In spring, 2019, with the 12 V batteries in the oldest i3's beginning to fail, I got nervous about the 12 V battery in our 2014 BEV potentially failing and my having no choice but to pay ~$210 for a replacement at our BMW dealer. So in May, 2019, I ordered an AUX18L from a local East Penn Manufacturing retailer ($150 including shipping to Honolulu) and bought a Foxwell NT520 PRO+ to register the battery which was before BimmerLink added that capability and before I learned that battery registration doesn't do much on an i3. At least, I should be able to read and clear BMW-specific diagnostic trouble codes (DTC's) with the Foxwell. Today, I would license BimmerLink instead.
The original 12 V battery shows no signs of failing, so my replacement 12 V battery is still sitting on a shelf in a closet. I top up its charge twice a year, so it's ready to go when needed.