BMW i3 Hybrid Electrical Gremlins, Aux Battery Suspect?

Hey there! I have some experience working on hybrids and faced something really similar with my 2011 BMW i8. Actually, my cousin also had comparable issues with his ActiveHybrid 5. When I got those same error messages and weak starts, it turned out my auxiliary battery was indeed the culprit. Took it to my trusted workshop last summer, and they confirmed my suspicion. The 12V auxiliary battery was completely drained and couldn't hold a charge anymore. Total fix was around 290 Euro including diagnostics and replacement. These auxiliary batteries in hybrids are pretty critical, they power all the basic electronics before the main hybrid system kicks in. Even a partially failing one can cause all sorts of weird electrical gremlins. Before I can give more specific advice, could you share: Are you getting any specific error codes? How old is your current auxiliary battery? Does the issue happen more often after the car sits for a few days?

Thanks for jumping in! Just had my service about 3 weeks ago at 199553 KM. I am getting error code 0x4B8D39 consistently. The aux battery is actually the original one that came with the car, so yeah, probably well past its prime at this point. And you hit the nail on the head, the problems get way worse when the car sits unused for 2-3 days. Been having to jump-start it a couple times in the past month. Your experience sounds exactly like what I'm dealing with, so I'm thinking I should just go ahead and replace the aux battery. Really helpful to know about your repair costs too, gives me a good idea of what to expect.

Ah, great to hear back from you! Yeah, after reading your details, I'm even more convinced it's the auxiliary battery, exactly like what happened with my i8. That error code 0x4B8D39 is the same one I kept getting. And wow, you're still running on the original auxiliary battery? That's actually impressive it lasted this long! Mine died around the 7-year mark. The jump-starting situation you described is super familiar. I remember having to keep jumper cables in my trunk for those annoying "won't start" moments, especially after the car sat for a few days. Such a pain! Since you mentioned your recent service, did they run a battery test? Sometimes they miss checking the auxiliary battery and focus more on the main hybrid system. In my case, I ended up just getting the battery replaced and literally all those problems disappeared overnight, no more error messages, start-stop works perfectly again, and starts like a charm every time. Quick tip from my experience: when you do get it replaced, make sure they use the correct battery spec. These hybrids are pretty picky about their auxiliary batteries. I learned that the hard way when I initially tried to save some money with a generic battery, big mistake! Had to replace it again with the proper one a week later. Let me know how it goes after you get it replaced. I'm betting you'll see the same night-and-day difference I did!

Thanks everyone! After hearing about your experiences and considering my 10-year-old original auxiliary battery is causing similar issues, I've decided to go ahead with the replacement. Really glad I asked here before spending money on extensive diagnostics. I've already called my mechanic and scheduled the battery replacement for next week. They confirmed they'll use the proper BMW-spec battery, so hopefully that'll sort out all these electrical gremlins once and for all. Will definitely update you all on how it goes, but feeling pretty confident this is the solution. Thanks again for sharing your experiences, super helpful in figuring this out! Keeping my fingers crossed for a smooth fix like you had!