"Bill, as IBNFISHN eluded to,
"Bill, as IBNFISHN eluded to, Mercs charging system is "unregulated" on most of their motors.
Intead of a voltage regulator, they simply use a rectifier. It changes the AC produced by the charging windings on the stator into DC and uses the battery as the regulator/capacitor.
Because of that (somewhat unstable voltage) a (maintenance) battery has the ability to off gas if the electrolyte gets boiled a bit by wild voltages. And while that is not good for the battery and will definately reduce it's lifespan, that is infinately better than using a sealed (maintenance free) battery that's only option may be to "explode" if it can't handle all that unregulated power.
You just need to occasionally check your "maintenance" batteries and if the levels are getting a little low you can top them off with some distilled water and perhaps once a year you add a solution to balance them (sold under a variety of names) available at marine and automotive places..."