How much water did you need to add to the battery last month, and the one before that? Letting one go six months without at least checking the electrolyte level isn't a good idea, as just normal evaporation will cause battery cells to get low. Depending on the state of charge of the battery, terminal voltage can be anywhere from 13 volts to 14.7 or slightly more. Voltmeters are good, ammeters are better, but the best way to check a charging system is a hydrometer. A fully-charged battery will have electrolyte specific gravity of 1.250 to 1.265, depending on the strength of the acid that was installed when the battery was new. High specific gravity with minimal water loss is the ideal situation, but I can't think of any modern charging systems that have an adjustable voltage regulator.
Jerry
