car wouldnt start on wed morning. hmmm, so a mate jump started me. car started no problem, after 30 secs of jump start....
ran it for ten minutes, then turned it off. went down to the car 4 hours later, battery completely flat.

so... battery is about 12 v.

when the engine is running, it meters at about 14 v, so the alternator seems to be ok

so im thinking- either a duff battery, or something is draining the battery when its left on its own....

suggestions?

The battery has probably gone west, but check the daft things like boot or glove box light stuck on first,
Steve.

I've had a lengthy lecture on batteries from a guy i used to work with who develops hybrid electric cars for people. Some of it might help.

Apparently seeing 12V at the terminals means very little about the actual health of the battery. It is related to the way in which the charge is stored in and on the plates inside the cell. The only way to check its real condition is to monitor the voltage drop whilst drawing current. If it drops to 10V or less when you're drawing 20A or so (maybe just your lights/stero on?) then it's dead.

He did explain a way of reviving them which involved briefly charging them with a ridiculous current in order to 'burn off' the sulphation on the surface of the cells but i don't think that's something to try at home.

Something else to consider is that a lot of modern cars have various electrical systems which prepare the car for driving in one way or another and so can continuously drain the battery. The new-new Mini can do this quite badly and also some Mercs and BMWs can do it. I once worked on a 5 series BM which would pre-activate various electrical systems if it sensed that the owner's key fob was within about 4 metres of it. One night we left the key near it on a bench and it was dead the next day. After a bit of investigation we found it drew 7A constantly if the key was just near to it, not even in the car.

Daft!

just charged the battery with a posh "charger/rescuer"

metered after - about 12.8 V

then tried to start the car - half a laboured turn, then nothing

then metered battery - 11 V

car starts happily from a jump start though...

looks more and more like the battery ...

If you can take the filler tops off, get someone to crank it and observe the indvidual cells, you will possibly see one gassing dramaticly, iff so that will prove it,
Steve.

If you take the battery to a decent place they can put a load tester on it - a huge resistor with a voltmeter attached. That puts a heavy load on the battery during the test, and is a good indication of its fitness.

In my experience I've found that modern batteries either work properly, or they're knackered; there doesn't appear to be an intermediate state of "sort of working".

I'd budget for a new battery - don't discount the dealerships though - I was looking for a new battery for my daughter's Golf Gti, and the VW dealer was the cheapest by quite a few pounds, clearly beating Halfrauds. I had the same experience when I went to the Toyota dealer for my own car a while ago.

[Edited on 5/5/07 by David Jenkins]

You should be seeing well over 13volts on a good battery, sounds like one of the cells has given up to me....

I'd put money on the battery being screwed.... I seem to recall seeing something about a trick they do in Cuba which involves hooking the battery up the 110v AC (ie mains) which blasts off the sulfate....

Obviously a very bad idea though (if the battery explodes you get sulphuric acid everywhere).....