Making car battery charger is easy thing to do, all you need is an old PC power supply and one car battery to test. Car battery chargers have output of 14V, but PC power supply have only 12V, so we need to increase that. Easy way to do is to use some resistors as show on the picture.
After adding 1 resistor of 220 ohm you will get output voltage of 14~14.5 which is enough for charging car battery. In PC power supply there is always short circuit protection that will cut out power if current get above 15 Amps.
For all those who want to build car battery charger from zero, here is a shematic.