In my case the i3 didn't warn me that the battery had a problem until it was dead. I got the "battery discharging" warning and that afternoon while out it as died (also gave me some parking break error code). I called BMW roadside assistance to get a tow. While on the phone with them I tried it again and boom she started. Lucky I was able to get it home, because when it dies you can't move the car. You cannot get it into Neutral, so, need to find a flatbed with a trolley to get the back wheels off the ground. Glad I didn' have to do that.
So the 12v battery has died; its been 5 years, so I'm not upset about that.
First I called around looking for a the OEM battery. Couldn't find it locally, but could order it on-line for roughly $150 including shipping. But, days to deliver it. My BMW dealer wanted $200 and said he could have it in 3 days. I was good with that, but didn't want the car sitting waiting for a battery to arrive, so I contacted another dealer near me and they had one in stock. As asked what they wanted to install it. They said $400 (2.5 hrs). I said that can't be right, they said it was. I drove down, bought the battery and didn't consider having them install it. The other dealer wanted $360 for battery and installation. So, I planned on going there with the battery in hand.
The next morning the car was dead. Thank goodness I was able to open the frunk, which gave me the opportunity to replace the battery myself and get the car running again. I'm no mechanic, but the whole process was about 40 minutes. Everything went as planned. Car is up and running. Only problem now is the "battery discharging" message is still there. Turns out the new battery has to be registerd with the car. I have no idea why this is necessary. So, now I have to make a trip to the dealer so they can tell the car a new battery is in there. Don't know what they will charge, but my expectation is that it will be way more that I think it is worth.
So, when you have this problem. 1) address it quickly becuase you will be surprised how quickly the car will become useless, 2) if you have a 12v battery that is over 3 years old have it tested, if its anywhere around 8v buy a new one and replace it, 3) don't pay BMW to do it -- it doesn't require unique tools and is straight forward.
Here is the link to the replacement video on YouTube: https://www.youtube.com/watch?v=lKRj8gLcsfI
Hope this helps.