What Causes Battery Failure?

VRLA Batteries, very common in UL 1778 and UL 924 UPS and Emergency Lighting systems, should last 7 to 10 years. But most batteries don't die of old age. They are usually damaged or severely weakened because the environment they work in. While the temperature ratings of most power conversion electronics are 104F minimum, the batteries are not going to last in that kind of environment. Most utility rooms or closets are not ventillated, not to mention air conditioned. So when batteries have to reside in heat about 95 degrees F, they immediately age prematurely.
Another reason batteries fail is due to being discharged and then left for days at a time. Batteries can be discharged very rapidly or slowly; but in either case they should not be left in the discharged state. This is not a problem for Lithium-Ion cells, but it most definitely is a problem for Lead-Acid whether it's pure lead or lead-calcium or some other alloy.
Finally, frequent cycling combined with one or more batteries reacting differently can result in some batteries being starved while others become bloated (literally!) from getting to much charge. While an UPS or other battery-based energy storage systems should be able to come online as needed, the batteries can still get away from pack. The image this brings to mind is "cat wrangling" where every create has a mind of it's own. No herd instinct here! No, one battery will not accept a charge very well and results in damaging other batteries. Also, any time there is a battery failure the result can be damage to the battery charger system.