Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

The resistance of the battery is $~0.43~\Omega$
We can find $R_b$, the resistance of the battery: $R_b = \frac{V}{I}$ $R_b = \frac{9.0~V}{21~A}$ $R_b = 0.43~\Omega$ The resistance of the battery is $~0.43~\Omega$