In a combination having cells combined in parallel, the combination has the emf across it equal to the emf of cel having maximum potential difference across it as the rest of the cells will act as resistance.
in case of serial combination, the total emf is the sum of all the emfs
Actually the thub rule in all the cases is, the potential at a point creates a reference point for all other connections.Eg.
say point two batteries AB and BC are connected in series having emf as 2V and 3V. let point A have 0 potential thus due to potential difference of AB point B will have potential 2 V and similarly C will have potential 5 V
thus total potential difference in the combination across AC is 5V