::.. Finding Battery Drain

[Qube]

Not on my car, but this should be generalist knowledge nonetheless. Have a Subaru Impreza whose battery goes dead (new batteries tested) every 2-3 days of non-use.

I'm pretty certain :) that there is an electrical drain on the battery with the key off. I'm going to remove the neg batt cable from the battery, then use a 12-volt lamp (or maybe voltmeter if I can find it) between the cable and the batt terminal. Basically hook one end to the negative battery post the other end to the negative batt cable disconnected. Unfortunately since I do not have a baseline for what is normal for that car, I'll have to guesstimate. I'll remove and replace the fuses one by one until the voltage or light dims.

Does this seem like a viable plan?

[SRR2]

The 12V lamp sounds like an adequate crude approach. You'd probably want nothing larger than a 3W side marker light without already knowing the size of the current you're chasing. Bleeding down an 80AH capacity battery in a couple of days translates to a nontrivial current, so this ought to work.

What are you going to do about lighting that's on when trunk/door/etc. are open? How long does this car take to go to sleep after shutdown? That can be messy to deal with on BMWs when you're trying to run down a parasitic draw.

[dacoyote]

Not on my car, but this should be generalist knowledge nonetheless. Have a Subaru Impreza whose battery goes dead (new batteries tested) every 2-3 days of non-use.

I'm pretty certain :) that there is an electrical drain on the battery with the key off. I'm going to remove the neg batt cable from the battery, then use a 12-volt lamp (or maybe voltmeter if I can find it) between the cable and the batt terminal. Basically hook one end to the negative battery post the other end to the negative batt cable disconnected. Unfortunately since I do not have a baseline for what is normal for that car, I'll have to guesstimate. I'll remove and replace the fuses one by one until the voltage or light dims.

Does this seem like a viable plan?

Yes

[Qube]

In a pinch, I'm going to hook up my inverter between the neg connection and get the digital voltage readout, then pull em one by one.

Hmmm how about I put an ammeter in place of each fuse and test?.... maybe a light bulb in a pinch. Would that work?

[SRR2]

The ammeter can work, but there may be ramifications. The problem with ammeters is ranging, internal resistance, and internal fuses. If you substitute the ammeter for the circuit fuse you just removed and you have the ammeter set for, say, 20A FS, you might not see the draw if it's too small. On the other hand if you set the meter for 200ma FS and the circuit parasitic draw is 500ma, you may well blow the internal fuse in your multimeter, and replacing those is usually a PITA.

All things considered, the 3W light bulb with a voltmeter in parallel across it would probably be the safest way to go. If the bulb lights or even glows with anything over a volt on it in a circuit that ought to be off, you have something to investigate further. If the bulb stays completely off and the voltmeter reads close to 0, it should be ok. No matter what happens, you can't hurt either the car's elex, the bulb, the voltmeter, or yourself.

[632 Regal]

the alternator has leaky diodes, have it checked.