Best Strategy for 13.8V 2A Lead Acid Battery charger Using Transformer that Puts out 12.4V / 25V

Thread Starter


Joined Jan 19, 2015
I salvaged a 240V --> 12.4V or 25V transformer and would like to create a 13.4V 2A charget for my lead acid car battery.

Which Voltage should I use and - any recommended strategy?

I know I need to add a rectifier and capacitor in there is about it so far after days of research.

Thanks in advance for any help, I'd be happy to provide more info and looking forward to head downtown and buy what I need :).


Joined May 3, 2013
Can that transformer output 2 Amps @ 25 Volts?
Wire up the transformer as a 25 Volt.
Head downtown and pick up a DC-DC Switching Regulator that takes an input from 24 to 36 volts and gives an output 13.4 Volts (should'nt it be 13.8 Volts?) @ 2 Amps.

Normally, these Regulators have a control to trim the output voltage and current limit.


Joined Mar 14, 2008
The simplest approach (which is what inexpensive chargers do) is to put a full-wave bridge on the 12.4V winding and a resistor in series with the battery to limit the charging current (no capacitors needed since batteries don't mind pulsating DC current). The resistor will need to be a power resistor with the power determined by the resistance required and the voltage drop across the resistor.

Alternately if the transformer is a center-tapped 25V unit then you can use a full-wave rectifier circuit with a diode on each each winding, the diode outputs connected in parallel, and the center tap going to common, again with no capacitor and a series resistor.


Joined Jun 22, 2012
I take it the transformer has two 12.5V windings and you can put them in series or parallel to create 25v, 12.5v , or is it one centre tapped winding. The parallel combination will give the biggest charge current, thes series will give you the biggest voltage.

If its centre tapped you can use two diodes on the outer wires and use the centre as the common.
Last edited: