BMW M3 Battery Drain: More Than Just a Dead Battery?

Hey there! I have some experience working on cars, and I faced almost the exact same issue with my 2005 BMW M5 last year. My 330i also had similar problems before. The symptoms you're describing sound incredibly familiar. Initially, I thought it was just the battery too, but my mechanic discovered it was actually a faulty battery sensor. This little device monitors the battery's condition and charging status, and when it goes bad, it can cause all sorts of electrical gremlins. Got it fixed at my regular workshop, they diagnosed it quickly and replaced the sensor. Cost me about 315 Euro, but it solved all those issues: no more unusual battery drain, error messages disappeared, and the electrical system went back to normal. Before I can offer more specific advice, could you share: Have you noticed any particular pattern when these issues occur? Are there any specific error codes showing up? When was the last time you had your battery replaced?

Thanks for sharing your experience! I've got 37899 KM on my M3, and just like your case, I've been noticing these issues getting worse since my last service. The error codes are pretty unusual, they come and go, but I've been seeing the battery warning light more frequently lately. Honestly, I'm relieved to hear about the battery sensor possibility because I was worried it might be something more serious with the electrical system. Did you notice if replacing the sensor also improved your starting times? Mine's been a bit sluggish lately, especially in the morning. I'll definitely get my mechanic to check the sensor when I take it in.

Hey again! Yes, absolutely, when I got the battery sensor replaced on my M5, it made a huge difference in starting times. The sluggish morning starts you're experiencing are exactly what I was dealing with. I remember before the fix, my M5 would sometimes take a couple of tries to start, especially if it had been sitting for a few days. It was super frustrating, and I kept thinking it was just the cold weather or something. But after getting the sensor replaced, it was like having a different car, starts right up every time now, no hesitation. One thing I forgot to mention in my previous reply, my mechanic also recommended doing a full diagnostic on the charging system while they had it in. Turned out to be a good call because they found some minor corrosion on the battery terminals too. Got those cleaned up while they were at it. Given your mileage and symptoms, it really does sound identical to what I experienced. Just make sure your mechanic checks both the sensor and the charging system. Mine also threw unusual error codes before the fix, it's typical when the car's computer isn't getting accurate readings from the battery sensor. Keep us posted on what you find out! Always interested to hear how these fixes work out for others with similar BMW issues.

Thanks everyone for the replies! Just wanted to update you after getting my M3 checked out. You were spot on, it was indeed the battery sensor causing all the trouble! My mechanic found the sensor was completely shot, and there was some nasty corrosion on the terminals too. Got both issues fixed, and wow, what a difference! The car starts like a dream now, all those unusual error messages are gone, and the electrical system is working perfectly. Total repair cost came to about 380 Euro including labor, which honestly wasn't as bad as I was expecting. Took them about two hours to complete everything, including a full diagnostic of the charging system just to be safe. They also cleaned up the battery connections while they were at it. Really appreciate everyone's input on this, you saved me from unnecessarily replacing the whole battery. And special thanks for recommending the full diagnostic check. It's such a relief to have this sorted out. Guess these BMWs can be a bit finicky with their electrical systems, but at least now I know what to look out for!