This is actually a spin-off of my other thread about testing for electrical shorts... I have been researching things online and wanted the opinion of some savvy electrical-minded folks out there...

I have a 2005 MCSc with 55K. My problem is that my battery has started draining down overnight. The battery is a Duralast and is only 4 months old. I had it load tested today, and was told it was "good" but had a low charge.

I have NOT had flickering lights, or stalling, or any weird electrical gremlin activity. The car will start and run normally if I use the jump pack, or - if it sits overnight, the battery is dead. When the battery is too low to turn over the engine, I can still hear the click-click-click of the solenoid, trying...

Checked all fuses in engine bay and footwell: no blown fuses.

I listened for the the PS fan to stay on after engine is off - don't hear it (I assume it's fairly audible to hear from outside the car?)

Multimeter testing across the battery posts showed this:

Ignition off - battery reads 12.5V (after jumping it)

Ignition on, RPM at idle, no load: ~ 14 - 14.3V

Ignition on, RPM at idle, full electrical load (lights, wipers, AC, rear window defroster, radio) : 12.4-12.5V

Autozone guy tells me that this shows alternator is bad. I would like to think it's that simple, but I don't think it's the alternator. I say this because I have read that 12.5V under full load is not bad. It's undercharging, but that's with the engine at idle speed, too (temporarily on my own, no one else to rev engine while I check!)

If it were a failed diode in the alternator, then I should be getting over 1V AC reading at the battery. Even with full electrical load, I am getting only 0.15 or so Volts, which I am assuming is just AC ripple. So, I don't think the diodes are bad.

I am now inclined to think it's a loose connection, bad wire, bad ground strap/earth strap somewhere, that's draining down the battery. How do I go about testing these? Where are they? Is there a ground strap(s) for the engine? Has anyone else had a similar issue?

Open to suggestions!!

 

55k on an 05....
Sounds like the car sits a bit.
You might want to consider a battery matainer, or a trickle charger...mini's do tend to have lots of parasitic drain...that having been said, a full battery should last a month+, and a partially charged battery overnight, no problem.
Any sounds with the car off... Humm or buzz? Some cars have the electric ps fan stay on or even the electric ps motor...or the bcm could be draining the battery if there is an internal short of sorts,or any other sort of load. Do you have a tester to check the load off the battery with the car off?! This info vs factory spec would be very useful.

 

Might be the voltage regulator since voltage drops when loaded......not sure what model alternator we have on this car, but in the past I used adjustable voltage regulators from Dave Barton with good results.

[EDIT] Dave Barton's will not work since we have Denso alternators in our car and not the more standard Bosch.....

 

Are you driving mostly short drives, with perhaps high loads...driving lights, stero amp, maybe listening to the radio while you wash it?
The #'s you quote are not all that bad...

 

My other car is a 330xi, which i drive mostly during our winter (read rain) out here. The Mini has been almost exclusively driven from March till now - and I do a two hour (one hour each way) - with just radio (and on drive home, lights). Don't have the radio on when I work on it or wash it. This has been my patter since I got the car and I never had to worry about battery drains before, so I know somethings definitely wrong!

 

PS fan cutting off?

 

Here is what I would do, 1. Remove the connections to the battery and scrub the posts and terminal lugs until they appear to shine. Use of a battery brush available at any auto store is a great aid.
2. Charge the battery fully with the battery leads removed, I use a C-TEK 3300 which helps reduce sulphation problems.
3. Replace cleaned battery cable connections and tighten securely at the positive post the negative post just moderately tight. When the negative lug touches the post you may hear a slight tic or even a spark as the battery supplies current to the residual load of the Mini. Be sure it is only the residual load, that is-- ignition off key out, car doors closed, hatch closed, etc., etc..
4. Measure the voltage at the battery terminals, a freshly charged battery will indicate a voltage probably between 12.8 and 13.2 volts if not much time has elapsed since the charger was removed.
5. The residual load on an idle auto with doors closed would probably be less that 20 ma. If you have a current meter place it in series with the negative post and the negative terminal lug. Be careful here as many current meters can be ruined if you are not. Your meter probably has switchable ranges so start high and go to the lower ranges. A 100 ma range would be great to switch to after you determine the current load is within range. If you read less than 20 ma with only the residual loads you are good.
6. If your residual or parasitic current load substantially exceeds 20 ma remove fuses one at a time until you find the fuse that drops the current to a reasonable value (~20ma or less) and that would be the culprit circuit. Be sure to replace each fuse after checking the load before removing the next fuse in this Easter egg hunt.
Best done with two people as you do not want to be opening and closing car doors and possibly ruining your current meter by drawing current other than residual.
Good Luck

 

Did you rev the engine with the full electrical load (your alternator idle with load voltage seems quite low)? If so the voltage should have been around 14.0+. If it were mine I would first remove and clean the battery terminals, then I would do a full charge and check the battery voltage. If problem still not found I would check to see that a glove box, under hood or some other interior light is not stuck on (check for a parasitic load). If I still haven't found the problem I would suspect the internal voltage regulator in the alternator. I had a defective one a while back and it would drain the battery overnight, but it would charge enough while driving to keep the battery up.

Been there done that.

Hope this is helpful to you.

Bob