I use these on three golf carts, which (with the factory caps) have a serious problem with spilling electrolyte from the vents, lowering the SG of the electrolyte, and corroding battery tie-downs.
The only other recommended solution is to underfill the battery cells, but then topping the batteries off becomes a hot-or-miss proposition.
The Water Misers work just like the description, and reduce spills and cart corrosion. They'll certainly be a part of every golf cart I ever own!
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