The capacity of the battery is 80Ah at a 1C discharge rate. The document defines 1C as the rated capacity of the battery. At a discharge current of C/2, which is half the rated capacity, the battery current would be 80/2 = 40A. Given a discharge current of 12A, and knowing the capacity at C/2 is 40A, the document can be used to calculate the internal resistance R of the battery as 0.3 Ohms.

2. Given
Capacity of a battery be 1C=80A h. (1C usually denotes rated capacity of
the battery.)
Battery discharge current Iss = V/R
= 12/R
Capacity depends on the magnitude of discharge current.
For discharge rate C/2 Iss = 80/2
R = 0.3 Ohms