Charge 48V battery from a 12V source

Thread Starter


Joined Dec 23, 2011
I have a 48V inverter powered by 4 lead acid batteries in series. I want to be able to charge the them with a 12V source. What I don't want to do is use a DC - DC converter to step the voltage up.

Hear is my idea. Have 4 sets of transistors used simply for switching each battery in or out of the load. That way I would charge each of the 4 batteries one at a time. Then have some sequencer circuit that will rapidly cycle from battery to battery. Some kind of logic circuit to make sure there was no way more than on of the transistor pairs could be on at a time would be good.

The most power I would be supplying to this device would be 30 amps. So each transistor should be able t handle a spike of 30 amps and an average of 7.5 amps.

Is this something I would need to build from scratch or can I find parts that or the whole thing already built?

Any recommendations would be helpful.

Chris W


Joined Feb 5, 2010
There is no way I am aware of to charge one, or more batteries while they are loaded and supplying current to an output device, unless you supply the FULL voltage to the string(for 48 volts it would be around 52-57 volts) needed to charge them AND supply MORE current than is required by your output device.

I can, and do, often charge my 48 volt arrangement at 12 volts, but NEVER while they are loaded and operating equipment.

A four pole double throw switch can be arranged to place the batteries in series for operation and then in parallel for charging. I'm thinking of large switches akin to the old style knife switch I'm using for this purpose currently.

Sample image(not mine)

Last edited:


Joined Mar 14, 2008
Why do you want to rapidly sequence from one battery to the next? You just need to charge one battery at a time and when the last one is charged you stop the charge. You only need to do a trickle charge if the batteries are not going to be used for a long time period.

The difficulty in controlling transistors is the floating nature of the battery load. That makes it a problem to use a common control ground.

The easiest way to connect the four batteries to one charger is to use three DPDT relays. Two relay outputs are connected to the batteries and the third switches between the two relay inputs. (See below). If you wanted to go solid-state you could use solid-state DC relays but that would be considerably more expensive.

The easiest way to generate the logic to control the relays would be a microprocessor controlling transistor drivers to drive the relay coils.

Battery Relay Select.GIF