Internal resistance of a battery cell

if a cell is running down, there would be a constant drop in p.d. across the cell, does the internal resistance of the cell increase or decrease?

 

[CWatters]

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
E=emf
I=current
r=internal resistance