While it's a relatively new battery, there also exists the possibility that an internal lead plate jumper has a crack. (mechanics.stackexchange.com)
They have six two - volt chambers, called cells, that contain a grid of lead plates submerged in sulfuric acid. (mechanics.stackexchange.com)
Hydrogen gas is a byproduct of electrical energy created from a chemical reaction of lead plates submerged into the acid and water. (mechanics.stackexchange.com)