DIY: Smart Battery Charger
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You can damage your rechargeable batteries by either overcharging them or overheating them when you connect them to a battery charger. You can avoid this by using smart battery chargers that automatically switch off or switch to a trickle charge that will not damage your batteries.
Smart battery chargers are expensive, but you can build your own for a fraction of the cost with a single transistor circuit. This circuit uses a Siemens windows discriminator TCA 965 B to set the maximum and minimum voltage levels for your battery. The charger will start charging when the battery drops to a certain voltage and switch off when it reaches the battery's ideal voltage. Following instructions from a circuit's diagram (see Resources) is easier than reading a written description of a circuit.
Solder the resistors R1 to R6 to the windows discriminator so that R1 is soldered to pin number 8, R2 to pin 11, R6 to pin 3 and to a blue LED, R5 to pin 14 and a red LED, R4 to pin 13 and a green LED, and R3 to pin 2 and a yellow LED.
Connect the green and yellow LEDs to each other and to the transformer. Connect the transformer to the blue diode, the red diode, the resistor R2 and to the positive pole of the automotive lead acid 12-volt battery that you wish to charge. Connect R1 to R2.
Connect P1 to pin number 6 and P2 to pin number 7. Connect P1 and P2 to each other and to R1. Join P2 and P1 to the capacitor and to the AC mains. Attach P3 to pin number 9 and to the negative pole of the car battery. P1 and P2 function in this circuit as discriminators for the Siemens IC TCA 965 discriminator. If the voltage of the battery falls above or below the settings of P1 and P2, the circuit is switched on or off as required.