I don't know why an op amp is needed. It's probably better to use a mux. I have some DG408 parts available. The 4 cell voltages can be multiplexed. The current shunt voltage also needs to be accounted when calculating cell voltage. The ads1115 has 4 adc inputs. The shunt gets a direct input to the adc while the 4 cell voltages get multiplexed.
It's not needed per se.
Without a mux you'll need an ADC per cell, so yeah, usually it's far simpler and easier to use a mux
You then have 2 ADC inputs free... and 4 on the mux... that begs to add things to be measured ?
I don't think using a "set" upper resistor (like 100k) for all the dividers is the correct approach. The input impedance to the adc is the Thevenin equivalent impedance. Using the resistors mentioned above, cell 1 impedance is 50k (100k//100k), while cell 4 has 12.5k (100k//14.3k). Perhaps the resistors should be sized to obtain identical impedance? The input leakage current errors will now be equal. One filter capacitor at the output of the mux will give identical corner frequencies for all 4 dividers.
Well, it's not the perfect approach but a good enough one as else the BoM complexity and cost increases, but as you'll probably just make one it's not a big deal.
The impedance is non-critical as long as it's lower than the max calculated (that's why I ignore the other resistors) and same with the corner frequency, really not critical. Then you can be anal about it and use different resistors, that's ok too
I used the high BoM re-use approach because I have a lot of those resistors and they are pricey because they are precision ones. That's also part of why I chose to use op-amps instead of dividers (and because the accuracy degrades for the higher cells), that way I could use only one value instead of dozens of them. But for a 4S BMS all that is much less a problem than a 16S one
NB: the mux has a pass resistance of 100 or 120 Ohms max IIRC, don't forget to account for that if needed.
Designing adc circuits is uncharted territory for me. I wasn't planning on doing any cell voltage averaging. But that may change after getting real data. My goal is to get a voltage measurement that is as good or better than Chargery's. Their granularity is 1 mV. With ADS1115 full scale at 2.048V and 32768 bits, voltage resolution is 62.5 uV. That should be a significant improvement over Charger.
Well, the good thing is that's just software so easy to change in the future if needed
Well, their resolution is 1 mV but the accuracy is far less than that. Like you'll have a 62.5 µV resolution but I bet the accuracy will be 10x worse (which is still far better than the Chargery and what you might need so no problem).
NB: as you're new with ADCs I recommend to read about mixed signals PCB routing (basically you star ground with the ADC as the center and you avoid mixing analog and digital in the same vicinity on the PCB) as it's quite important when you're in the µV territory.