Here is a story which might be helpful to some later model i3 owners. I have an i3S 2019, 16k, made Feb of 2019, so just 3 years and two months old. Most of the stories about the 12 volt battery seem to involve earlier cars, 2014-16 &c. Anyhow, I came out to my car in a tight garage the other morning after having put it away on charge and it was a brick, no response to key fob, no lights, horns &c. I thought it likely to be the 12 v battery and having got it towed the dealer, that was what it was. But instead of giving warning signs, quirky lights, windows, &c. it just went completely dead.
So a new battery and I am back on the road, but typically these problems have occurred in 4-5 year old batteries, so owners in the three year range might want to consider getting one of the batteries and keeping it on trickle charge. Or just change it at 3 years as a matter of prevention.
Fortunately my car was under extended warrantee and the dealer had a battery so I was OK for the towing and the work.