Deviation from Ideal Battery Model
In practice, the voltage of a battery decreases as it delivers more current.
Note that the voltage of a battery also decreases as it discharges, and we won’t model that here.
Our ideal voltage source model doesn’t show this voltage decrease.
Battery Model with Internal Resistance
We can model a battery as an ideal voltage source in series with a resistance.
This resistance is called the internal resistance.
This model shows a linear decrease in the voltage with increasing current from the battery.
Simple Battery Model
Voltage Divider
We can see this reduction in voltage by using a voltage divider as a model.
The current across the internal resistance creates a voltage that subtracts from the open circuit voltage.
Open Circuit Voltage
The voltage measured at the battery terminals when no current is flowing.
Short Circuit Current
The current delivered by the battery when the terminals are shorted together.
IR in Series
Adding batteries in series increases the overall internal resistance of the pack.
The resistances add and the overall resistance is the sum of the individual resistances.
IR in Parallel
Adding batteries in parallel lowers the overall internal resistance of the pack.
We use the rule for parallel resistors to calculate the overall resistance.