How do you know if you have a bad alternator or car battery?

Answer 1

One should start by measuring the voltage of the battery with the engine off. If you measure a low voltage then it is pretty easy to understand that this will not start the car. At this point, you could charge the battery, remove the battery and have it tested (which is free at a car parts store) and if necessary replace it by a good one.

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Your car's volt gauge should read about 13.5 to 16 volts with the engine running. It should read 12 volts with the engine off. You mentioned the old battery had a lot of corrosion on the cable connections. Corrosion can cause a voltage drop to the starter, causing it to turn the engine over slowly or not at all. Auto parts stores have testers for batteries.

These simulate the load of the starter on the battery and measure whether the battery has enough juice in it. The machine can also test the alternator while it's on the car to determine if it is charging when the engine is running.

Also there is a quick and dirty test you can do with a multimeter (Note: these are pretty inexpensive and are very useful in other DIY projects involving any electricity.) . Set it to the "Volts DC" scale. With the engine running, touch the red probe of the meter to the positive "+" post and touch the black probe to the negative "-" post of the battery. It should read 13.5 to 16 volts. If it is less, the alternator may not be charging.

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One should never disconnect either battery cable with the vehicle running/on, as it could cause an explosion, leakage, or serious electrical damage to vehicle.

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You need to hook it up to a battery charger for several hours at 3 or 5 amp slow charge- then bring it to a battery store or maybe a kragen or pep boys auto store and they load test it for free.

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How old the battery is generally your first clue. Most only last about 5 years. Second, are you in hot or cold climate? Cold weather drains a battery so the cold cranking amps on it have to be high enough to allow cranking of the battery in really cold wheather.

Age of the battery can reduce the power of the amps needed. Heat in really hot climate can also kill a battery over the years. If you can take it to a auto parts store they can put it on a machine to check to see if the battery is fully dead or is still good. If it is still good and just has a low charge, they can fully recharge it. If it is still in the car and you want to check it without going to the store, try to crank the car. If it drags a little and you are not in a cold climate it just may just need a boost (If in a cold winter climate it probably needs recharging).

You can boost with another car and cables or buy the instant boost sets they have out now (no other car needed and they work great). If it cranks with a loud turnover noise and the car just doesn't start it could be another problem. You also need to remember that the alternator is the equipment that charges your battery. If your alternator is bad the battery will just drain like a flashlight battery until it is dead. The alternator keeps your battery recharged as your car uses it while the engine is running. Just hope its the battery because an alternator is much more expensive.

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