How to Test an Alternator

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How to Tell if a Car Battery is Dead....5

Make sure an alternator is charging a car battery. Learn how to keep a new car battery working in this free video.

Part of the Video Series: How to Change a Car Battery
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Video Transcript

Hi I'm Aaron Gregory on behalf of Expert Village, and we're going to talk about what happens when your car battery dies. As soon as I put the key in the ignition, I'm going to look for some of those other indicators of whether or not the car has battery power. Good sign. When I turn the key the dash lights come on and the clock comes on. If I turn the headlights, I hear them pop up quick and they're bright. If I turn the key to crank, crank's the engine fast, needs to crank fast to start. Sounds good! Now we need to figure out why that battery died in the first place because we don't want to sacrifice another battery to a car that's going to destroy it. Maybe there's a problem with the charging system, such as the alternator. A simple way to test the charging system in your car is to do a comparison of voltage at the battery when the car is running and when the car is not running. The voltage in the battery and in the electrical system should be higher than normal battery voltage when the engine is running. We're going to start our simple test of the charging system by testing the battery voltage with the car turned off. We're connecting our multimeter to the battery again, positive and negative, and I see about twelve volts in the battery. Twelve volts is about average for battery voltage. Sometimes that varies up to as high as 12.8 for a new battery, and a little lower for an old battery. A battery under ten volts shouldn't be under your hood. Let's start the car and compare the running voltage with our eleven volt reading that we just took. Okay, now that the engine is running, the alternator is turning and it should be charging the battery, and providing enough spark to keep the spark plugs firing. The battery is mainly just for starting the engine. So let's do that check again. Negative to negative, positive to positive, and if you look at the meter we have about 13.5 volts. That means that the charging system and the alternator are doing their job, and this battery will not die before it's time. Today we went over how to change a battery in your car. From turning the key and nothing happening to looking under the hood and checking for corroded wires, loose cables, to disconnecting the battery, testing out the multimeter, selecting a new battery. We used some simple tools. We have an analog multimeter that goes for about $25.00 at just about any electronics or auto parts store, and it's very useful for diagnosing burnt fuses, dead batteries, many other electrical things. It comes with thorough instructions so you can learn how to use it when you buy it. We also used a common 3/8 inch ratchet and extension with a 10mm deep-socket for the long J-bolts on the battery, 10mm open-end wrench, and a 13mm wrench for the battery cables. If you do have a dead battery, I hope that it's a project you take on yourself and you learn something and gives you a good chance to do some preventative maintenance on your car, and save some money in the process.


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