This is a simple circuit of a 12v/24v battery charger. I had originally a 12v battery charger however the transformer burnt. Instead of throwing everything away, I salvaged the charger box and built a new charger, capable of charging also a 24v battery.
I used two identical transformers, 240v Primary (although in actual fact they are a number of tappings on the primary to better control the output voltage). These came from old cheap UPS systems. I used a tapping of one of the transformers (T2) to control the HIGH/LOW voltage.
The charger has got an output voltage selector switch (SW2) together with a HIGH/LOW selector switch (SW1). The relay is used to add the 2nd Transformer output (T1) in series with the 1st Transformer (T2). Both bridge rectifiers are the block type, 25amp current output. Resistors R1 and R2 have been included across the transformers output to dump any stray voltage. I could measure 50v on the digital multi-meter when in actual fact the output would have been 30v!
Internal close up of the charger wiring.
Transformers used. As mentioned these came from 'old' UPSs
I added a heat-sink at the back of the charger to better help in dissipating the heat generated by the rectifiers
Close up picture showing the rectifiers...
I also changed the charger cables to a pair of sturdy 10mm cables together with larger crocodile clips
The original charger box...