Help! Why does my bettery keep dying?

[aerosam]

I went away for the weekend, parked up my BMW on the Friday, went to go to work yesterday, battery flat. Got a jump start and drove to work (about a 15 min drive). Stopped for petrol, it re-started fine. At the end of my 8 hour shift I came back to the car to find it dead again.

Got another jump start and drove home, left it running and tested the battery voltage - 12.6v at idle rising to 14v when revved. Now if I remember correctly the voltage needs to be 13.3-14.4v to charge the battery. So it seems that the alternator should be ok - right?

Well it's dead again this morning, now sitting on the drive on a battery charger. It's gotta be either the alternator, battery or a dodgy circuit leaking all the charge.

Help! I can't keep begging jump starts and leaving it running all the time.

[AshG]

cant be the alt as the voltage goes up when its running. its either a dud battery or something switched on sucking all the juice. try a battery off another car to eliminate.

[Bonzo]

Got another jump start and drove home, left it running and tested the battery voltage - 12.6v at idle rising to 14v when revved. Now if I remember correctly the voltage needs to be 13.3-14.4v to charge the battery. So it seems that the alternator should be ok - right?


Unfortunately not always the case. :(

An altermator may still charge when running but cause the battery to drain when not turning .

What you have described is a classic symptom of alternator failure.

As Ash said, try to borrow a battery from somewhere & double check that nothing is left on to drain the battery. The boot light is a favorite.

I have changed 3 alternators this year, all of them doing the same thing.

Pitty all three customers had been out and bought expensive batteries that they did not need. :(

If you have an auto electrician nearby it is worth having things tested, it may save you money in the long run. ;)

[londonsean69]

An altermator may still charge when running but cause the battery to drain when not turning .

This happened to a brand new alternator on my Mums Fiesta. The diode pack was blown, and you can't buy them seperate.

Sean

[aerosam]

I've managed to get the thing going again so i can get to work, now I'm here I've disconnected the battery in the hope that it will hold charge till the end of my shift so I can get home. This should also prove whether it is the alternator or the battery at fault.

If it's the alternator then I have a spare at home on my other engine (destined for my roadster) and I'll start changing it tomorrow. Problem is getting access to it, (FYI it's an E34 BMW 540i V8) space is extremely limited and the bottom of the engine bay has more guards than my mum's fireplace.