Charging lead acid car is fairly simple. You need to take care of charging current which should be around 0.1C and charge up to 13.4V battery voltage. Having this in mind, it is easy to build charger out of few parts available. Pityukecske have shared his version of battery charger on instructable. He has chosen a PIC12F683 microcontroller which controls relay switching charging current to battery and also monitors battery voltage on one of its ADC inputs.
Current circuit doesn’t have much flexibility – charging current is basically driven directly from transformer and diode rectifier. It is visible on panel ammeter. So transformer is chosen to meet current requirements. Microcontroller monitors battery voltage. Since it is over 12V there is a resistor voltage divider used to scale voltage to microcontroller working limits. Once voltage reaches 13.4V, microcontroller shuts current by deactivating relay. Couple status LEDs indicate charging state. Simple solution that works in most cases.