Apparently the OEM i3 12 V AGM battery is made by East Penn Manufacturing under the Deka brand. Its model number is AUX18L. Local dealers can be found here
for those of you interested in replacing the battery yourselves. I have just sent my local dealer an email message requesting availability and price. If this dealer can sell a battery to me, I would have another source than my BMW dealer, hopefully at a lower price.
Now I need to figure out the best way to register the battery. The Carly tools apparently require a Windows operating system to install BMW software. I don't do Windows, so Carly is not an option for me. An independent BMW mechanic would likely charge less than my BMW dealer, but I would like to be able to register the battery myself. What's the least expensive tool that could register a new battery?