I thought I made the slickest thing. https://github.com/bkw777/REXCPM_UPS https://photos.app.goo.gl/i87E4wzimexCR3wL6
I was able to get 60mah of battery onto the system bus interface board for REXCPM, and theoretically that should be able to keep the sram memory for at least a year, but it looks like it will only last about 2 possibly 3 days.
So plan B is a separate "keeper". A separate thing with a much larger battery that you connect to the rexcpm when not in the computer.
But that's only half of what I wanted. What I was really hoping for though was to have the battery built-in, so that when the 100 batteries die sitting on the shelf, the on-board battery is still there for at least another year.
Both of these boards do work as merely ordinary rexcpm bus adapters, whether the batteries are installed or not.
The 102/200 board also allows one to use a REXCPM on a 102 or 200 without soldering any wires or opening the case, although it does mean using an external wire which is not exactly robust.
-- bkw
