Hrm, not sure how to approach this. I want to measure the voltage on each of four 6V batteries that are in series for 24V. These are part of a UPS backup system, and I want to be able to track each battery in the bank. I also want to track the current going through them, which can range up to 200A DC.
My initial thought was to use a pair of muxes to connect the -ve and +ve terminals to AGND and a microcontroller A/D input, but I'm fairly sure that won't work ... AGND is internally connected to GND, and suddenly having 12V or 18V whacked on it couldn't do it any good.
I'm guessing at differential opamps, one for each battery, with a voltage divider in front to drop the voltage down to the AGND-AREF range of the A/D (2.5V). They would have to be able to handle high common-mode voltages, something like an AD8206, which can handle 42V ...
That one happens to have a fixed gain of 20, so the divider would have to drop 7V down to 125mV. (R1 = 10k, R2 = 180R) I'm sure there are other opamps out there, this just happens to be one I saw in an Analog bulletin.
I also want to measure total current through the bank. I have a 300A shunt that puts out 50mV at full rating (system does 200A normally). Again, if this is in the +ve line, the conditioning opamp would have to be able to discern some 35mV riding on top of 24V. Question: Can/Should the shunt be put into the GND/0V/-ve line ?
The AD626 looks good for this. The gain can be set anywhere up to 100 - I would a gain of 83 to amp 30mV up to 2.5V.
It can handle a CMR of 24V when operating from a 5V supply. For headroom I guess it should run from a 12V supply.
Is this the right direction to be looking in ? Is there an easier way to do this ? That is, measure large (200A) DC currents using a 50mV shunt, and measure 4 different battery voltages that are in series.
Thanks for any tips :)