One of the things I realized looking at Tom's planes is I am over-thinking the design. I have to look for a simple solution that meets all criteria - and to cut the criteria to the minimum for safe flight.
Sitting in front of the TV, I sketched out a minimalistic backup batttery controller. It has to
- Provide power even if the main bus shorts out - Provide 6 fused circuits for backup - Indicate if operating on backup power - Allow charging of a dead batter - Limit or carry charging current
I sat down at my computer today and converted much of it to a schematic. I got stuck on the charge current. Bottom line, I have to either disallow charging of a dead battery or provide a high current path. I decided that the simple way is to put a small CPU to control a FET and PWM to limit current. Simple for me. For safety, the CPU will only be on the charge side and will have backup fuse in case CPU gets lost.
Once I lock down pin-outs and form factor, I can continue with the panel and do the controller later.
