Disconnect the negative terminal to protect your car's electronics, and then connect the charger's clamps before you plug it into a power outlet. Check the settings and then turn your charger on. Depending on how weak your battery is, it can take longer than 4-8 hours to charge. Let's go through the steps to connect your. Car battery chargers vary significantly. Trickle chargers, smart battery chargers and battery maintainers — it might sound like a marketing ploy, but. A fully charged car battery will have 12.88 volts. Cars use a 12-volt electrical system, and the difference between a fully charged battery and a completely dead one is just 1.04 volts. In fact, the difference between a fully charged one and a battery that might fail in a couple of weeks is. You may need to charge your car battery if 1. you jumped your carrecently. 2. you notice odd behavior in your accessories. 3. you left an interior light on. It takes about 4-8 hours if you're using a typical battery charger and the battery isn't completely dead. You could charge it faster with an industrial.