There are lots of ways to model a battery cell. One way is with a constant or variable EMF and a variable internal resistance that increases as the cell discharges. That's not a bad model for alkaline cells.
However different types of cell (eg different chemistry) need different models. There isn't a one model that fits the behaviour of all cells.
Emf of a cell is the potential difference between it's terminals when terminals are connected externally.
∆V=E - I*r