Output impedance

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. 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:

$$R_B=\left( \frac{Vs}{I} \right)-R_L$$

or

$$R_B=\left( \frac{V_S-V}{I} \right)$$

where

$$R_B$$ is the internal resistance of the battery

$$V_S$$ is the battery voltage without a load

$$V$$ is the battery voltage with a load

$$R_L$$ is the total resistance of the circuit

$$I$$ is the total current supplied by the battery

Internal resistance varies with the age of a battery, but for most commercial batteries the internal resistance is on the order of 1 ohm.

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 energy wasted by the so-called "internal resistance" of that cell. This wasted energy shows up as lost voltage. Internal resistance, r=(E-V)/I

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