Hi,
sorry for being late,. @DVDdoug : you are rigth I'm atting below the full picture (still WIP).
The idea is to have 8 opamps do measure the voltage of each cell (between 2.5 and 3.65volts), use an analog multiplexer as I will need more ports on arduino for other porpuses. FInally I'm amplifing the signal to have a wider range of voltages.
The circuit:
