Why do some AGM or GEL batteries bulge?
To prevent the permanent loss of gases so that the recombination process has time to take place, each cell can hold approx. 1.6 pounds per square inch (psi) of pressure without venting. (the battery has a VALVE-REGULATED lead-acid design).
Large batteries with very large cells will bulge somewhat as this normal pressure builds. This is especially true in higher temperatures because the polypropylene case is pliable. Therefore, a certain amount of bulge is normal. The valves only let the gas out, never in.
A partial vacuum (sucked-in appearance) can form within a sealed battery under various circumstances.
Battery temperature and ambient pressure play a role, but predominantly the recombination and discharge reactions are responsible. After charging ends, the recombination reaction continues until most oxygen in the battery is consumed. The total volume within the battery decreases slightly during a discharge. Deeply discharged batteries often have a “sucked-in” appearance. Batteries with large cells may display this appearance even when fully charged. If a battery bulges severely on a charge, it may indicate a blocked valve or an overcharged situation. When a battery is in this situation, it should be removed from service.