I thought about a few different combinations of MCU/Modules in the beginning (Arduino Pro Mini + ESP8266 + SIM800 or ATtiny85 + ESP8266 + SIM800) but I settled on only using the ESP8266 and SIM800.

I wanted to use the Pro Mini/ATtiny because they have 5V ADCs whereas the ESP only allows a maximum of 1V at its ADC-Pin. The Pro Mini would also have been the brains of the whole BMS not like the ATtiny that would only have been used for measuring the different cell voltages and sending them to the ESP.

While I'm waiting on the parts to arrive I started the schematic in EAGLE (nowhere near finished, spent most of the time searching for librarys). 

I found a great library for the SIM800 module I ordered

Couldn't find anything for the LoLin v3 so I created my own...

I hope the circuit will fit on a single-sided PCB as I want to etch it myself.