For old-school, non-sealed flooded-type lead-acid batteries, you need to keep an eye on the water level. If the water drops below the top of the lead plates, sulfation will occur, lowering the battery’s capacity and leading to an early death. Check the fill lines on the side of the battery and make sure the water level is in between those lines. If not, fill each cell with distilled water to the fill lines. If you’re going to charge the battery after filling, put only enough water to cover the plates because the level will rise as it charges.