Some lead acid batteries including AGM and Gel batteries have a natural swelling. So, due to the way that they are built by enabling the absorption of gases that are produced during the chemical procedure inside the battery. These types of batteries are tightly packed and when the cells plates increase in any way then the battery case takes the added pressure. Of course, this is described as “recombinant”. So in this type of battery then slight swelling can be a common feature with little consequences. However, overcharging the battery could be another matter. Overcharging can be dangerous and make the battery casing swell way out of proportion. Of course then overcharging should be well avoided.