Rick's Free Auto Repair Advice

How long to recharge your car battery?

How long to recharge your car battery by idling versus driving?

First, it’s never a good idea to recharge a dead battery by idling

Your alternator is designed to top off your battery. It’s not designed to recharge a completely dead battery, especially at idle speed. Trying to recharge a completely dead battery by idling is a great way to overheat your alternator and kill it. Keep in mind that a battery charges costs around $70, while a replacement alternator costs around $500 to as much as $1,000 with installation.

But you asked how long to leave a car running to charge battery

Your alternator only outputs about 1/3 of its rated amperage at idle speed. So it takes almost 4 hours of idling to fully recharge a dead battery. And that assumes the battery is in good condition. If the battery was discharged due to a shorted cell, it will never fully recharge and the shorted cell can damage the alternator.

How long does it take to recharge a dead battery by driving?

Again, we’ll assume that the battery is in decent condition but is fully discharged. In that case it takes at least 30-minutes of driving at highway speeds to fully recharge a dead battery. In other words, a 15-mile outbound and return trip at highway speeds the entire time. And that’s without other electrical loads on; like the blower motor, defogger or heated seats.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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