On my continuing saga of battery management and specifically cell voltage measurement for long string batteries, i have moved away from a commercial cell voltage measurement chip due to it's high cost and very poor availability (if you want any component that say VW or FORD want too, they will always get the ones being made in times of supply constraint.........) and have a dev board being made up using a basic switched capacitor type arrangement where an analogue switch couples a small cap to each cell in turn, then brings it down to the ground ref of a 12b ADC. This is a pretty cheap and easily sourcable (multi sourcable atually using std components available from multiple manufacturers) solution, but i am worried about things like charge leakage and charge injection. Ive done a lot of simulation and think i have some reasonably optimium starting values and a decent(ish) system.
However, i am a bit worried that it would be "better" to use a opamp as a simply voltage shifter to achieve the same things, or perhaps to even just use (switchable-offable) resistive dividers and a higher resolution ADC etc? Both approaches seem simpler, but have some issues such as CMRR and resistor matching that is not that hard to ignore without using expensive high tolerance parts.
So, back to the start, i have:
4 cell string, of between 2.5 and 4.25 Volts for a string max of about 17v.
Want to measure cell voltage with better or equal to 1mV resolution and about 0.5% absolute accuracy across a temperature range of -10 to 65 degC (accuracy can get worse outside of these limits)
There will be significant common mode noise in a spectrum spread between approx 50 Hz and 1MHz
The system should be not be damaged by a voltage of 60Vdc appearing between any of its connections continuously
The system should ideally not be damaged by a voltage of 300Vdc appearing between any of its connections for up to 10ms
The system should fail gracefully when gross and continuous voltage application outside of those cases is applied
The current drawn from a single cell i the string should be limited and each cell should be equally loaded
The current drawn from the string should be used to power the circuitry, ideally i'd like to keep average current at around 50mA when measurin and sub 1mA when not measuring
WHat would people do? Any experience?
1) 14 noise free bits and switchable 0.1% resistor dividers would do it with a suitable accurate voltage reference
or
2) Low offset opamps as voltage subtractors would enable the same resolution with a 12b ADC (makes it possible to use an on-board ADC in a uC) but resistor spec and CMRR start to look a bit problematic
or
3) other architectures?
I'm going to continue with the switched capacitor architecture through to some basic test h/w, but i'd very much like to understand other options and to have a "fall back plan" all queued up and ready