BMW 3 Hybrid Alarm System Dead, Battery or Module Issue?

Having faced a similar issue with my 2019 BMW 5 Series Hybrid, I can relate to your situation. The auxiliary battery failure was indeed the culprit in my case. The symptoms matched exactly, no alarm response and stored error codes. Got it fixed at a certified shop last month. The auxiliary battery is separate from the main hybrid battery system and handles the alarm and some other auxiliary functions. The repair wasn't particularly complex, basically a battery replacement with some system reprogramming. The interesting part was that the technician mentioned these auxiliary batteries typically last 4-6 years, depending on usage and climate conditions. This wasn't a major repair compared to other potential hybrid issues, but it needed proper diagnostic equipment to reset the system afterward. If your symptoms mirror mine, I'd suggest getting it checked soon, as a failing auxiliary battery can sometimes affect other electrical systems down the line.

Thanks for sharing your experience! This sounds exactly like what I'm dealing with, same symptoms and all. I'm definitely going to get it checked out soon. Just curious, do you remember roughly how much you ended up paying for the whole fix? And has everything been working smoothly since the repair, or have you noticed any other quirks with the electrical system?

Hey again! Happy to give you an update on my repair experience. The total cost came to 330 € for replacing the aging battery, and I'm pleased to say it was worth every penny. The "Additional battery alarm system defective" warning that was driving me crazy is completely gone now. I've had zero issues since the fix, the alarm system's working perfectly, and all other electrical systems are running smoothly too. It's been about two months now, and honestly, it feels like having a new car again, at least in terms of the security system. One thing I really appreciate is how the mechanic explained everything during the repair. They showed me the old battery and how it had degraded, which made the cost easier to swallow. And trust me, getting it done at a certified shop was the right call, they knew exactly what they were doing with the reprogramming part. Just a heads up, once you get it fixed, you'll probably wonder why you didn't do it sooner. The peace of mind knowing your alarm system is actually working properly is totally worth it.

Thanks so much for all the detailed info! I actually got it fixed yesterday and wanted to share my experience. The mechanic I found was super professional and diagnosed it right away, definitely an auxiliary battery issue, just like you said. They had it done in about two hours. Mine ended up costing a bit more, around 400 €, since they had to order a specific battery model. But honestly, after dealing with that annoying alarm problem for weeks, it was totally worth it. The mechanic even showed me the old battery, it was pretty shot. The best part? Everything's working perfectly now. The alarm responds like it should, and all the electrical systems are running great. Such a relief to have it sorted out. Really appreciate you helping me avoid another bad mechanic experience. Sometimes it's just about finding the right mechanic for the job!