The optoisolaters consumes a lot of current, which would require that you can disconnect them between each measure unless we are talking about very large batteries.

It's way better to either have an ADC with enough resolution to measure all the individual voltages relative to the common ground and then subtract the value of the previous cell. Or have a mux that does mux both sides of the battery. Just detecting cell failures does not require you to have a huge resolution for each cell. If you have 5 bits for the cell, you can measure 32 levels. For lead-acid, full range for a cell would be 2.5V so 80mV / ADC step. And a 10-bit ADC would manage 32 cells.