The task is to measure the voltage of individual cells in a 48-cell battery pack of AA cells in series.
The measurements serve to log performance statistics, warn the user if/when a cell dies, or that a cell is otherwise unbalanced compared to the whole pack. I'm thinking about how to wire this up to the MCU. My current design idea is as follows: connect the minus of the battery pack to GND, and then place a voltage divider from each tap of the pack towards ground, so that the voltages of the taps are brought within the 0..5V range. After that, place 6 analog multiplexers of 16:1 configuration, so that any two taps can be addressed individually. These two selected channels are amplified with an opamp in differential fashion, and fed to a PIC. The analog muxes, the opamps and the PIC are powered by a battery/LDO combo.
It isn't hard, it just seems overly complicated, so I wonder if I miss a simpler solution?
Best Answer
Use high voltage multiplexers first then get one pair of signals that can be fed into a single (albeit expensive) high-common-mode differential amplifier.
The MAX14802 is an analogue switch (16 channel) that uses 200V technology and typically can be run from +/-100V or +200V/0V or +40V/-160V etc.. Digital interface is 3.3V/5V.
It has a data input that controls which switch is active and using 2 devices you can select the individual battery/cell to be measured. Obviously, for 48 cells you'll need 6 devices but because they are high voltage devices you'll only need one set of high-precision resistors and an instrumentation amplifier to do the measurements.
I think the AD8479 looks like it'll get close to being accurate for the differential/instrumentation amplifier and is only a few dollars.