I have a 2003 Ford Mustang GT, I had the battery tested and it will take a jump and start up so naturally I am thinking the alternator is bad. I take the alternator out and have it tested 3 times and it passes all 3 tests. At this point I take out all fuses underneath the hood to make sure there are no bad or blown fuses all look to be in working order. Terminals have been cleaned and new terminal connection have been installed recently.

I haven't driven the car in around 4 months so I thought the battery might be bad, but what has me stumped is the battery will take a jump and run for about 30 minutes then I can't start it again, like the alternator is not charging it but as stated above the alternator passed the test.

Is there anything I should look at before admitting defeat and taking it to a mechanic?

***UPDATE I had the battery on the trickle charger for around 24 hours now, but it doesn't event half enough power to work the locks, I am thinking a new battery is in order.

  • Where did you connect the jump leads to on your car, did you connect both leads directly to the flat battery?
    – HandyHowie
    Commented Jan 1, 2016 at 22:54
  • 1
    What is the voltage at the battery with/without the engine running? Commented Jan 2, 2016 at 1:27

3 Answers 3


Your description sounds very much like the battery is totally dead and needs replacing. When you jump lead a car, you are replacing the installed battery with the one from the other car for starting the engine. Once the engine is running, all the cars electrical functions will be running from electricity generated by the alternator.

At this point, a healthy battery will charge from the alternator and will be ready to start the engine next time. Your battery sounds like it is not accepting a charge and is therefor useless and needs replacing.

The only thing that confuses this is that you say you had the battery tested. How long ago did you test it and how was the test performed?

If the battery is truly in good order, then can you tell me where you connected the jump leads to on you car, did you connect them directly to the battery?

  • I'm not sure why this was downvoted ... I +1'd it for you nonetheless. Commented Jan 2, 2016 at 1:47
  • I think you might be right and the battery does need totally replaced, I put it on the trickle charger overnight around 24 hours and theres not enough power to even run the power locks or turn on the interior lights let alone crank the engine. The last time it was tested was around 2 weeks ago, the test was performed at my local autozone store. The leads were connected to the battery terminal cables that were connected to the terminals themselves.
    – Traver
    Commented Jan 2, 2016 at 15:09
  • If you had connected the jump cables to some other terminal on the car, I may have suspected a bad connection somewhere, but since you put them directly to the battery and it started ok, then the battery does look like it is at fault. Presumably the battery positive and negative connections are clean and tight.
    – HandyHowie
    Commented Jan 2, 2016 at 15:27
  • Are you able to test the battery voltage after you charged it?
    – HandyHowie
    Commented Jan 2, 2016 at 15:27
  • went and picked up a new battery that seemed to be the issue.
    – Traver
    Commented Jan 2, 2016 at 20:24

Do a load test on the battery. You will need a multimeter or voltmeter and a fully charged battery.

  1. Connect the meter leads to the battery posts (positive with positive and negative with negative).

  2. Put the meter where you can see while starting the car (on the windshield laying down works).

  3. Make note of how many volts does the meter read with the car OFF.

  4. Start the car.

  5. Make note of how many volts did the meter read while the starter was working. This is where you are doing the load test.

  6. Make note of how many volts the meter reads **while the car is running in idle.

    • What was the reading during the load test?

      Anything under 11.8V means your battery is bad.

Repeat the test multiple times. Making sure you give the battery enough time to recharge between tests. Meaning: don't test it twenty times in a row.

What if the test comes out OK?

Take your car to the mechanic. :)


Check the cables running from your alternator. Both cables follow them all the way through and make sure they're not burnt and have good connection.

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