Parasitic or Battery Drain at Rest

[retromodcoza]

• This post has been spun out of another post at LTPs suggestion. Its for record purposes for folks who face this problem in future.

It may help you to find the source of your parasitic drain. This is when the car, when off and locked, draws more power than it should, draining the battery faster than it should. Result

- Flat battery
- Battery that needs constant charging
- Battery that drains itself in a few days.

You'll need a multimeter. Then following these instructions to get the multimeter measuring the draw at rest
https://www.wikihow.com/Find-a-Parasitic-Battery-D...


I asked AI (Claude in this case) to list the components of my 2009 Vantage in the probable order that they would cause parasitic drain, then went pulling fuses.I did this by feeding claude the service manual which I found online. I put the voltmeter in series with the battery and measured the draw, then pulled each fuse one at a time to see what might be causing the issue.

After pulling 22 fuses, I found that 0.49A of the 0.56A draw was being caused by the alternator. According to AI there may be a leaky rectifier diode causing the draw at rest.

I'll need to replace/refurbish the Denso SC1 120A alternator, but at least my battery maintainer won't be working overtime anymore.

See my worksheet below.

[bogie]

Great, we should add reference to this thread in the FAQ post in this forum, under "Battery" section (not sure who can edit the faq I tried previously and failed)

[LTP]

I didn't need the credit, but thanks.

I'm glad you did this because, as I said in the original thread, I had no idea that a faulty alternator could flatten the battery of a car that's sitting parked, with everything turned off and the keys out.

In fact, I was sceptical to the point where I had to do some light Googling to check the AI findings above and confirm that they were correct - a faulty diode in the voltage regulator of an alternator can give a (comparatively) large quiescent current drain and flatten batteries.

[quench]

Thanks for posting! Alternator as the culprit - who knew...

Will be bookmarking this.