How Long Does It Take to Charge a Dead Car Battery?

Dead batteries happen to all of us at some point. You may inadvertently leave lights on in your car for a long time or miss signs of a weakening battery on your last few starts. Or maybe you haven’t maintained your battery or driven your car for some time. If you’re really out of luck, you might be in a situation where there’s no one around to give you a jump start when you car battery is dead.

Some people will keep a battery charger around to help them in these situations, but what should they expect when they decide to use it? And how long does it take to charge a dead car battery? There’s a little more to understand here than just how to jump your car, and this is a subject not typically touched on in a driver education course. Here are a few things you should know about charging your dead car battery.

Understand What Type of Charger You Have

The length of time it takes to charge a battery has a lot to do with the type of battery charger you have, said Benjamin Jerew, an ASE master mechanic since 2001. A high-power boost charger, for example, will charge a lot faster than a simple 1-amp float charger, which could take anywhere from four to 12 hours to recharge a battery.

Depending on the condition of the battery, some will be able to take a charge better than others. But remember, you don’t need to charge the battery all the way up. It’s a case of getting it charged enough to start the car and let the alternator fully recharge the battery, usually with about a half-hour of driving at highway speed.

“A deeply discharged battery, given 15 to 30 minutes on a 40-amp charger, should be able to start the car, giving the alternator a chance to finish charging the battery," said Jerew. "But you should never charge at such a high rate longer than this. If needed, switch to a lower amperage, 5-10 amps, to prevent overheating."

Other Things You Should Know

Many batteries can indeed be recharged, even when we think they are completely exhausted. In fact, a lot of batteries can last four to seven years before they are too weak to start the car, Jerew added.

It’s not just poor driving habits that take down a battery. Even a smart defensive driver can do it. Sulfation is the biggest thing to watch for when it comes to draining batteries, and in many cases, people may not even realize they’re allowing it to happen.

Sulfation occurs when a battery can’t fully charge. If you are constantly idling at low speeds in a car with many electronic features, it’s a tremendous drain on the battery, and not getting the car up to a higher speed for a length of time doesn’t allow the battery to full recharge. This accelerates the battery’s decline.

“If the battery keeps going dead and the alternator and cables are in good condition, it's likely the battery is too far gone,” he clarified.

The master mechanic also offers one more tip to keep your battery working well for the long haul.

“Batteries don't last forever, but a little maintenance goes a long way,” he said. “Drive at least 30 minutes straight, once a week. Otherwise, use a float charger to maintain a full charge when the car isn't being driven. Finally, clean and protect battery terminals and cables, rechecking at least every six months. If you have to use a battery charger, slower is better.”