Cut Out Battery Charger

Here is the outline for the autoloader I've been using for the car battery of my children. The charger is a small molded unit that probably provides no more than an amp and this circuit has much more problems. No current limit is provided by this circuit - is based on the charger for that. The circuit can be modified to provide more current by lowering the 470 and 330 ohm resistors in the base circuit of 5195 and 10k in the collector 4401. A relay could also be used in place of the pass transistor.

That's how it works: When the battery voltage is low, the voltage at the base of the first 2N4401 (right) is not enough to turn and the second 2N4401 is biased by the 10k resistor. The power transistor is on and LED lights. When the battery is fully charged, the voltage exceeds a somewhat arbitrary "over-voltage" value slightly below 14 volts and the regulator is disconnected. The 470k feedback resistor gives the circuit some hysteresis so it will not turn back on until the battery voltage drops below 13.5 volts. When the battery is nearing full charge, the light starts to blink and after a few hours the only light that comes on from time to time. This occasional overvoltage jolt that seems to keep the batteries in great shape