How much of battery weight transfers to tongue?

I can't remember the exact numbers but last year while measuring my tongue weight on a TW scale I set a Honda 2000i generator on top of the propane tanks to see how much it would effect TW. What I remember was it was only about half what Honda says the generator weighs. I expected almost 1 to 1 weight because it was so close to the ball. Most batteries are 2-4 ft back from the ball so the impact may not be as much as a person would expect.