Renaissance Charge, LLC., used these caps in the Electric Porsche Vehicle. After over one year of daily use, no water was needed to be added.
You need three per 6V battery.
The Hydrocap replaces the regular cell cap. When the cell is gassing,
the hydrogen and oxygen gasses are vented into the Hydrocap. Inside the
Hydrocap, a catalyst of platinum and other platinum group metals
recombine the gasses into pure water. This water is then dripped back
into the cell. The Hydrocap recycles the water that the cell gives off
as hydrogen and oxygen gasses. This eliminates the danger posed by the
hydrogen gas and vastly reduces watering the cells. When the cell is
gassing, some of the recharging energy is not being stored in the cell,
but is breaking down water into its constituent elements- hydrogen and
oxygen. Some of the energy used in the conversion of water into
hydrogen and oxygen is retrieved by the Hydrocap. When the Hydrocap is
operating it gets warm. This heat energy is a by product of the
catalytic recombination of the hydrogen and oxygen back into water.
While this may seem just an interesting aside, we found the Hydrocap's
warmth very useful as an indicator of the cell's state of charge. 