Can the resistance between two points in air be calculated? How?
It's more likely that the surfaces of the battery will form a better conduction path than the surrounding air. For instance, the surface resistivity of Nylon 66 is about \$10^{11}\$ ohms per square. See this document for other common plastics.

Given the formula for volume resistivity (R = \$\rho \cdot L \over A\$) you could calculate what an equivalent volume of air would introduce "in parallel" with the surface resistance of your battery enclosure.

However, all the notes that I find on recommendations for measuring surface resistivity suggest that the air must not be humid when taking measurements as this will lead to errors. Given that non-humid air doesn't cause a significant error when making surface resistivity measurments it's probably reasonable to assume that the dominant conducting material around the terminals of your battery is the surface of the battery's material and not the air.

So, I would say that it is an interesting but fruitless exercise to calculate the resistance of air.

How would you calculate the resistance of a volume of air given that the resistance between two points may be X?



See also Phil Frost's attempt at humour in one of his comments LOL