Keeping your car battery constantly charged when the car is not in use appreciably increases the life of the battery. Charging is, of course, normally possible in your garage. The charger described here provides a constant charging current that may, for example, be fed to the battery via the cigarette lighter.
Automatic battery charger circuit detail:
The charger consists of a mains transformer, Tr1, bridge rectifier B1and smoothing capacitor C1. The charging current through the regulator, IC1 and the switched series resistors is 107 mA (47Ω); 230 mA (22 Ω); 500 mA (10 Ω) or 1A (5 Ω).
Diodes D1D4 indicate the position of the switch. Transistor T1, resistor R1 and diode D5 ensure the brightness of the diodes. When the battery is not connected the relay is not energized and the mains is switched off.
Automatic Battery Charger Circuit Diagram:
When the battery is connected C3 is get charged, T4 is switched ON and the relay is energized. The mains is then switched ON and the battery is charged via D7. The consequent voltage drop across D7 causes T3 and T2 to be switched ON, so that the relay remains energized although, since its collector is at +12V, transistor T4 is switched off. Resistor R5 ensures thatC1 is kept charged so that T4 remains off.
To ensure that the charger works with flat batteries, the relay contact may be bypassed by S1 which enables the charger to switches ON manually.
Note that during constant charging of lead acid batteries there is the risk that water dissolves into hydrogen and oxygen and this will reduce the liquid in the battery. Since sealed batteries cannot be topped up, the present charger is not suitable for this type of battery.
Also do not use a current higher than necessary; in most cases 100 mA is ample. The larger currents are intended for charging large NiCd batteries.