Found this project via a link at diyaudioproject forum.
Self explanatory, quite similar to the one I did earlier except this one is only using a single diode hence a 1/2 wave rectification which I think is good to remedy the sulphation problem. Cheapest charger indeed! Come to think of it, this charger can charge other batteries too like Ni-Cd and Ni-Mh... just watch the current.
So, what are you waiting for? Just becareful of LETHAL mains voltage! Enough said.
Extract from the posting:
The advantages of this circuit are:
1. Did I mention it’s cheap?
2. It’s very easy to build.
3. It can charge about any voltage of battery, it doesn’t care.
4. It helps prevent sulfation in the battery.
5. Charge current is adjustable.
6. As a side effect, it provides good light
$3 battery charger! Thank you gogetumnow for sharing!