Technical info - Bulging


Why do some VRLA batteries bulge? Why do some VRLA batteries appear “sucked in”?

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. Batteries with very large cells - such as the BCI 4D, BCI8D, DIN250, JIS150, JIS200, GC, electric vehicle and scrubber types - 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. If a battery bulges severely on charge, this is not normal. It is an indication of a blocked valve or an overcharge situation. If the charger works properly, such a battery should be removed from service.

A sucked-in appearance can also be normal.

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 of the 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. 

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.