The 12V battery finally gave up the ghost and needed replacement. But the $277 price from the local BMW parts department was too much. So I found a 12V substitute, Deka ETX18L, for $136.00, in stock at "The Battery Store" (256-5361845 or Facebook 'TheBatteryStoreHuntsville".) After one modification to the battery, the car started up and appears to work just fine.
The BMW and Deka batteries are identical shape with the exception of a gap between the lead base and a raised, hollow, plastic wall toward the center. The BMW gap is ~0.5 inch (~12-13 mm) but the smaller Deka gap was ~0.25 in (~6-7 mm.) Using a box cutter/rasor knife and Dremel tool, I removed lead from the Deka positive terminal to make a BMW sized gap. See the battery image from this URL:
https://www.batteriesplus.com/productde ... bMQAvD_BwE
The separate, BMW brass, positive terminal has a screw-on, center post surrounded by a circular gap and raised, key-shaped outer wall. The red plastic, encased cable clamp has two 'sensors' but no exposed electrode to touch the brass ring. What these two sensors do is unknown so lead removal on just the positive base allows the brass terminal to fit. I suspect one sensor is a temperature and the other a 'presence' sensor... a mystery. The negative clamp has no such sensors so a simple post was used. Note that I will try to use Google drive to hold viewable images and/or a YouTube video.
The takeaway is the BMW and Deka ETX18L batteries are identical except for the size of the gap on the lead, positive and negative electrode base and hollow, plastic wall. Only the positive terminal clamp appears to have two sensors that may use the key shaped outer, brass ring for unknown purposes. Removing part of the soft lead allows the BMW brass center post to mount. The term 'over engineered' springs to mind.
For the next couple of days I will re-torque the 10 mm bolts and hex-head screws as well as continue tests. I do have a Duracell replacement on order but will inspect before purchase.
ps. The YouTube video "UK BMW i3 - 12 volt battery replacement ..." shows the brass terminal fittings.
pps. If I do it again, I will trim the positive brass fitting so it will fit on any of the equivalent batteries.