The potential difference greater than emf when battery is being charged.
Explaination: We know that V=E-IR
When another cell with higher emf (electromotive force) is connected in opposite direction with the main battery the new battery produces a current in opposite direction so the current in the previous battery becomes negative. 
When the current 'I' becomes negative, the equation is changed and it is :		   
V=E-(-I)R 
V=E+IR
that is E is less than V

Context