Is my battery knackered or is there a short somewhere?

[Robin427]

The battery in my tintop (BMW) keeps going flat. It is the original (13 year old) battery so I'm pretty sure that is the culprit. However, a year or so ago, a similar thing happened and it turned out that the battery was shorting through the alternator (which I replaced). What's the best way to check if my battery needs replacing?

[Wilburn]

Hi,

We currently have same fault on our Audi. When you are not going to use the car for a day or two, disconnect the battery, then when you come to start the car a day or 2 later, re-connect the battery and if it starts then there is something draining the battery. If it doesnt start then your battery is probably dead and you will need a new one - assuming that the battery is charging correctly!

[Grease Monkey]

Your local Kwik Fix (Why didnt the blasphemy checker pick that one up??) will have a load tester that they just pop on the top of the terminals and it'll give you an indication.

Either that or remove one terminal and put an ammeter between the cable and the battery pole. You should get a minimal discharge to cover running the clock, radio backup, alarm receiver etc. As long as its in the milliamp range you should be ok.

Next, get it fully charged, ie 60 or 70amp/hours worth, check you've got a good 13 volts, then leave the headlights on for an hour. Headlights are what, 55watts each = 110watts, plus 10watts tail lights plus 10 watts front side lights, plus dash lights etc, you're looking at 150watts? at 12 volts thats 12 amps?

The battery should be able to do this without dropping past 50% discharged and retain a good 13 volts and still turn the engine over fine.

If it doesn't then it's not holding its full charge and is good for the bin.

(You could also check to see if there's any water in it....)

[Robin427]

I remember it did "boil dry" and although I topped it up, it was probably too late. If I can get the effing thing started I'll drive it to a motor factor and buy a new battery. Couldn't start it earlier today...

[SimonP]

BMW OE batteries are damn good. The only seem to fail through neglect or a charging fault.

Check the battery voltage with the engine running is in the range 13.8 to 14.4V - expect the lower end of that range on a BMW.

I killed the battery on mine by leaving the HDD mp3 player switched on for a week while I was away. I got it to charge up, but it was never 100%. Strangely enough it failed after about a year......

Simon.

[Andy302]

Once deep discharged car batteries do not recovery well, especially if boiled dry!!!

In your case Id go buy a new one.

Lack of care is the biggest killer of car batteries around.

FYI drop testers or load testers are not considered reliable or a good way to test a battery by people who make batteries! I use a tester at work was recommended to me by Yuasa which uses conductance and therefore does not take any current from the battery under test.

Also after charging leave the battery over night to stabilise before testing.

[Robin427]

I bought a new battery for £35 at the local motor factor - it's worth about the same as the car. The BMW battery went to the tip.

Thanks for your help guys... there's plenty more wrong with the damn thing so don't rest on your laurels yet!!!

[Grease Monkey]

Robin,

Why dont you stop spending money on the tin top and get the bloody GD finished.! Summer is just around the corner!

[Robin427]

I'm waiting for some parts, including a throttle cable which means I can't easily get the car out of the garage to work on it properly...

...besides, I needed the tin-top to be able to drive around to get some other parts for the GD