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Mux shield. Wow. That's a lot of I/Os. Does it use it's own ADCs (and if so, how many bits are they) or does it multiplex analog inputs to the ADCs in the Arduino?
The problem with using one chip on a series string is that you have to deal with upwards of 90V worth of potential voltage difference on a 72V car. Therefore, say max is 90V and you put it through a voltage divider to make 90V = 5V to the chip, that's only a 0.09V resolution with the Arduino's 8-bit ADC (1024 digital steps). I guess not too bad, but I would like better resolution than that, especially with a lithium pack. If the mux shield has a better ADC, say 10 bits, that would give you 4 times better resolution than 8.
This is why I suggest multiple Aduino chips, each taking care of only 8 inputs across a spread of only [4.5V X 8] 36V, yielding a 0.035V resolution (or even less - 4.5V X 4 = 18V with a 0.017V resolution....). Each of the two "reader" Arduinos (for an 18 cell system) can send PWM warning signals to the master board (that has 8 of its own analog inputs) via a single wire through an opto-isolator to keep voltage potential difference between two chips at different parts of the battery series string from letting the smoke out of things.
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