What you're asking for is a "Flying Capacitor" circuit.
Imagine a capacitor across each cell, which of course has the same voltage across as the cell.
Now remove the capacitor, measure it's voltage, put it back, remove and measure the next cap, etc. This can be done with small relays, and 20 cells can be measured, fully isolated from the battery, in less than a second. Have a look at this old post.
Leo..