mysterious flat battery?

[mk1magic]

Hi
Right I have a very random problem with my e21, it keeps flatterning the battery randomly? It started a while back when I kept leaving the stereo on (off the perminant live so to use in eveings at shows etc). This killed the battery so I gave it a charge and like a muppet I did it again a week later!
This really did kill it so I decided to change it with a good diesel equivelent from a reputable parts store near me in coalville (shepherd and wilson). I had it running sweet as for a good month then one very sunny sunday afternoon got in it to pop down to the shops and....click nothing!
I hadnt left the stereo on or the lights etc etc nothing?
I was about to go on my hols so left it for a couple a weeks. Got back and eveytime I wanted to move/use it I used jump leads....gave it a charge the otherday, popped it in started first time, just gone into it after 2 days and...yup its flat, nothing on etc etc

I'm meega confused and a bit miffed as it wasnt the cheapest battery to replace!
Any ideas?
Cheers
Arron

Read more: http://retrorides.proboards.com/index.c ... z1Vh2lp0wS

[Peter V.]

In the E21 there are not much things that work directly from the battery. You could leave your lights on but you did not. A big battery in the car is not a garanty that it has more capacity because the alternator gives a certain amount of amps and that's it.

Have you measured the charging of the battery when the engine is running? Try to disconnect your carradio with a switch so you can use it at a show but then have the possibily to disconnect it completly.

[Reck]

As Peter said - check the alternator is actually charging the battery enough. I had an alternator that was putting out less than the car was drawing. Most motor factors have a testing unit that they'll use for free if you don't have a voltmeter. I can't remember what the output of an E21 alternator should be - I think it was 14v+ ?

If it isn't putting out enough it could be something dead simple like a loose fanbelt.. In my case I had to get a new unit, but I think you can get the brushes seperately if you fancy taking it apart.

[Duracel79]

Is the battery definitely draining or could it be a sticking starter motor solenoid?

When it fails to start are there all the lights on the dash bright etc?

Ben

[TopCat]

Try taking out the boot light bulb- they can remain on with boot closed.........

[Reck]

Wish I had a bulb in the boot.. Can't see FA at night!