Car Battery Charging and Maintenance

Keeping your battery healthy so it can power all your high tech gadgets

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 functioning 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Unlike outdated electrical systems that used generators and could function without a battery, modern automotive electrical systems need a battery in order to function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 it's so important to understand how to correctly, and safely, charge and maintain a car battery.

Two people charging a car battery
Sam Edwards / Caiaimage / Getty

What Charges a Car Battery?

There are two ways to charge a car battery: with the alternator, or with an external charger. Under normal circumstances, the alternator charges the battery whenever the engine is running. When a battery dies, then an external charger is the best way to charge it back up.

This is why most normal battery usage, like running the radio or dome lights while the engine if off, is replenished naturally the next time you start your car. As the RPM of the engine increase, the alternator’s ability to generate electricity also increases, and any power that isn’t being used by accessories like your headlights is available to charge the battery.

The flip side of this is that, in some cases, your alternator may not be able to supply enough power to run all of your accessories. For instance, if you have your air conditioning, wipers, headlights, radio, and other accessories on when you’re idling at a stoplight, it's possible to create a load that's larger than your alternator can handle. When this happens, power stored in the battery will pick up the slack.

How Much Charge Can a Battery Hold?

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. After subjecting a battery to a load like that for that amount of time, the voltage will have dropped below 10.5 volts, and the battery probably won’t be able to start the car.

While individual batteries are rated to show how long they can provide power to a specific load before dropping below a critical voltage, it's a good idea to avoid actually subjecting a regular automotive battery to this type of use. If you drain a car battery too far, you can actually damage its ability to hold a charge in the future.

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.

Charging a Car Battery

When the alternator isn’t up to the task, or when the battery is drained to the point where it isn't capable of starting the engine, the other way of charging a car battery is to use an external charger.

Car battery 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.

How to Hook Up a Car Battery Charger

Hooking up a car battery charger is a lot like jump-starting a car:

  1. Check to make sure your battery charger is off. When in doubt, unplug it.

  2. Connect the positive lead from the charger to the positive terminal on your battery.

  3. Connect the negative cable on the charger to a good ground.

    If you connect the negative cable to the negative terminal on your battery, be very careful to avoid touching, moving, or removing the cable while the battery is charging.

  4. Set your battery charger to the appropriate voltage and amperage.

  5. Turn on your charger, or plug it in if necessary, and set the timer if it is equipped with one.

Charging a Car Battery With Jumper Cables

With that in mind, it’s also possible to provide a certain level of charge to a dead battery via jumper cables, although there is some risk involved. After hooking up jumper cables from a donor vehicle to the battery and engine or frame of a vehicle with a dead battery, starting and running the donor vehicle for a while will allow its alternator to charge the dead battery.

During this process, all of the accessories in the donor vehicle should be turned off, or the alternator may not have enough juice left over to charge the dead battery. Depending on how dead the dead battery is, a few minutes will typically provide enough of a surface charge to get things rolling.

After receiving a jump start, the alternator in the car with the dead battery will take over, and as long as there aren’t too many accessories running, simply driving the car around will allow the battery to charge back up. However, alternators aren’t really designed to charge totally dead batteries, so hooking up a battery charger is still a pretty good idea even after receiving a jump start.

Maintaining a Car Battery

Aside from making sure the battery maintains a good level of charge, primarily by not leaving the headlights on overnight, most automotive batteries require regular maintenance in the form of checking the electrolyte level and specific gravity.

The electrolyte, which is a solution of sulfuric acid and water, should always cover the lead plates in each cell, since exposing the plates to the air can cause issues over time.

If the specific gravity is low across all the cells, the battery is typically in need of a charge. If the specific gravity remains low after charging the battery, that's usually a good indicator that the battery should be replaced. If the specific gravity is only low in one cell, that indicates an internal fault, in which case charging the battery may actually be dangerous.

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.

Was this page helpful?