Battery Drainage Problem

Last updated: August 25, 2000

From: Steve Cirian (steve@ScuderiaCiriani.com
Date: August 25, 2000

>I've been wrecking my brains trying to figure out why my car won't hold a
>charge. I replaced both the alternator and battery and it still drains after
>a few days. I have no idea what can be causing this ie, stereo, turbo timer, etc

When did it start? Did you do anything to the car about that time? e.g.- add a new piece of stereo gear, turbo timer, lights, etc, anything else electrical?

The method usually used for tracing this sort of thing down is as follows:

Pull the positive cable off of the battery, w/ the key off. (Be careful not to ground the wrench!)
Put a multi-meter in-line with the battery cable and the post you just removed it from. Set it to current (Amps).
If it shows any reading, something is draining it (i.e.- the battery is probably good). Make sure all the doors, etc. are closed, so nothing could be causing a drain, such as the dome light.
If the voltage meter does show a drain, then pull the fuses one by one, and see if any one of them makes a difference. If you pull one, and the meter goes to zero, the offending item is on that circuit. Get out the shop manual and look at the wiring diagrams to see what is on that circuit.
If you pull one, and it drops significantly but not all the way to zero, then you may have more than one bad part and will need to repeat the following steps more than once, for multiple circuits.
Check the electrical parts/equipment on the circuit, and look for anything obviously wrong with these, such as corrosion. If nothing visible, then it might be something internal to the parts. If there is a particular part that you suspect may be bad, try replacing it with a new one or known good one.
After this, just to make sure that you really have found the problem, look at the multi-meter again. i.e.- you may have more than one bad part. If the meter shows zero, congratulations, you are done!
If the meter still not read zero, after checking all of the fuses, it is something that is not on a fused circuit. Check the shop manual for everything that is not on a fused circuit, and and look for obvious problems or test by replacing with a known good part.

If you STILL can't find anything wrong, then you could just choose to live with the problem, and buy a trickle charger. These are connected to the battery and give it just enough charge to "top off" the battery. However, I would recommend fixing the problem, so you don't come out after work some day and find the battery dead, or after a trip find the car dead in an airport parking lot. Not good if in a bad neighborhood or dark parking lot, either :-)

The trickle charger does have a place, however, for storing the car for long periods of time.

Cheers,

Steve

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From: Steve Cirian (steve@ScuderiaCiriani.com)
Date: August 28, 2000

>Won't there be some drain even in a car that is functioning perfectly?
>For instance, some current must be used to keep you radio presets, flash
>the security lights, hold the trip odometer value, etc. Perhaps it would
>be good to test a known good car first and see how much current it is
>running after shutdown.

Good point. There will be some, but there would be some experimentation that would be required to see what the "normal" drain is on a known good car.

I am guessing that if there is enough to drain the battery in a couple of days that it will be significant. i.e.- easy to spot, "aha!"

Maybe pull each fuse and watch the multi-meter and see what fuse has the biggest drain, and start there, working your way down if more than one fuse shows a drain.

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Date: Fri, 15 Sep 2000 01:21:19 -0700
From: Max Cooper (max@maxcooper.com)

There was a thread recently about someone whose car would drain the battery in a few days if it was left to sit. I just went through the same problem on my car and it has mysteriously fixed itself.

I read in the Mazda Factory Workshop Manual that the "dark current" should be 20 mA or less. This is the current you will see if you put an ammeter between the battery and the connection with the key out of the car. I imagine that aftermarket alarms and stuff might increase this number, but it should be in the ballpark.

The battery on my car recently died what seemed to be a natural death. It had been slow to start lately and I knew the battery was on its way out. It lasted over two years, which I thought was decent considering the previous owner replaced it every year.

Right at the end of my previous battery's life, I noticed that the cooling fans would turn on after I parked the car, which had never happened before. I had to jump start it twice in one day and replaced the battery that night. The fans would still turn on after parking with the new battery, and I noticed that the display on my PMC (fuel computer) stayed on with the key out, which it should not do. I also noticed that if I went over 5 psi, the car would cease making any power even though it seemed to run fine otherwise. I didn't have time to work on the car for a few days, and it quickly drained my new battery.

I checked my fuses and hit the brake pedal with the battery disconnected a few times to be sure that I would be starting fresh. I connected the battery again and jump started the car. This time, everything was fine. I can get power over 5 psi now, and the PMC turns off when I take the key out. It seems like I just needed to clear the memory in the PMC and everything was fine again. I am happy tonight.

Summary:

20 mA is the current you should get with the car off
Try hitting the brake pedal with the battery disconnected to see if that solves your problem.