Dead battery

[MiniMe]

I took the mini out of the garage for a spin last weekend. It had spent the best part of the last 4 months in the garage thanks to our lousy summer. The battery was flat so I gave it a jump and took it for the half hour spin into college. When I returned to the car after 3 hours, the battery was flat again so I got a good Samaritan to give me a jump. On the way home after about 20mins driving I called into a shop - battery was dead again.
There's no problem with electrics while the car is running.
Does this sound like a dodgy earth or something else like a knackered battery?

[tommy1]

maybe check the belt is turning properly and has teeth to charge the battery while driving. i had a similar prob a while ago and my belt was knackered, hope you get it sorted

[macker]

If their are as you say no problems while the engine is running it would suggest that the alternator is running perfectly and providing all the power needed to drive the car, which would mean that the belt is fine and not slipping,

could just be that the battery has had it and no matter what you do it won't hold a charge

[mark halford]

battery light would be on if alternator not charging battery mine came on when belt came loose id say battery had it no matter how long you drive it wont charge.

[MiniMe]

Thanks for the replies. I don't think the alternator is the problem for the same reasons you have described - no warning lights and no problems with electrics once the car gets started.
I'll try to get my hands on a trickle charger to see if that makes any difference. I've installed a battery isolator to help to prevent the battery from draining while the car is in storage. I don't think the cold weather from last winter and almost constant storage for the last year did it any favours.

[spud1979]

Im having same problem with one of my cars. I can't figure out why it is dead every time I come out to the car. The previous owner had the inner light disconnected. Would that have something to do with it?

[keith]

there might be a current "leak" allowing power to leak out of the battery slowly over time. or the battery could just be old and beyond its service life.
test by disconnecting the positive lead at the battery and measure any current flowing with *everything* switched off.
things that drain a battery over time include a the alarm/immobilser, faulty switches.

one way to guard against this slow battery drain is to disconnect the battery when leaving the car for an extended time. note that this may disable the alarm.

if the battery fails to charge in normal use, *and* there is 13.5 or more volts at the + battery terminal during charging, or if the battery still loses power while disconnected its probably beyond help.

*edit* its also good to keep the battery from getting too cold for too long - if you are laying up the car outside over the winter, pull the battery and put it in the house.