While your engine is running, your car's alternator charges your battery so it can power all the electronics in your car, including the spark you need to start it. If your alternator isn't working right, you could end up stranded in the middle of nowhere. Here's how you can test your car alternator for power, so that doesn't happen to you.

To test your alternator, you'll need a voltage meter and amperage meter, or a multi-meter that reads both voltage and amperage.

  1. Check the battery Before you can tell if your alternator is working correctly, you need to see what's going on with your car battery. With the engine off, connect the positive side of your voltage meter or multi-meter to the positive terminal of your battery, and the negative side to the negative terminal. The reading should be about 12.7 volts. If it is below 12.4 volts, the battery may need replacing [source: Autos].
  2. Check the alternator voltage Turn on your engine, but don't press the gas. When the engine is on, the alternator is supposed to send electricity to the battery. Check your voltmeter or multi-meter again and read the voltage. If your alternator is working, the voltmeter should read between 13.8 and 14.2 volts [source: 2CarPros].
  3. Check the alternator amperage Disconnect the voltmeter and connect an amperage meter (if you aren't using a multi-meter) to your battery the same way as you connected the voltmeter. Turn on all the electrical equipment in the car: lights, stereo, wipers and everything else you can. Let the engine idle at around 1200 rpm. Check your car's manual or parts manual to see what your alternator's maximum amperage should be. The amp reading on your meter should be near the maximum output. A 90-amp alternator will output approximately 88 amps. If your amperage is too low, then you probably need a new alternator [source: 2CarPros].