Rick's Free Auto Repair Advice

How long to leave a car running to recharge battery

How long to leave a car running to recharge battery?

How long does it take to recharge a battery with a charger?

If you’ve got a dead car battery and you’re wondering how long to leave your car running to recharge the battery, hear this; it’s much longer than you think. Here’s why. At idle speeds, your car’s alternator outputs less than 1/3 its maximum capacity. Plus, in late model cars with power management systems, the computer controls the charging rate, so it’ll only recharge the battery at safe rate. To get an idea of how long it takes to recharge a car battery, let’s take a look at how long it takes to recharge a car battery using a modern dedicated battery charger.

Car battery recharging times using a battery charger

car battery recharging time chart

So you can see by the chart that a totally discharged battery rated at 600-700 CCA takes almost 4-hours of charging with a 30-amp battery charger. At idle, that’s about how much your car’s alternator outputs, even though it’s rated for a maximum output of almost 160-amps (but that’s at 2,000-RPM, not idle).

That’s why your 1-hour idling period still leaves you with a dead battery.

Why you shouldn’t use your alternator as a battery charger

Most people think your alternator’s job is to recharge the battery. That’s partially true. Its job is to replace the power lost during startup. But its real job is to supply power to run your accessories. It’s not designed to recharge a totally dead battery and certainly not at idle speeds where the diodes and windings can easily overheat and jump ship.

Recharge a dead battery with a modern intelligent battery charger than can recondition your battery

Got an old charger from the 50’s or 60’s? Get rid of it. It doesn’t have the internal smarts to judge the condition of your battery and adjust the charging rate to recondition a sulfated battery.

A new battery charger like the Solar ProLogix is reasonably priced and great at putting your battery back in shape. For more information on car battery charging, see this post.

©, 2021 Rick Muscoplat




Posted on by Rick Muscoplat


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