BMW Alarm System Dead: Auxiliary Battery Suspect

Hi there! I have some experience working on BMWs, and I had a similar issue with my 2015 BMW 3 Series Diesel (also owned a 5 Series before). My alarm system acted up the same way, totally dead. After checking with my trusted workshop, they diagnosed a failed auxiliary battery. These backup batteries typically last 7-8 years before giving up. Got it replaced along with a system reset, and everything worked perfectly afterward. Total bill was around 330 Euro, parts and labor included. Quick question though, have you noticed any other electrical issues? Like central locking acting weird or interior lights dimming? This could help pinpoint if it's just the auxiliary battery or something else. Let me know your car's mileage and if you've had any other strange electrical behavior. Will help me share more relevant details from my experience.

Thanks for the detailed reply! My car just hit 183000 KM and had its last service about two months ago. Actually yes, I've noticed the central locking has been a bit inconsistent lately, sometimes takes a couple clicks to work. Funny you mention the interior lights too, they seem a bit dimmer than usual, especially when the car's not running. Sounds like I'm dealing with the same auxiliary battery issue you had. Really appreciate you sharing your experience, gives me a good starting point for the repair. Did you get any other electrical gremlins after the battery replacement?

Hey again! Thanks for the follow-up. Those symptoms you're describing are exactly what I experienced before getting my auxiliary battery replaced. The dimming interior lights and finicky central locking were like red flags, took me a while to connect the dots though! After I got the auxiliary battery replaced, all those little electrical gremlins disappeared completely. The central locking became responsive again with just one click, and the interior lights got back their normal brightness. It's been about 18 months since the replacement, and I haven't had a single electrical issue since. One thing I learned from my experience, once you start noticing these symptoms, it's best to get it sorted quickly. I waited a couple weeks thinking it wasn't urgent, and one morning, the alarm system just completely gave up. The mechanic told me the failing auxiliary battery can sometimes put extra strain on the main battery too. Since your symptoms match mine so closely (even the timing with the mileage), I'd bet you're dealing with the same issue. Just make sure whoever does the replacement also does a proper system reset afterward, that's crucial for everything to work correctly again. Good luck with the fix! Let me know how it goes. Always happy to share more details if you need them.

Thanks so much for all this detailed info! It's really helpful to hear that your car's been running perfectly for 18 months after the fix, that's reassuring. I'll definitely get this sorted this week then, especially after what you mentioned about the strain on the main battery. Don't want to risk having two batteries to replace instead of one! I've already called my regular mechanic and booked it in for tomorrow morning. Made sure to mention about needing that system reset after the battery replacement too, wouldn't have known about that if you hadn't mentioned it. Really appreciate you taking the time to share your experience. Will definitely post an update once it's fixed. Fingers crossed it'll be as straightforward as your repair was!