12 Volt charger
Best way is to get a normal transformer and bridge rectifier.
Match the TX voltage as close to the charging voltage of the battery, to minimise losses in series resistors etc.
A charging voltage to aim for is 14 Volts.
Allowe for 1.2 volts drop in the bridge rectifier, requires 15.2 volts dc.
A transformer with a 11 or 12 volts 20 Ampère winding is required.
Use a 35 Amp bridge rectifier on a heatsink.
Use a voltage comparator which switches a relay in the TX primary when the voltage reaches a predetermined value (14 volts) , and turn the charging cycle off.
Or go for a SMPS design which is more complicated.
Regards, Raymond