What is the maximum common mode voltage when using the ADC24 in differential mode?

I need to monitor individual cell voltages in a 5 cell series string but the 4th and 5th cell readings are incorrect - what is wrong?

When using the ADC24 in differential mode to measure up to a 5V differential voltage (with variable common mode voltage from 0 to 16V) -

are both channel 1 and channel 2 (channel -1) fitted with identical 2:1 resistor dividers as would be used if the 5V was measured single ended?