BMW Z4 Alarm System Dead, Backup Battery Suspect?

I've got some experience working on BMWs, had a similar issue with my 2004 Z3 last year. When it comes to car maintenance, I'd say I have low experience, but I do try to understand what's going on with my ride. The symptoms you're describing sound exactly like what I dealt with. My backup battery was completely dead, and the alarm system was acting up just like yours. Took it to my regular workshop, and they confirmed it was the battery aging. They replaced the backup battery and recoded the alarm system. Got everything sorted for about 330 Euro. Before I share more specific details, can you tell me if you're getting any specific error codes? Also, have you noticed any other electrical issues, like central locking problems? My Z3 had those symptoms too before I got it fixed. Hope this helps! Let me know those details, and I might be able to point you in the right direction.

Really appreciate your detailed response! My Z4 just hit 172393 KM and had its last service about 2 months ago. No issues with the central locking, but I'm getting error code 392 on the dashboard. Just like you mentioned about your Z3, my backup battery theory seems more likely now. I'll probably take it to the workshop this week, good to know it's a relatively straightforward fix. And thanks for mentioning the cost, helps me know what to expect. Did your alarm system work perfectly after the fix, or did you notice any quirks?

Yep, that error code 392 sounds familiar, exactly what I saw on my Z3! And with your mileage, it's about the right time for these backup batteries to give up. Mine was at around 165000 KM when it started acting up. After they fixed mine, the alarm system worked like a charm, no quirks at all. The workshop did mention that it's pretty common in these models. Just make sure they do the recoding after replacing the battery, that's crucial. Some skip that part and end up with weird issues later. One thing I learned from my experience, don't try to save money by getting a cheaper aftermarket backup battery. I initially considered that route, but my mechanic warned me against it. The original BMW part might cost a bit more, but it's worth it for the peace of mind. My Z3 has been running great since the fix, going on 14 months now without any alarm issues. Given how similar our problems sound, I'm pretty confident you're looking at the same solution. Let me know how it goes at the workshop!

Thanks so much for all the great advice! Just wanted to update you, I took my Z4 to the workshop yesterday, and you were spot on. They found the backup battery was completely dead, just like in your case. They replaced it with an original BMW part and did the recoding as you suggested. The total cost came to about 350 Euro, pretty close to what you paid. The alarm system is now working perfectly, and that annoying error code is gone. Really glad I didn't go for the cheaper aftermarket option you mentioned. Your experience with the Z3 really helped me understand what I was dealing with. It's such a relief to have this sorted out properly. The mechanic also mentioned it's a common issue, just like you said. Thanks again for taking the time to share your knowledge. Really appreciate the detailed guidance, saved me a lot of worry!