Rick's Free Auto Repair Advice

Charge car battery after jump start

How to charge a car battery after jump start

Every day I see people post engine starting problems on auto forums where they have jump started a dead battery. Then they think they can charge the dead battery by driving. The scenario is always they get jumped and then let it idle for just a few minutes or drive off to “charge their car battery.”

An alternator needs power to generate power

An alternator generates power by passing an electromagnet across a stationary coil of wire. In other words, it needs power to make the electromagnet in the first place. An alternator isn’t like a DC generator with a permanent magnet. An alternator needs at least 12-volts and at least 2-amps to create an electromagnet field. That means you have to start charging your dead car battery with a decent battery. As the rotor spins, the magnetic flux lines are cut by the stator and that feeds the generated power back to the battery. So you really can’t charge a dead battery with an alternator.

Here’s where things go bad. Right after jump starting a car, a battery charge a car battery by drivingmay show 12-volts. But since it’s “empty” it won’t have any amps. Volts without amps can’t create a magnetic field. So driving away after jump starting a completely DEAD battery will NOT charge up your battery. Even if the battery has some amps, most drivers drive off and crank up the blower motor, headlights, and rear window defogger. The discharged battery can’t output enough power for all those loads plus the alternator.

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 in the rotor. 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. So don’t even think about letting it idle to recharge the battery.

A battery charger costs $40. A new Alternator $350

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. Read this article to make sure you don’t do it wrong

© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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