SLA at high current

Thread Starter


Joined Dec 14, 2011
Hi Guys-

I've got a really dumb question about how car batteries and the like work. I'm a self taught hobbyist and didn't get much in the way of formal classroom instruction with electrical circuits, I just do some simple stuff that's fun (to me!).

I know that regulated power supplies you plug into the wall keep the voltage constant and delivers what amps your circuit needs up to its wattage rating. No worries there!

However I want to use a SLA battery to power a gadget I made that uses an air compressor (like the type you plug in your cigarette lighter to pump up your car tires), and some control circuitry. The compressor uses 10 amps, and the control box is about 0.5 amps, if I added up the numbers correctly. So if the compressor's running, the circuit takes up 10.5 amps, if the compressor isn't, it takes 0.5 amps. The compressor would only run about 15 seconds at a time every hour or so, so I'm not really worried about the power draw.

My confusion is how the battery will behave, when the amperage needs of the circuit is at extremes. Will the battery try to push out as many amps as it possibly can once there's a circuit made? Will the voltage and amperage be fixed, and I need to account for that? Or will the battery push out 12 volts and somehow only dole out the amps needed for the circuit? Do I need some voltage regulator in my design?

Sorry if it's a dumb question, I just never visualized how a battery works like this. I'd like to have some idea before I hook up the battery and have something explode!




Joined Oct 3, 2010
Assuming you are talking about a car battery or marine battery or the likes, that can deliver hundreds of amps on demand: 1. your project will only draw as many amps as it will draw. if it's 10.5A, then 10.5A will be drawn. 2. They have low internal resistance, so a 10.5A draw will not cause the voltage to sag any noticable amount. 3. This means, I think you will be fine. no regulator needed. Watch out for shorts though, because a short with a current capable battery is going to be very interesting.

If you are talking about a bunch of AAA batteries in series to make 12V, a 10.5A load will cause the voltage to sag probably to <1V, because of their high internal resistance.