How can you tell if it's your battery or alternator that's bad?

Answer 1

Battery or AlternatorThe battery provides the juice needed to operate the starter motor. It also provides power to the electrical and electronic devices in your car when the engine isn't running. Every time you start your car or play the radio or operate the power seats or windows while the engine is off, you drain the battery a bit. The alternator -- which is really a generator -- replenishes the battery when the engine is running and provides power to your car's electrical systems when the engine is running.

If your battery is totally drained, the car will fail to start. You may be able to "jump-start" the car using another vehicle. But why is the battery dead? If you left the lights on, even a small dome light inside the car, that could have drained the battery. Jumping the car and then letting it idle for a while can replenish it, but if the battery is old and near the end of its service life, it may not be able to take and hold a charge any longer.

Or the problem could be the alternator. Perhaps it is no longer able to recharge the battery. If that's the case, the battery -- even a brand new one -- will eventually run down, and rather quickly, because your car's electrical system will be running off battery power, instead of from power supplied the alternator. When the battery goes dead, your vehicle will stall, because there is no power to run the electrical systems.

Answerwith the vehicle running(even if you have to jump start it), remove one of the clamps off of the battery. if the vehicle dies immediately, you have a bad alternator. if it runs for a little while before it dies, a bad battery.

This might have worked on a car thirty or more years ago but is actually potentially very harmful for a modern car. The cars now are designed to have a battery as an integral part of the charging system and if it is disconnected can send voltage spikes through the system and burn out your computer. Also the alternator is designed to put out a certain amount of current and this test would only tell if the alternator is putting out enough current to keep the engine running, not if it puts out enough to keep the battery charged, run the lights etc. The only real way to tell if the alternator works is to put a voltmeter on the battery and see if the voltage goes up after the car is started. Then to tell how well it works, put an amp meter in line with the alternator and see how much energy it puts out when the various circuits are turned on.

A bad battery can be hard to detect sometimes. It can work fine then all of a sudden fail to start. Then next time you try it works fine again. The best way to test it is with a load tester at an auto parts store or shop.