next maker faire this thing needs to roll around and move the head and stuff. SO I needed a power bank. I wanted it to be a 2 cell battery, that is also balance charged. https://learn.adafruit.com/multi-cell-lipo-charging/fast-multi-cell-charger my schematics are a bit different, but the general concept is the same. Micro USB is all the rage to charge stuff, but mounting can be quite a challenge, so I went with a USB B plug. I've used two classic TP4056 modules with onboard protection (TE420) and a voltmeter. Bye bye weekend, but totally worth it!