BMW M2 Battery Woes: Trust Your Gut or Your Mechanic?

I had nearly identical issues with my 2016 BMW M4 (gas). Initially thought it was just a dying battery, but the actual problem was the Battery Sensor Module (IBS). The symptoms matched yours exactly, slow cranking, dim lights, and unusual electrical warnings on the dash. Had this checked at my regular garage last month. The diagnostic showed the sensor was sending incorrect readings to the car's computer, causing the charging system to malfunction. The battery itself was fine, but the faulty sensor made the car think it was either overcharged or undercharged. The fix wasn't particularly complex, but it required specific BMW diagnostic tools. They replaced the sensor and recalibrated the system. It wasn't a major repair, but leaving it unfixed could've eventually damaged other electrical components. Make sure they check both the battery health AND the sensor, they often overlook the sensor and just replace the battery, which won't solve the underlying issue if the sensor is faulty.

Thanks for sharing your experience! This is super helpful as I've been going crazy trying to figure out what's wrong. I'll definitely have them check the Battery Sensor Module. Quick question, do you remember roughly how much you paid for the fix? Also, has everything been running smoothly since the repair, or have you noticed any other electrical quirks?

Hey again! Yes, absolutely, I can tell you everything's been working perfectly since I got it fixed. The total repair cost was 315 € for replacing the faulty Battery Sensor Module and doing the recalibration. I know it sounds a bit steep for a sensor, but it was worth every penny. Since the repair last month, all those annoying issues have completely disappeared, no more slow starts, dim lights, or unusual warning messages on the dash. The car fires up instantly now, even after sitting for several days. It's like having a new car again! Trust me, getting the sensor replaced was the right call. Looking back, I wish I'd done it sooner instead of worrying about whether it was just a battery issue. Just make sure they use the proper BMW diagnostic equipment when they do the work, that's really important for these kinds of electrical fixes. Let me know how it goes after you get yours checked out. I'm curious to hear if you've got the same issue I had!

Thanks again for all the details! I actually just got back from my mechanic and you were spot on! Had the Battery Sensor Module checked and replaced. Cost me around the same as yours, and honestly, what a difference it's made. The funny part was, my mechanic initially tried convincing me it was just the battery. I insisted on checking the sensor after reading your advice, and sure enough, that was the culprit. He had to order the specific part and tools, which took a couple of days, but it was worth the wait. Car's running like a dream now, starts instantly, all lights are bright, and those annoying dashboard warnings are gone. Really glad I didn't waste money replacing the battery first. You saved me from a potential headache and unnecessary repairs. Thanks for steering me in the right direction. It's such a relief to have this sorted out properly!