I used to have one of those fillers when I had my gas station. You just shove the nipple down into the battery hole and push. You will hear gurgling if the battery needs water. When it's just at the right level it stops. If the batter didn't need water in the first place, it doesn't do anything. How it works is, inside the nipple, there's a ball. This ball acts as a checkvalve. When the water level gets high enough, the ball floats up and plugs the hole and stops the water flow. ---Anthony
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