How to tell if it's my alternatorr or battery?

(2011-02-05 15:20:33)

1. Meena Says: Last night my car wouldn't start. It's been sitting in the yard for around two weeks now without being started so I figured it was just a weak battery so I asked my husband to jump it. The car still wouldn't start but it sounded like it was trying to and the dashboard lights got brighter. We took the battery out and got a garage to charge it for around 40 minutes, brought it home and the car started right up. I let it run for about 15 minutes, turned it off and it started fine again. Then I turned it off and left it for about 2-3 minutes, tried it and it wouldn't start again. My question is, do you think it's the battery or the alternator? I just replaced the started and the alternator last year so I'm hoping it's just the battery. What do you think?

2. Paul S Says: START CAR..DISCONNECT 1 BATTERY CABLE WIRE,IF CAR STAYS ON THEN ALTERNATOR IS GOOD...THATS THE BEST TEST...ALSO HAVE YOU CHECKED THE ACID LEVEL IN BATTERY CELLS,IF LOW OR EMPTY FILL WITH CLUB SODA..LOW ACID LEVEL BATTERY CAN'T MAINTAIN A CHARGE...

3. Chris Says: The starter and alternator should be ok, batteries don't last forever (4 or 5 years is good). Check all the terminals and connections are tight first, try a battery swap to start up, if that works it looks like the battery has gone to 'battery heaven'.

4. bandit_60 Says: does the amp light go off when you first start it? if the amp light goes off when it,s started it means the alternator is working. if you have a dead cell in the battery or a weak cell then it won,t hold the charge. i say it,s the battery.

5. some1 Says: i am not a car wiz but one of mine did this they told me if the head lights had been diming while it was running that is a sign its the alt. But if not i would think you have a dead battery

6. haze Says: sounds like a dead cell in your battery. to check, crank the car and pull the neg battery cable. If it stays running your alt is fine and its the battery.

7. pedro7of9 Says: battery..also make sure cables r clean and tight
next time it running drive to auto store for test of battery,,alternator..its free
and idleing a car does NOT make enough energy to charge a battery..

8. john hobart Says: I think that you should have done voltage checks to make a accurate decision right away. before you go spend $50+ for a battery or an alternator, go buy yourself a digital voltmeter at radio shack and check the battery resting voltage in the morning before you start it and check to see that if/when you get it started that you are getting at least 13 volts while it's running. you can find your answer in just a few minutes.

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=multimeter&origkw=multimeter&sr=1

It does sound like your alternator though. letting your vehicle just idle for 15 minutes after it was just jumped is not enough time to get the battery charged up, assuming that your alternator is actually working. you would need to get the RPM's up for several minutes to charge the battery back up to full. The best way to do that is to drive it and if it's a manual, don't stall it.