The relationship betwen terminal voltage and electromotive force of a battery is given by:
[tex]V=\epsilon - Ir[/tex]
where
V is the terminal voltage
[tex]\epsilon[/tex] is the electromotive force
I is the current
r is the internal resistance of the battery
In our problem, V=8.4 V, [tex]\epsilon=12.0 V[/tex] and the current drawn is [tex]I=70 A[/tex], so we can find the internal resistance of the battery by re-arranging the previous equation:
[tex]r= \frac{\epsilon-V}{I}= \frac{12.0 V-8.4 V}{70 A}=0.051 \Omega [/tex]
