E39 - 4 month old battery flat

[Origin Unknown]

Replaced the battery around 4 months ago (proper Bosch battery).

Just about managed to start the car this morning but thought little of it. Just been out now and it's doing that clicking thing and all lights flickering. I.e. flat battery

So what has likely caused this then? I do a 70 mile commute a day so it's getting a good run. The alternator was replaced 2 years ago so I reluctant to suspect that. Final Stage unit/Resistor? How do I test for this?

[E30M3SE]

Pull the large 40A fuse (76) behind the glove box, charge up the battery and see if this resolves you battery drain issues.

http://www.bmw-planet.com/diagrams/release/en/zinf...

You should really put a meter into the supply circuit and measure the parasitic drain, should be no more than 40mA after 16 minutes, it may not the FSR.

[Origin Unknown]

Thanks. Div question but what does this fuse control BTW?

[E30M3SE]

The heater blower.

[rassi]

Known E39 problem is the final stage resistor ("hedgehog") - may not be your problem but if your heater and speed of the blower is acting erratically then this could be the problem. Do a search for FSR.

[4rephill]

Origin Unknown said:

The alternator was replaced 2 years ago so I reluctant to suspect that.

Whilst you're reluctant to, don't rule it out!
Too many times people rule out parts that have been replaced in recent years only to eventually find out that the replacement part has failed.

If you can get the car started, check the battery voltage with the engine running, it should read @ 13.5~14.5 volts.

If it's reading less than this then the alternator is not working properly.
If it's reading higher than 14.5 volts then the alternator is over-charging and could have damaged the battery.

[Origin Unknown]

In the end, I jumped it using my wifes car and took it for a god run for 1/2 hour. It's been completly fine ever since. No idea what killed it but thanks for the advice above.

[Sump]

The extreme cold finds weak batteries.