Battery drain

DUANE SKILLEN

MEMBER
2001 BMW 740
92,000 MILES

I have a drain of 12.6 volts with everything turned off.
I have unplugged each fuse under the hood and in trunk. I have unplugged the alternator to check diodes.
The only time I got a reduction in drain was when I disconnected the positive wire from battery to the fuse block above the positive terminal. The drain went from 12.6 to 11.7.
What else could it be?
I am checking for drain with negative cable unhooked and using voltmeter between cable and negative post on battery, set to 20 volt setting.
The battery charges when started up to 14 plus volts.
On off the battery shows 12.8 volts
If I leave battery connected it goes out in couple days.
Battery tests good and will charge to full.
This is a recent issue after twelve years of ownership.

1 Reply

STEVE W.

MECHANIC
13,465 POSTS

Do not test for voltage. You need to test for amp draw. That is because there will always be some drain on the system, for the clock and alarm system mainly. So you will always measure battery voltage.

Try the same test with that meter set up to measure amperage. Be advised that most meters have a limited amp range, some are 5 amp others might be 10 or a bit higher.
But also be advised that pulling fuses can lead to a false answer. Many cars have modules that will wake up when power is re-applied, so say you pull a fuse and see a big drop, then you plug it back in and see the big drain again. Is it really that fuse or did you wake a different module?

A much better way to test for parasitic draw involves using the meter to measure the tiny voltage drop across the fuses, then using a known draw chart to calculate the power a circuit is using. This way you do not wake a module or need to reprogram half of the things in the car from pulling the fuse.

This is a video by Charles on the topic, it explains the charts and how to test.
Be sure the chart you use matches the fuse type you have.
https://www.youtube.com/watch?v=lRcj1fQcWwU