Andy, the energy in your storage battery is stored primarily in the
compounds on the surfaces of the lead plates, and "topping off" the
electrolyte with distilled water won't seriously "dilute the
charge"!  I suggest that you _use_ the battery when you play L'Orgue
Mecanique for your relatives, and then give it another charging
cycle to prepare for the North Tonawanda Band Organ Rally.

Alternatively, you might rent a quiet and small Honda "Put-Put"
generator for the event, or borrow a generator from a motor home.