

Sure. The traditional way is a low value series resistor. Some use a light bulb which has the advantage of giving a visual indication of charging (bright for high charge rates, dims as the charge progresses). Auto tail light bulbs are often used.

Doug Owen

Doesn't that lower the voltage too??

There's only a voltage drop across the resistor when there's current flowing. As the charge current goes to zero, so does the voltage drop so the battery voltage matches the charger voltage.

It just takes longer for that to happen.....which is what you wanted, right?

Doug Owen

Best way to charge 12 volt SLA batteries is regulated voltage at 14 volt. This is exactly what alternator of car does.

Actually, if we're talking 'best' here, it has to be a lot tighter than 14.0 Volts. The per cell voltage varies strongly with temperature, say from 2.55 at zero C, 2.43 at room temperature and 2.25 at 50 C (like under the hood). This leads makers to recommend things like Panasonic's "15.4 Volts at 0, 14.7 at 25 (room temp more or less)and 14.2 at 40 degrees C for Cycle use, the Float numbers are 14.1, 13.7 and 13.4 respectively.

And of course and even better charger has provision for equalizing charges as needed. And I guess an even better one would cycle off and come back on as needed or on a timer.

Far from simple fer sure.

Doug Owen