We have a BattEstimate.lua script that might be a good starting point.
Re building your own battery, Huibean (VimDrones) and I are working on a smart battery for my aerial photography copter with the hardest part being the open source BMS using AP_Periph (hardware definition is here). I don’t think the VimDrones BMS board itself (e.g. the hardware) is open source but the software is.
I’ll properly announce the BMS in the coming months once it’s more fully functional