Question: I have what I think is a simple question. In the photo I sent in you can see the batteries on my boat with clear hoses connected to them.
When I bought the boat I didn’t think too much about these hoses, I just assumed they were to assist in topping up the cells with water when and if they ever got low on fluid. The trouble is the clear hoses connect to another hose you can see in the photo, and then that hose is routed upward behind the settee and the cabinet behind it. The hose basically disappears behind this cabinetry. So I’m guessing my original thought about the hose going to a point where I could pour in more water to top up my batteries is not right. Can you explain what I’m looking at here? Where does that hose end up anyhow?
Answer: You are off-base in your thinking. The batteries on your boat are of the flooded but sealed variety. This means that yes, they do contain liquid electrolyte, but they should never need to have water added during their normal service life.
The batteries are not necessarily completely sealed either. In fact, the hoses you see are plugged onto small hose nipples mounted on the battery cases.
Just inside the case there is probably a small check valve that is designed to open only in the event of excessive pressure build-up inside the battery case, typically around three psi. This excess pressure will occur if the battery gets exposed to an overcharging event caused by a faulty battery charger or voltage regulator on your engine’s alternator. Either way this is a problem that would need immediate attention, as it would damage the batteries rather quickly. The hoses are there to vent the excess pressure.
Hoses like these are often led outside the boat or attached inside the boat to a tubular fitting mounted in the base of a lifeline stanchion, so that the hydrogen gas that is being released will leak out into the atmosphere via the stanchion. I’ve seen this same methodology used to vent fuel tanks as well -- it gets the unwanted gasses outside of the boat and into the atmosphere where they can dissipate quite rapidly.