cavalier battery running down-Alternator or battery?

[specialbeat]

Hi all.
I have a Vauxhall Cavalier (J plate) and was wondering if anyone knew about the electrics?
Basically I went to get in it the other day (Not been driven for approx 4 days)and it would not fire up (Starter is ok!) So I proceeded with Jump starting. I started the engine and left it running for about 10-15 minutes (I didn't dare go for a drive incase I stalled it). Stopped the engine and immediatley started again. Fine!. I went back about and hour later and the d**mn thing wont fire up again. Is this my BATTERY OR ALTERNATOR? Please help me.

 

[IanDB]

Take it down to your local garage/battery stockist and get them to test it. They can do a battery discharge test and check your alternator charging capability.

 

[shaggy]

I'm assuming the first time it wouldn't start was because the battery voltage was too low. When you jump started it and left it running for 10 mins that wasn't long enough to charge it, so the voltage was still low, hence it didn't start later. Although it started imediately after running for 10 mins and switching off the engine, the engine and oil were warm so required less energy to turn over. If you have a cheap voltage meter you can do a basic check on the alternator. You should see the voltage rising to something like 14+ volts as you bring the revs up to perhaps 3k. Have you looked to see if the red charge light is off?
The only thing about letting the battery stockist check the battery is that he would like to sell a new battery.

 

[keyplayer]

Check that your glove box/boot etc lights are going off correctly.

 

[jofly24]

Make sure that your earth straps are not loose or broken. There is one from the alternator to the alternator bracket and also one from the gearbox to the bodywork. If any of these are broken or loose this can give a problem like a flat batt or faulty alt.