This car battery charger circuit can be used to charge 12V and 6V batteries. If it is used a transformer that can deliver 4A to 5A at a voltage between 12.6V and 16V then we can get rid of the switch for 6V or 12V batteries.

12V car battery charger circuit schematic

The car battery charging current is automatically limited to 4.2A. If there is a 600mV voltage on R1 (4A thru it), then the T1 transistor starts to conduct. Excessive charging current is avoided because the current value on T3′s base is limited. The difference between applied load current (at T4′s collector) and real voltage of the battery is balanced thru T4′s collector-emitter junction.

The power input of T4 (2N3055) is the product of load current and voltage difference already mentioned. When charging 6V car battery this power reaches a maximum of 40W. The rectifier diodes must be able to deliver 4A at 40V. T4 2N3055 must be mounted on a good heatsink in order to dissipate the heat.