This is a circuit for battery charger. But the circuit can places in a car. This is a simple design and useful to make it. In this circuit there is facility for monitoring the charging current and voltage. This is the figure of the car battery charger.
The circuit is based on the IC MC78T12ABT. The IC is nothing but a 7812 in TO-3 package with 3A capacity. The transformer T1 steps the mains voltage to 15V AC and diodes D1&D2 does the job of rectification. The transformer T1 can be a 230V primary; 15-0-15V, 3A secondary step down transformer. Capacitor C1 does the filtering and C2 acts as a decoupling capacitor. The ground terminal of IC1 is lifted to 2.1V using the diodes D3, D4 and D5. So the output from the IC1 will be a regulated 14.1V (12+2.1).The battery is charged via diode D6.The D6 blocks reverse flow of current from battery to charging circuit when the mains power is not available.
Many cheap battery chargers usually assume that charging the battery slowly is OK although the battery is already full of charges. All they do is usually charging the battery with constant current no matter if the battery is almost completely discharged or has been fully charged. Off course the battery wouldn’t be damaged immediately after few charging cycles, but actually they’re damaged slowly and the life time can’t be maximized. This is one of solution for the problem above.
This circuit is automated that you don’t have to manually set the alarm timer to wake you up when your battery has been fully charged so you can unplug the charger to prevent overcharging. This battery sense the voltage while charging the battery, and automatically stop the charging when it’s fully charged. This is the figure of the circuit.
To set the R8, turn the variable resistor R8 to its maximum value (highest resistance), connect a fully charged Nicad battery (about 1.44V) to the batteryn terminal. Press and release the start push button, and make sure the LED D1 is turned on after releasing the start push button. Now turn the R8 slowly toward its minimum value and stop turning exactly when the LED D1 turned off. Now your automatic Nicad battery charger is ready. Place a discharged battery and press the start button, the D1 LED will turn on to indicate that the charging is in progress. LED D1 will turn off after the battery has been fully charged. To provide multiple battery charging, make few more similar circuits shown in the dashed line box, and you can charge up to ten Nicad battery (50 mA charging current per battery) using 1 Ampere 9-12 volts transformer.
