Question: OK, I’m totally miffed. I was checking around the battery compartment on my new boat the other day and took the photo you see here. What’s that hose coming out of my battery? Is it supposed to facilitate pouring distilled water into the battery to replenish any used electrolyte? The battery does seem to have a sealed top, so that makes a bit of sense. Trouble is, I can’t find the other end of the hose shown (the black and orange hose) and I’m not sure how to figure out how much fluid I would add when needed.

The black and orange hose vents the sealed battery in case of overcharging. Don't pour water through it!

Answer: Yikes! Don’t even think about trying to pour water into that hose! The hose is acting as a vent, in case excess pressure builds up inside your battery due to overcharging. The hose is probably routed to the base of a lifeline stanchion or some other outside fitting on your boat. Some clever builders use stanchions as piping for vents to things like fuel and water tanks, so why not a battery?

You’re right, you’ve got a sealed battery here and you don’t want to add water to it. As long as it’s not exposed to overcharging due to a faulty voltage regulator or battery charger, the fluid inside the battery will re-combine and essentially never need to be re-watered.

The hose is attached to a check-valve type vent fitting that will only open if excess pressure (about 3 psi) builds up inside the battery. Then, and only then, will the vent hose be called into action. Pouring water into the hose would be futile anyhow, as the check valve will prevent the water from entering into the battery.

- Ed Sherman