A device which will detect a difference in voltage
I'm not sure I understand the configuration, especially the "not relative to ground". Does that mean the cells are in series?
In that case there's a simple solution. Connect a 1:1 resistor divider between \$V_-\$ of the lower cell and \$V_+\$ of the higher cell. \$V_+\$ of the lower cell (= \$V_-\$ of the higher cell) goes to the inverting input of the comparator; the midpoint of the divider goes to the non-inverting input. This forms a Wheatstone bridge with both cells in one branch, and the resistors in the other. If the lower cell's voltage is higher the output of the comparator will be low. Both cells will have voltages close together, so some hysteresis may be needed.