This is a quick scribble of the power:
These are the parts I used:
12-5 Buck
3S charger ?https://epartners.co.nz/products/jo8001
BMS https://epartners.co.nz/products/st4001-1?_pos=1&_sid=770963d6a&_ss=r
12-12 converter
Relay
It's not the most efficient way to do it, but gives the flexibility to charge from 12v or USB-C - and I had these boards all in the shack!
ZL1SGS