There are already videos out there on how to replace the 12v auxiliary battery from the BMW i3, but I wanted to include some important details on exactly how to do it, especially for newbies who aren't used to working on their cars. Don't be overwhelmed. It's doable and my video starts here. Let me know if you have questions. I'm better reached on my YouTube channel in the comments but will check this thread every so often while I'm working on this.
You know you're looking for a Part 2. Part 2 Includes the full demonstration of each step. Includes preparation of the battery as well. I've also included info on the OBD2 reader, discussion on how to use it to register the battery for cheaper than what the dealer charges. Step by step. Get started here.