Sail with New Freebooters

While researching the subject of electrical installations on boats, I came across warnings about poisonous and corrosive gases that could be given off by lead-acid batteries in various situations. They were always just baldly stated, with no references to chemical or medical literature to corroborate them, so I have been doing a bit of research to answer the following questions about each gas:

1. Can lead-acid batteries really cause the gas to be given off?
2. If so, how?
3. What are the effects of long or short term exposure to the gas?

The gases I have seen mentioned are:

• chlorine
• arsine
• stibine

Here’s what I have managed to dig up so far. If you can provide references to scientific or medical literature that adds to it, please do.

Chlorine

The claim is that if battery acid is mixed with seawater, chlorine will be given off. During the period in which Thomas Edison was persuading the US Navy to switch to his alkaline batteries, there are many claims that the sulphuric acid electrolyte in lead-acid batteries gives off chlorine gas on contact with salt water. I have done a quick search of scientific and medical fora and papers, but I can find no justification for this claim.

As far as I know, the reaction only takes place in the presence of manganese dioxide. However, perhaps this was commonly present as a contaminant of either the steel used for ballast tanks or the paints used at the time?

Chlorine is not given off as part of the charging and discharging processes, either - even if the batteries are boiled dry by overcharging.

In any case, concentrations of chlorine well below the lethal dose will immediately have you choking and rushing for the exit, holding your breath. Trust me - I’ve done it. If chlorine really can be given off, it’s a concern for submariners rather than for yachtsmen. If you do the kind of sailing that puts you at risk of getting trapped inside an upturned boat, there are plenty of other reasons to use AGM batteries rather than wet cell ones to avoid any spillage in the first place.

Arsine (arsenic trihydride) and stibine (antimony trihydride)

The hydrogen given off during charging can bond with traces of metals to form their hydrides. Arsenic is a common contaminant of zinc, which is often a constituent of either the plates or their connections to the battery posts, so there is a real risk  of arsine being generated during overcharging. Stibine (mistakenly spelled stabine in one online article I found) is also likely, since thin plates are sometimes stiffened by addding antimony to the lead.

Both these gases poison you by destroying red blood cells, and they are deadly if you inhale them for several minutes at a concentration of 10 parts per million (carbon monoxide needs to be two thousand times as concentrated to be lethal).

Stibine smells of rotten eggs, like hydrogen sulphide, so is unlikely to catch you unawares, but the garlic odour of arsine is very hard to notice at 10 parts per million - so arsine could be considered more dangerous, especially if you are asleep or suffering from a cold.

So are the risks real?

In my opinion, the risks are greater on sailing craft rather than motor cruisers. Sailing boats usually keep batteries low down and do not have forced ventilation of battery compartments.

Whether you are worried about poisoning, explosion of hydrogen, or corrosion from spilled acid, there is a lot to be said for keeping your batteries in a compartment that is sealed off from the living accommodation.