I have a battery cell monitoring device that currently uses a COTS cell stack monitor, the LTC6811 from analogue devices.
This part is becoming expensive, difficult to find, and i'd like to have more controls felxibility in terms of the data we can get from our cell monitoring system
I'm looking at options, which currently involve some sort of micro with isolated coms (CAN, RS485, isoSPI etc) and to keep costs down i'd like to try to use the on-board ADC of that micro.
Realistically this probably means a 12b ADC resolution, perhaps 13 with a bit of s/w oversampling, but i'd also like 1mV of resolution on my cell voltage measurements (1+-5mV absolute accuracy would be fine)
For now, consider a fairly short series sting of cells, putting cell voltages within say 20 volts of the local ground reference for the ADC, that means with cell voltages up to 4.5v (and down to 2.5v, no requirement to measure a true "zero" voltage)
So, the issues are measuring a differential voltage of say 2 volts on top of up to 20vdc of common mode, and maybe signifcantly more AC CM (due to high frequency system noise etc)
I have no great requirement to absolutely minimise either overal current consumption (current pulled from the 20v string) or individual cell discharge as our cells are large (hundreds of Ah) but i'd like to try to keep the drain even as possible between cells to avoid differences in discharge over storage periods (which could be reasonably long, ie 6 months perhaps)
I've considered using a relatively high voltage instrumentation amp (INA) as the front end (36v is fairly easy to come by) and this means a unity gain INA (low cell discharge) but requires a true rail to rail input INA for the top and bottom cells in the stack (Top cell pos is at stack pos, bottom cell neg is at stack neg). Follow that INA with a subtracting opamp to remove say 2.5v, and we get 0 to 2v (easily set with playing with the gains) for 2.5 to 4.5v cell differential voltage, so decent resolution even from 12bits. Downsides are the higher cost (and limited availability these days!) of that high voltage R2R INA and they tend ot be current hogs to some degree (i can turn off the supply to the INA when the device is not actively monitoring the cells, so overall stack current draw is not that important)
Other options are an upfront high value resitive divider on the cells, to bring all the voltages down into the range of a more conventional 5v (single supply) INA, but those resistors are going to increase individual cell drain currents and add some common mode offsets, particularly in the AC domain
I know there are amplifiers designed for high side current shunt measurement that have a high common mode capability, but i don't have a feeling for the precision of such devices and i'd have to resistively divide the cell votlages down considerable to get into the sensible gain range for such devices (typically 20, 50 or 100V/V)
Another option i've not yet thought through is using the differential inputs of the ADC, and whilst this gets me the cell voltage, it looses me some resolution because it's harder to offset some of the voltage i don't need to ever measure (ie < 2.5v)
Oh, i'd like to be able to get the cell voltages roughtly 50 times a second, so an analogue b/w of something over 300Hz looks to be in the ball park
Is there an approach i've missed? I've not considered multiplexed front ends, some sort of switched capacitive divider or anything like that. Interested to hear opionions, suggestions and experiences?