I have a 12v car baterry 13plates, I want to make a home made baterry charger. Anybody can give me a schematic circuit diagram, at least simple one... Anyway I have already a 12v 12amp stepdown transformer, is it usable?

Hoping for a replies. Thanks...

 

Do you want a simple dumb charger or a complex smart charger?

A dumb charger, similar to commercial chargers, can be made by adding a Schottky bridge rectifier to the transformer output and connecting its unfiltered output to the battery through a low value power resistor to limit the current.
You likely want to include an ammeter to monitor the charge current (and possibly a voltmeter), along with a fuse/circuit breaker to protect against accidental reverse battery connection.

 

For the power resistor, you could use a headlight globe. Or a tail light globe for lower current. A handy source of power resistors.
Just remember, they have non linear resistance, depending on the heat, so are not a replacement for power resistors in most uses, but for the battery charger they will be ok.