Hi, EMF of a cell is the pd across its terminals when no current is drawn from it. Terminal voltage is the PD across a cell when a current is drawn from it. When the circuit drawn current e.m.f is larger than terminal voltage. During charging terminal voltage lags behind the e.m.f.
Terminal voltage V=E-Ir , when nod charging. Where,E:EMF of the cell. I:current. r:internal resistance of the cell.
Terminal voltage,V=E+Ir , when charging.