Why do some AGM or GEL batteries appear sucked-in?

A sucked-in appearance is quite normal. A sucked-in battery should be charged, but if it remains sucked-in after charging, the appearance can safely be ignored. However, if only a single cell displays or lacks this appearance, a load test would be prudent. A partial vacuum 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. To prevent the permanent loss of gases so that recombination 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).