I'd like to measure the voltages of individual lithium based cells (LiPo, LiIon, LiFePO4) in a battery pack (up to 4 cells in series), using an ADC. I was thinking about the following approach, using an analog multiplexer and an amplifier:
- The multiplexer is powered by the battery to prevent the cell voltages to be above the multiplexer's supply voltage.
- The multiplexer routes each cell's positive and negative terminal to the amp's inputs.
- The amplifier is also powered by the battery pack.
- The amplifier has a gain of 1 and turns the selected cell's voltage into a single ended signal that is then divided down to ADC range.
I'm looking for an accuracy of 10 mV.
- Is this a reasonable approach?
- What should I look out for in the amplifier? So far it would have to support rail-to-rail inputs, unity gain and high CMRR. Any recommended types for this application?
- Channel select and inhibit inputs could simply be driven with an N-channel FET and a pull-up to +BATT. Should I consider other approaches?
- What kind of filtering should I have at the amplifier's inputs?
- Are there any caveats when the pack has less than 4 cells (in the schematic: BT3 and BT4 shorted for a pack with 2 cells)