Thevenin equivalent circuit of a battery
When you say that the battery "draws a maximum of 10A" do you mean that when a true short circuit is applied to the battery, so that the terminal voltage is zero, that the supplied current is 10A? If so then your calculation is correct. If that is the maximum rated current you should draw from the battery then you need to know the voltage at the battery terminals when 10A is being drawn.

If you want to determine the Thevenin equivalent voltage and resistance without overloading the battery, then apply some known resistance \$R_L\$ and measure the output voltage as \$V_L\$. Measure the voltage without a load as \$V_{OC}\$. The voltage divider equation tells us that

\$ V_L = V_{OC} \frac{R_L}{R_{TH}\times R_L}\$

Solve for \$R_{TH}\$, and you know that \$V_{TH} = V_{OC}\$.