Asked in AlternatorsCar Batteries
How do you tell if the battery or the alternator is bad?
July 24, 2010 8:54PM
Buy a digital multimeter ($10-15). with the car off check voltage at the battery. Should be 12-12.5 volts. If not start the car (boost if needed) and check the voltage again. Should be 13-14 volts at least with engine running then the alternator is usually o/k. If less than 12 volts you can suspect the alternator. Use the multimeter and check the big power wire at the back of the alternator. With the car running (BE CAREFUL OF YOUR FINGERS!!!!!) put the positive (red) probe of the meter on the terminal on the back of the alt and neg(black) on the neg terminal of the battery and check the voltage again. If still less than 12 volts check to see if the voltage regulator is in the alternator (call parts supplier if need be) most are in the alternator, and if it is replace the alternator. If it has an external voltage regulator check it also or just replace it depending on cost (most are cheap).
Most of the time people have a bad battery which kills the alternator. The vehicle runs off the battery and the alternator recharges the battery. If the battery is only at 50% for example the alternator has to work overtime to keep it charged which works the alternator to death and leads to premature replacement.
with 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.