I’ll direct you to battery building 101: Battery Basics for Beginners [Serious]
and once you finish that, 201: The battery builders club
As a quick note:
If you take the battery out of your device and see just 2 or 4 (thick) wires, that’ll just be charge (and/or) discharge, and banance is handled by the pack (or not at all in the ones prone to fire…)
If you see 2(thick)+{9-11}(thin) wires, that’ll mean balance is handled externally to the battery (by the motherboard as you mentioned, then you’ll be able to buy a BMS to plug into the 9-11 pin connector).
And a heads up: you’ll also get a lot of push to dump the hoverboard battery entirely since it’s probably pretty light duty and not likely to be terribly great in either discharge or capacity…