BMW M3 Battery Chaos: Multiple System Failures Strike

I had almost identical issues with my 2017 BMW M4 last winter. The symptoms started with unusual electrical glitches and that annoying start-stop malfunction warning. After a few days, the car barely started in the morning. Took it to the garage, turned out both the main and auxiliary batteries were failing. The auxiliary battery primarily supports the start-stop system and other electrical components, while the main battery handles the core starting functions. When one starts to go, it puts extra strain on the other. It wasn't just a simple battery replacement though. The technician had to recalibrate the battery management system and clear all the error codes. The whole thing took about half a day to fix, but it was worth it. The car has been running perfectly since then. I'd definitely suggest getting both batteries checked out. It's not a catastrophic issue, but it's not something you want to ignore either, as it can leave you stranded or cause damage to other electrical components.

Thanks for sharing your experience! That sounds exactly like what I'm dealing with right now with my M3. Glad to hear you got it sorted out. Would you mind sharing how much you ended up paying for the whole job? Also, has everything been running smoothly since the fix, or have you noticed any other electrical quirks pop up? Really appreciate the detailed explanation about both batteries needing attention. I'll definitely get them both checked out rather than just assuming it's only the auxiliary one.

Hey there! Happy to give you an update. After getting it fixed, everything has been running like a charm with my M4. The total bill came to 290 € for sorting out the Auxiliary battery malfunction, honestly, it was money well spent because all those annoying electrical gremlins disappeared completely. Since the repair about two months ago, I haven't had a single issue. The start-stop system works perfectly now, no more warning lights on the dash, and the car starts up instantly every time. The technician did a thorough job with the battery management system recalibration, which I think was key to solving the problem completely. You're definitely on the right track getting both batteries checked. In my case, they found the auxiliary battery was the culprit, but having them inspect both was the smart move. Just make sure they do the proper recalibration after, it makes a big difference in how the whole system performs. Let me know how it goes with yours! Always good to compare notes with fellow BMW owners.

Thanks for all that info! I actually just got back from the mechanic today, and boy, do I have an update to share. Turns out you were spot on about the battery issue. The auxiliary battery was completely dead, but thankfully my main battery was still in decent shape. The funny part was when they hooked up the diagnostic tool, the mechanic's eyes went wide seeing all those error codes! Had to wait around for a few hours while they did the replacement and recalibration. Total bill came to about 310 €, a bit more than yours but not as bad as I'd feared. The drive home was like having a completely different car. No more warning lights, start-stop working perfectly, and that hesitation when starting is completely gone. The mechanic also gave me some tips about battery maintenance that I hadn't known before. Really glad I asked here before going in, helped me know exactly what to expect. Thanks again for your help, you saved me from another potential bad mechanic experience!