Any linear electronic circuit or device which supplies a current may be modelled as an ideal voltage source in series with an impedance. This is helpful in analysing the voltage drop which occurs as current is drawn.
Batteries
Internal resistance is a concept that helps model the electrical consequences of the complex chemical reactions inside a battery. When a current is flowing through a cell, the measured e.m.f. is lower than when there is no current delivered by the cell. The reason for this is that part of the available energy of the cell is used up to drive charges through the cell. This wasted energy is the so-called "internal resistance" and shows up as lost voltage.
It is impossible to directly measure the internal resistance of a battery, but it can be calculated from current and voltage data measured from a circuit. When a load is applied to a battery the internal resistance can be calculated from the following equations:
or
where
RB is the internal resistance of the battery
VS is the battery voltage without a load
V is the battery voltage with a load
RL is the load resistance
I is the total current supplied by the battery
The above equations calculate the difference between the expected resistance with no ouput impedance, as given by Ohm's law, and the actual resistance.
Internal resistance increases with the age of a battery, but for most commercial batteries the internal resistance is on the order of 1 ohm.
It should be noted that the above only applies to ideal batteries under ideal load conditions and does not directly relate to real world internal resistance of batteries due to the chemical nature of the cells.
