The relationship between the terminal voltage V of a battery and its electromotive force [tex]\epsilon[/tex] is given by
[tex]V=\epsilon - Ir[/tex]
where I is the current and r the internal resistance.
In our problem [tex]\epsilon=12 V[/tex], and when a current of [tex]I=89 A[/tex] flows, the terminal voltage is [tex]V=8.3 V[/tex]. So we can find the value of the internal resistance:
[tex]r= \frac{\epsilon - V}{I}= \frac{12 V-8.3 V}{89 A}=0.042 \Omega [/tex]
