Components List:

R1 = 1Kohms 	 D1  = 1N4001                      T1  = 220V/17V 4A Transformer
R2 = 1.2Kohms 	 D2  = 6.8V 0.5W zener             LD1 = Green LED
R3 = 470 ohms 	 TR1 = 4.7Kohms  trimmer           LD2 = Red LED
R4 = 470 ohms 	 Q1  = BTY79 or similar 6A SCR     M1  = 0-5A DC Ampere meter
R5 = 10Kohms 	 Q2  = C106D  SCR                  S1  = 10A D/P On ? Off Switch
C1 = 10uF 25V 	 GR1 = 50V 6A Bridge Rectifier     F   = 5A Fuse

The above circuit claimed have ability to prevent battery overcharge that make electrolyte lost due to evaporation. This circuit will eliminate the problems by monitoring the battery’s condition of charge through its retroactive control circuit by applying a high charge current until the battery is completely charged. When charging is complete, it turns on the red LED (LD2) and deactivates the charging circuit.

This circuit is drawn to charge 12V batteries ONLY. Certain emphasis should be taken when wiring up this circuit. They are the connections of the transformer to the circuit board, and those supplying current to the battery being charged. These connections should be made with cables having a large cross-sectional area to prevent voltage-drop and heat build-up when current flows through them.

Source: http://users.otenet.gr/~athsam/car_12v_battery_charger.htm
Please visit the original source for detail explanation.