Yep It’s archaic (approx. 20yrs old). I found it in my Dads garage during a clean out. It was sat in a pool of water together with a JVC amp, both of which live happily in my workshop! This is as you can tell is a car battery charger so is not quite what we need to charge Gel Cells but it (and many others like it) can be modified. The problem with car chargers is that they are commercial units made as cheaply as possible to maximize profits for the companies concerned, so they lack a few components that turn them into a decent chargers. Fist of all is the rectifier, what this does is to turn the AC current form the transformer into DC that is required to charge our battery’s. As you may expect there is a cheep version (half wave) and an expensive version (full wave) of rectifiers. Have a look at the diagrams below, you can tell why the full wave is more expensive.

How do they work? Click here for an explanation.

In my experience it is better to use a charger that already has a bridge than to modify a half wave rectifier into a bridge (due to cost mostly) because you will need extra diodes that are capable of handling  up to 5A which also means heat sinks will also be required! (Tip, try to use an old one, they tend to be a higher spec than a new one)

If you have a look at the inside of this charger you will see the rectifier (LHS reddish coloured grid thing), the smoothing capacitor (above the bridge), and the transformer.

After disconnecting all of the rusty wires from the rectifier pack I could then go ahed and check to see what type of rectifier I had and also to check for any blown diodes! Below is a the layout that I found, connect both of the + terminals and you have a bridge (drawn slightly different than the one above)

Now that I know that I have a full bridge then I have to add another component that is missed out from car battery chargers. This component is a capacitor. Its function is to smooth out the voltage wave form that comes from the output of the rectifier into a flat(ish) line (steady voltage). Basically choose one with a high capacitance and a voltage above 12v.(mine is 87000uF @ 56v) I chose this one because it was the only one that I had that suits the criteria.

What I did next was to replace all of the wiring and re solder all of the joints and that turned my 350mA output back into a 5A max output suitable for charging the Gel Cells! Click here to see a full schematic.