Connecting the cables the right way is an essential part of safe car battery charging. Sam Edwards / Caiaimage / Getty

Aside from the alternator, the battery is the most important component in any car’s electrical system. It provides the juice to run all of your fancy electronics when the engine isn’t running, and when the engine is running, it plays an essential role in the proper functionality of the alternator’s voltage regulator. Unlike outdated electrical systems that used generators and could function without a battery, modern automotive electrical systems need a battery in order to function properly.

A dead battery means a car that won’t start, and an alternator that will have to work too hard—potentially to the point of failure—which is why the subject of properly charging a car battery is so vital in terms of modern automotive technology. Although a vehicle’s alternator is capable of keeping its battery charged under normal circumstances, batteries do go dead for a variety of reasons, and there also comes a time in the life of every car battery when it’s just time to move on.

What Charges a Car Battery?

When properly charged, and in good working order, a car battery will typically read at about 12.4 to 12.6 volts and have enough reserve capacity to power a 25A load for anywhere from nine to 15 hours, at which point the voltage will have dropped below 10.5 volts, and the battery probably won’t be able to start the car. Extreme temperatures, and wear incurred through the normal cycle of charging and discharging, can reduce the reserve capacity, which is why you may come back to a dead battery after leaving your headlights on while running a short errand, while in another situation, you may be able to leave them on all day and still start the engine just fine.

In any case, there are two ways that a car battery can be charged: by the alternator, or by an external charger. Normal battery usage, like running the radio or dome lights while the engine if off, will be replenished naturally the next time you start your car. As the RPM of the engine increase, the alternator’s ability to generate electricity also increase, and any power that isn’t being used by accessories like your headlights will be available to charge your battery.

In some cases, like when you’re idling at a stop light, there may not be enough power to run all of your accessories, in which case the battery will actually discharge further instead of receiving a charge.

Charging a Car Battery

If the alternator isn’t up to the task, the other way of charging a car battery is to use an external charger. These chargers run off AC power and provide 12V DC at relatively low voltages, which is the best way to charge a completely dead battery. Charging a dead battery with an excessively high voltage can increase the off-gassing of hydrogen, which can, in turn, result in a hazardous situation where the battery might explode. This is why it’s important to take the same care when hooking up a car battery charger as you would when hooking up jumper cables, and also why it's often a good idea to use a trickle charger.

Fact:

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