Car Battery Charger

My garage is not heated, and one cold. snowy morning, I bundled up and weni out to the garage to start the car. Nothing. Since I couldn’t get out, I spent the morning in my basement work shop putting this charger together. I mounted a socket on the car’s engine block that the charger plugs into, and now when I get home at night, I open the hood and plug the car in.

Take a look at Ernie’s circuit, in Fig. I. That high-performance charger will quickly charge gelled lead-acid batteries and then turn off at full charge. At the start, the current is limited to two amperes. As battery voltage rises, current decreases. When current falls to 150 milliamps the charger automatically switches to a lower float voltage to keep from over charging. When you hit full charge, transistor Q1 lights the LED to indicate the charge status.