electric circuits - Relation between Terminal Voltage and EMF of a cell while the cell is being charged

I know the relation between the terminal voltage, V and emf of the cell, V' while charging given by

V = V' + Ir

where I = Current in the circuit and r = Internal Resistance of Cell

Can anyone please explain why in charging a cell, the sign changes from - (which is when the cell discharges) to + in detail by deriving the formula ?

Thanx in advance

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@Farcher Can u explain/deduce that formula(V = V' + Ir) on the basis of following diagram:-

The diagram shows a cell of emf, V' and internal resistance, r being charged up by a charger.