Question: Why do some gel batteries bulge? Are there any visual signs of a faulty or plugged pressure relief valve? Answer: In order to operate property and for the recombination of hydrogen and oxygen to take place, each cell is under approximately 1 1/2 psi of pressure. Batteries with very large cells, such as the 8G4D and 8G8D, will bulge somewhat as this normal pressure builds. This is especially true in higher temperatures, because the polypropylene case is more pliable. Therefore, a certain amount of bulge is normal. However, if a battery bulges severely, to took like a football, this is not normal and is an indication of a blocked valve. Such a battery should be taken out of service.