Batteries should be filled with distilled water while battery is at a FULL state of charge. Fill to just at or below the lower edge of the fill tube. The tube extends down about an inch from the top of the cell. There are several aftermarket fill kits that will do this for you. I utilize one that fills all 4 house batteries simultaneously to the correct height.
Lot's of info available. A decent place to start:
Battery School | Batteriesnorthwest.com | General Battery Care Procedures
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