BMW Z4 Battery Drain: Dim Lights & Starting Issues

I have some experience working with BMWs, and I had a very similar issue with my 2011 BMW 335i (also owned a 135i). The symptoms you're describing sound exactly like what I experienced last year. The battery sensor was indeed the culprit in my case. My car was showing the same signs, battery warning light, dimming lights, and difficult starts. Took it to my trusted workshop, and they diagnosed it as a faulty Intelligent Battery Sensor (IBS). The repair included replacing the sensor and recoding it to the car's system. Cost me about 315 Euro total, but it fixed all the issues completely. Before you proceed though, could you tell us: Have you checked if the battery itself is still good? Are you getting any specific error codes? How old is your current battery? This would help narrow down if you're dealing with the same issue I had.

Thanks for sharing your experience! Mine shows identical symptoms to what you described. Just had my last service 3 months ago at 110311 KM, and they said the battery tested fine back then. Haven't checked for error codes yet, but planning to get it scanned tomorrow. The battery is about 3 years old. Really helpful to know that you solved it with the IBS replacement, gives me hope it won't be something more serious. Will definitely have my mechanic check the sensor first.

Hey again! Glad to hear back from you. Yeah, those symptoms definitely match what I experienced with my 335i. Since your battery is relatively young (3 years) and was tested good recently, I'm even more convinced it's the IBS. Just wanted to add, after I got mine fixed, I learned that these sensors can actually cause premature battery wear even if the battery itself is good. My mechanic explained that when the sensor fails, it sends incorrect charging signals to the car's computer, which then doesn't charge the battery properly. Pretty annoying design flaw, if you ask me. Quick tip from my experience, when you get it scanned, look specifically for codes related to the charging system. In my case, it threw a "Battery Management" fault code. Also, make sure whoever does the replacement knows how to properly code the new sensor to the car. My first attempt with a general mechanic didn't work out because they didn't know about the coding requirement. Keep me posted on what the scan reveals. Would be interesting to see if it's the same issue I had. These BMW electrical gremlins can be tricky, but at least this one's a straightforward fix!

Thanks so much for all that detailed info! You're absolutely right, I just got back from having it scanned and sure enough, it threw the same "Battery Management" fault code you mentioned. My mechanic confirmed it's most likely the IBS sensor and quoted me around 300 for the full fix, including coding. Really relieved it wasn't something more complicated or expensive. I appreciate the heads-up about making sure they know how to code it properly. I double-checked with my mechanic and they confirmed they're familiar with BMW IBS coding. Going to get it fixed next week. Really grateful for your help, would've probably spent way more time and money trying to figure this out without your insight about your similar experience. Will make sure to keep an eye on the battery health going forward too, now that I know about that potential wear issue you mentioned. Thanks again for taking the time to help me out!