Hi,
Is MUX OK:
every MUX has it´s datasheet. Read them.
Especially:
* introduced charge
* leakage currents
I assume the sampling rate is very slow, therefore the introduced charge may cause no significant error because it is compensated by the source impedance.
I assume the leakage currents of analog MUX will also have no signaficant influence on your measurement precision.
Source impedance:
Maybe you use 1M - 50k resistor divider. Then the source is about 50k .. not that high when buffered with an OPAMP
The 1M - 50k divider brings down 80V to about 4V. On a 5V system this leaves some headroom for high charging voltage.
Guard ring:
For yure you can do.
I call it useless, because with a usual (clean) PCB one can expect leakage currents in the range of some 10 nanoamperes.
It won´t cause much error. I´d keep the sensitive node short and increase the distance to other tracks.
(If you use a guard ring, then you need an actively driven around the center node of the voltage divider. Too much effort)
1M and PCB:
1M is the input resistance... but there is no node that is that high ohmic. (unless the input is left OPEN ... but then the measurement is invalid)
Your highest ohmic node will be around 50k (multiplied with 50nA is 2.5mV, x 20 gives about 50mV cell voltage error...worst case)
1.6mA.
Your math is not correct (you realized it already). It is about 0.84mA.
What is the expected charge to charge period time in hours? Multiply this value with 0.8mA and get the drained out charge.
What is your cell charge? Calculate the relation of both values.
Klaus