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Some will tell you to disconnect either the positive cable with the car running to troubleshoot the problem and determine if it is the battery or alternator. This is very poor advice that could damage your vehicle. Never disconnect the battery cables while the vehicle is running for the following reasons:

  • you should never disconnect the battery with the engine running because the sudden change in voltage could damage your car's ECU (electronic control unit) or its EMU (engine management unit).
  • disconnecting either the positive or the negative wire from the battery whilst the engine is running tells you nothing about the battery: the battery could be bad or good. The engine will stay running with a bad battery disconnected as a good alternator will supply the power to keep it running.

The best advice is to start by measuring the voltage of the battery with the engine off. If you measure a low voltage - less than 12 volts - 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.

Your car's volt gauge should read about 14 volts with the engine running. It should read at least 12 volts with the engine off. 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 14.5 volts. If it is less, the alternator may not be charging.

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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.

For more information see the answer to the Related Question shown below.

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Q: How do you know if you have a bad car battery or alternator?
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Related questions

How do you know you have a bad alternator?

Turn car on remove positive battery terminal if car turns off alternator is bad


How do you know when alternator is bad?

Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps


What can make a car battery die when running?

It sounds like the alternator is bad. If the alternator is bad the battery will not hold charge when running


How do you know if your altinater as gone in your car?

If your battery doesn't hold a charge while running then your alternator is bad


How do you check if my alternator is bad or my batter is bad in my car?

If the car won't start, jump it off. While it is running, remove the positive cable from the battery. If the car dies, then your alternator is bad. If it keeps running then you may have a bad battery.


My alternator is not getting power from the battery The wiring from the battery to the solenoid and from the solenoid to the alternator is good is this a bad solenoid the car will start and run?

Check for a bad fusable link at alternator


Why will a car not start unless it is jumped started?

bad battery, and bad alternator not keeping it charged - also bad connections to the battery or loose wire connections on the alternator.


How to know if the alternator is gone in a 98 mercury mystique?

let the car run and then pull the negative battery cable off. if the car stalls, the alternator is bad! do you know where the bank 1 o2 sensor 2 is?


What are the reasons for the car battery light to go on?

its either the alternator the battery is bad or a loose or bad wire on the starter


Can a car start with a bad alternator?

Yes it can as long as the battery is charged . The purpose of the alternator is to put the power used back into the battery.


Can you start your car with a bad alternator?

Yes it can as long as the battery is charged . The purpose of the alternator is to put the power used back into the battery


MY car battery light is still on after the alternator and the battery has been changed. The battery cables are clean and well attached. Now what?

Battery light usually means alternator not charging. Most common solutions would be be a bad alternator, bad connection, or loose alternator belt. A replacement alternator can be bad. If it is ok than most likely a poor connection to the alternator.