Help diagnosing an electrical issue

[Jimswift]

About my car:
I've had my 2006 Z4M for two years now and its sitting at roughly 13,000 miles. Last year I was having a few strange issues and with the help of this forum figured out that the battery needed to be replaced. Symptoms went away and I figured the problem was solved as it never troubled me for the remainder of the year.


The Problem:
Fast forward to this year as I recently took the car out of storage. I park in an underground apartment parkade so I was not able to leave the car hooked up to a battery tender full-time (though I did hook it up for a few hours every couple of weeks). When taking the car out of storage the battery was fairly dead, as expected, but after leaving it hooked to the charger for a few hours the car started up perfectly. I took the car out for a good hour long drive and things were great. The next time I went to start the car less than a week later the battery was dead. There was just enough power to unlock the car and show that the computer hadn't lost it's settings. I hooked it to the charger for a few hours and again the car started up fine. Went for a 40 minute drive thinking this would help charge up the battery again, then came home and parked. The next morning, the car struggled to start due to low power, but it did turn over. I drove to work and parked the car. By lunch the battery was dead and the engine wouldn't turn over. (Yes, this wasn't a good plan but it was sunny and I really missed driving the car!)


Diagnosing:
Since the battery is accepting the charge from the trickle charger I feel the issue isn't with the battery. After a few hours charging the battery reads ~12.5 volts on the multi-meter. When I start the car the voltage goes to ~14.3 volts which I believe indicates that the alternator is also working fine.

Tonight I am going to hook up the meter again but this time measure current and see what the draw is when the car is off. I'm hoping I can then narrow it down by pulling fuses and see which one reduces the draw from the battery to a normal amount (assuming I find an abnormal draw to begin with). Other than that I am feeling out of options. Thoughts?


I love this car and I really hope I can resolve this before summer gets here. Thanks in advance!

[Geauxkart]

See if you can get a helper, and test the voltage during cranking. Even though you are getting a light charge in the battery, it may still be bad. If it drops below about 10V cranking, I think you have a bad battery.

But let us know about the current draw at rest, cause that sounds like a probable theory as well.

Good luck!

[yousefnjr]

Sounds like the battery to me. Even though it's only a year old, if it's been drained for an extended period of time, maybe it's been damaged beyond what recharging can fix? I remember my old auto shop teacher years ago explaining something with the inner lead plates and sulfation, maybe something to consider.

[pungo]

+1 on the battery, you need to charge it overnight then perform a load test on it to check the health. A few years ago, I had a battery going bad and bought a Charger/Maintainer/Desulfator and now my battery is 12 years old and still going strong. Here's the Charger/Maintainer/Desulfator I got. I move it around from vehicle to vehicle to tractor and all my batteries are happy.

[Jimswift]

Well, I measured the current tonight and it looks like the car is only drawing 15mA when sitting which is a good sign. I've got the battery hooked up to the tender and I'll leave it on for ~24 hrs and check again tomorrow night. Thanks for the advice everyone. I'm really hoping that its just the battery.

[Vanne]

+1 on the battery.
Btw, u have a 2006 z4 with 13K miles??
That's less than 2000 miles per year??

Wow

[Jimswift]

Alright, so following the advice here I removed the battery and hooked it up to a CTEK tender overnight. I'm not sure if this desulfates the battery, but it did charge up nicely overnight and today with the charger now showing a green light. I installed the battery right after work and everything started up fine, but the truth will be in how the charge holds. Just wanted to say thanks again for the advice. I'm pretty happy the issues seems limited to the battery so far.

I purchased the car with ~3000 miles about two years ago. The car is currently sitting just over 11,000 miles actually, so I'm getting ~4,000 miles per year since I've owned it (it's not my daily driver). Loving every drive!

Now where is that sun?