Thevenin resistance of a car battery?

Question

If I have a 1.3 ohm load on a car battery and measure 12.46 volts at the battery terminal and then I also turn on the radio and measure a total of 20 amps of current flowing and now have 12.3 volts at the battery terminals; What is the Thevenin resistance and Thevenin voltage associated with the battery

Answer 1

You're taking 10Amps, increase it to 20Amps and get 0.16V extra drop at terminals.
So on 0 Amps the terminal voltage would be 12.62 volts.
The internal resistance is 0.016ohms or 16milli-ohms

Answer 2

Change in voltage divided by change in current