/diy/ - Do It Yourself

It it possible to make a load-sharing charging circuit for multi-cell batteries?
I can find tutorials for single-cell load-sharing circuits, but I can't find anything for multi-cell.
I mean, I know it's possible, otherwise laptop batteries would all be single-cell, I just can't figure out how to do it myself.

I'm trying to make a battery power hat for a Pi so I can throw it in my backpack as a portable computer, without having to power it through the USB port and a separate USB battery pack.