Why do some AGM batteries develop a concave appearance?
Sometimes the outer view of an Absorbed Glass Mat (AGM) battery will appear withdrawn. It usually is due to atmospheric compression, which is common in Valve Regulated Lead Acid (VRLA) batteries. The valves in this type battery serve to retain gas generated during the charging process so that it can recombine into liquid and maintain the proper ratio of liquid in the cells. When the battery discharges during normal use, the valves also prevent outside air from being drawn into the cells, thus possibly creating a concave appearance. It doesn’t always occur, but if it does, it is not an automatic indicator of battery condition.
After recharging a battery with a concave appearance, the case usually will return to normal. Proper environmental temperatures (below 100F ambient) will also prevent concaving, due to the pliable nature of the plastic used in battery construction. Either way, good practice is to observe batteries on a regular basis to assure best performance.