Question

What is the internal resistance of a 12.0-v car battery whose terminal voltage drops to 8.4 v when the starter draws 70 a ?

Answer 1

The relationship betwen terminal voltage and electromotive force of a battery is given by:

`V=\epsilon - Ir`
where
V is the terminal voltage

`\epsilon` is the electromotive force
I is the current
r is the internal resistance of the battery

In our problem, V=8.4 V,
`\epsilon=12.0 V` and the current drawn is
`I=70 A`, so we can find the internal resistance of the battery by re-arranging the previous equation:

`r= (\epsilon-V)/(I)= (12.0 V-8.4 V)/(70 A)=0.051 \Omega`