Just adding to this thread.
I've used Hubauer Shop for parts direct from Germany. Normally a 2 week wait, this is ordering in Europe.
My wife's BMW i3 REx (2015) 60Ah battery, as far as I know is on the original 12v battery. I've no indication to think it's failing, but am looking all the same. Weather here in Scotland is cold, has been for the last couple of months, but car is used daily during the week for 50 mile round trip, so I know the main battery will be be managing the 12v starter.
The car also gets preconditioned while hooked up to the mains, so again all helps.
When searching, Hubauer list two batteries, 20AH original (61219321815) and 40amp (warranty only) (61217635788).
Links here: https://www.hubauer-shop.de/en/carparts/i-models-parts-catalog/i3-meg/1z41-i01-ib1/i3-60ah-rexa-mega-city-vehicle-vehicle-electrical-system-141-/141-613350-batteri/
So couple questions, what's the 40AH version for, and if you fitted would it last any longer, or do these batteries simply fail with age regardless of AH rating.
If I ordered but didn't fit immediately, can I just top up with 12v trickle charge? Just want to have it rather than risk being let down and forced to go to main dealer. Also looking at a way to register it.