Rick's Free Auto Repair Advice

How to charge a car battery after a jump start

Learn how to charge a car battery after a jump start

If you think you can charge a battery after jump by letting it idle, think again. It you think you can recharge a dead battery by driving around the block, that’s also not going to cut it. And then there’s idling a dead battery to recharge it; recharging by idling takes a minimum of 4-hours!.

From Interstate Battery:

“You can charge your car by driving, but only when you drive at highway speeds for a significant period of time. The alternator charges your car’s battery, but some factors may affect the alternator’s ability to charge the battery, including:

The amount of current diverted from the alternator to the battery
How long the current is available (the drive time)
The temperature of the battery
The age of the battery
………..the alternator is not a true car battery charger. It’s running all the onboard electronics. The best way to charge your battery is with a charger. The following situations may also keep the alternator from adequately recharging your battery:Battery drained because an interior light was left on
Battery drained because a vehicle was not driven for a month or more
Car only driven at slow speeds or in stop-and-go-traffic”

Late model vehicles use a LOT of power when idling so you can’t charge a battery after a jump just by idling

And your alternator only produces about 1/3 of its maximum output at idle speeds, so there’s little or no power left to recharge your battery. Don’t believe me, see this post on how much power late model cars use when they’re running.

Charging your car battery with a modern digital battery charger will give you the best results

A dead battery often has sulfation issues. Many digital battery chargers have a recondition mode than can reverse minor sulfation. It’ll reverse the damage and fully top off your battery. I highly recommend investing in a modern digital battery charger, especially if you live in cold regions.

Your alternator is NOT a charger

The alternator’s job is to supply the power needed for all electrical items on the vehicle, plus replenish the battery from the last start up. But the alternator is not a battery recharger so much as it is a battery maintainer. If the alternator has to recharge a discharged battery, the alternator will become over-worked, which will shorten its life. That’s especially true if you try to recharge a dead battery by idling your engine. That puts a tremendous load on the alternator, which causes it to overheat and fail prematurely.

Maximum Alternator Output Only Occurs at high RPMS

After you start your car with jumper cables, the voltage regulator sees a discharged battery and commands maximum field current to the alternator. But at 600 RPM, the alternator can only provide about 1/4th of its rated output. Let it idle for a long period and all you’ll do it overheat the rotor windings and burn up your expensive alternator.

A 110-amp alternator can only output 110-amps at RPMS of 2,500 or more. Your car needs 30-50 amps at idle just to run the fuel pump, ignition coils, computers, and safety system. So there’s really not much power available to charge the battery. See this post on how much power a car consumers when running. So don’t even think about letting it idle to recharge the battery.

A battery charger costs $60. A new Alternator $600

Not exactly brain surgery, is it?

The correct way to deal with this situation is to jump the battery (using a jumper pack is much safer than jumper cables) and driving it to a place where you can place a REAL battery charger on the battery. Once the battery is fully charged, you can conduct a full charging system test. Here’s how to do your own.

Connect a digital volt meter to the battery terminals. A fully charged battery should read about 12.7-volts. Start the engine. The reading should jump to around 13.5 volts. Leave the engine running and turn the blower motor to HIGH. The reading should dip down and then rebound back to 13.5 or more volts. Leave the blower on and turn on the headlights. It’ll dip again and rebound. Then turn on the rear defroster grid. Next, sit and watch the meter for about 10 minutes with the engine running and all those accessories on full blast. If the voltage stays the same at 13.5 or higher, the charging system is fine. A weak alternator will start pulling the voltage down in small increments because it can’t keep up with the electrical drain. The biggest mistake DIYers make is slapping on a volt meter, seeing a 13.5 or greater volt reading and calling the alternator “good.” You have to fully load it to test it.

There’s a right way and wrong way to test an alternator.

© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat



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