I’ve got some downtime waiting at the vet so I’ll expand a bit on my setup.
I’m repurposing an MPPT solar charge controller, it’s intended to take a PV array (solar panel) as input and it’s output is essentially a programmable CV/CV Li-ion charger.
I’m simply using another battery pack in place of the solar panel and using it as a DC to DC charger.
The particular ‘mppt’ model I’m using is step up only, not true MPPT, so you need a 10S to charge 10S-13S, etc, it won’t step down to charge a lower battery voltage. But it’s cheap, compact, and it works.
Wiring setup is simple:
10S5P Backpack Battery (w/ it’s own BMS and charge input) >> MPPT >> Charge cable to board.
This is my “Do Not Try This At Home” picture of my hastily thrown together test setup. I’m currently rebuilding it all with a 10S5P and enclosure.