Marty, the exploding battery issue is not caused by low battery electrolyte level. The problem is, in my opinion, caused by the constant overcharging of the battery by Generac's charging circuit in some generators.
A maintenance-free battery would not tolerate the overcharging well. I do believe I've heard of that battery type exploding as well.
Skipp, I will agree and semi-disagree with your thoughts. The over charging can and will cause the electrolyte to "boil" out. When the liquid level gets below the cells they can and will arch to each other when put under a load such as cranking. This arch is what sets off the hyd. sulfide gas making it go boom. That is what happened to me when trying to start a 40kw. And it was a maint. free battery. Now before I even attempt to crank one I check the level. Between charging and hot summers I have found some that I needed to add up to 32oz of electrolyte too. We are now in the process of when a battery needs replacing we are going to gel cell batteries. But with about 180 generators it will take a while.