Measuring battery resistance
Some comments and suggestions:


Measure the voltage at the battery's terminals, not across the load resistor, and do not use needle probes that are touched to the battery's terminals. Use probes with sufficient surface area that can be tightly bonded to the battery's contacts to minimize contact resistance. 
Depending on the probing setup, the Seebeck effect could affect your voltage measurement data.
A resistor's value changes with temperature.  When current flows through a resistor it dissipates power, which causes the resistor to heat up, which changes the resistor's resistance.  This change in resistance value might need to be taken into consideration in your calculations.
A battery's internal impedance is temperature dependent. As current flows through the battery it heats up, which increases its internal impedance.
When measuring low value resistances (10 ohms or less), a 2&#8209;wire ohms measurement is inadequate and should not be relied upon for accurate resistance measurement. For low value resistance measurement, always use a ohmmeter that performs ohms measurement via a 4&#8209;wire "Kelvin" connection. 
(NB: Simply subtracting out the fixturing resistance in a 2&#8209;wire ohms measurement is usually insufficient. See the section titled "Why not just subtract fixturing resistance?" in the CIRRIS Systems article (1) referenced below.)


(NB: In technical writing, the device that is being measured is commonly referred to as the device under test (DUT). In your case the DUT is the battery.)

REFERENCES


CIRRIS Systems, 4&#8209;Wire Kelvin Testing [Online]. Available: https://www.cirris.com/learning-center/general-testing/special-topics/40-4-wire-kelvin-testing
J. Fischer, "Kelvin Type Connection", Electrical Engineering Stack Exchange, June 15, 2019 [Online]. Available: https://electronics.stackexchange.com/a/443829/79842
J. Lockhart, Real Batteries, Lecture 12, PHYSICS 230.01 [Online]. Available: http://www.physics.sfsu.edu/~lockhart/courses/p230l12.pdf
Electropedia, "Battery Performance Characteristics", Battery and Energy Technologies, 2005 [Online]. Available: https://www.mpoweruk.com/performance.htm