BMW i8 Battery Drain & Electrical Gremlins Need Help

I had a similar issue with my 2018 BMW i3s Hybrid last winter. The symptoms were identical, dead battery after sitting idle, unusual error messages, and inconsistent lighting behavior. Initially thought it was just the battery, but my mechanic diagnosed it as a faulty Intelligent Battery Sensor (IBS). This sensor monitors the battery's charge state and controls the charging system. When it fails, it can't properly regulate the charging, leading to battery drain even when the car isn't running. The repair involved replacing the IBS and recalibrating the system. While not the most complex repair, it's specific to BMW's electrical system and needs proper diagnostic tools. The issue was serious enough that it could've eventually left me stranded somewhere if not addressed. After the fix, all electrical issues disappeared completely. I'd recommend getting it checked by a BMW specialist who has experience with hybrid systems, this isn't something for a general mechanic. BTW, it looks like your i8's mileage is getting up there, so it might be worth having them check the main battery health while they're at it.

Thanks for sharing your experience! That's really helpful. My symptoms sound exactly like what you described, and I'm glad you mentioned the IBS, makes total sense given the weird behavior I'm seeing. Would you mind telling me how much you ended up paying for the IBS replacement? Also curious if you've had any other electrical gremlins pop up since the repair? I'm trying to budget for this and want to make sure I'm prepared if there's more to it.

Hey there! Happy to follow up about the repair costs. I paid 315 € for the IBS replacement and calibration, and I can tell you it was totally worth every penny. The best part? Haven't had a single electrical issue since getting it fixed, the battery's holding charge perfectly now, all those annoying error messages are gone, and the lights are working exactly as they should. Must say, felt pretty relieved when everything was back to normal. Those unusual battery drains were really starting to stress me out, especially not knowing if the car would start after sitting for a few days. But since the repair? Rock solid. The mechanic really knew what they were doing with the hybrid system. Just make sure you get a qualified BMW specialist to do the work, they'll have the right diagnostic tools and know-how. Once they fix that faulty IBS, you should be good to go. My i3s has been running like a dream since then!

Just got back from the mechanic, and wow, what a difference! You were right about finding a BMW specialist. I found one who really knew their stuff and had actually worked on several i8s before. The whole experience was way better than my last mechanic visit. Turned out it was indeed the IBS causing all the trouble. The repair cost me a bit more than yours, around 400 €, but they did a full electrical system check while they were at it. They showed me exactly what was wrong and explained everything really clearly. Got the car back yesterday, and it's like a completely different machine! No more weird error messages, lights are working perfectly, and the battery's holding charge like it should. Such a relief not having to worry about whether it'll start every morning. Thanks again for pointing me in the right direction with the IBS suggestion. You saved me from going through a bunch of unnecessary repairs trying to figure this out. Really appreciate everyone's help on this one!