Black Stain Around Battery and Sulfur Smell

Electrical and electronic topics for small boats
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Black Stain Around Battery and Sulfur Smell

Postby robbyrosa » Mon Aug 01, 2016 3:40 pm

I have a 1983 Montauk with a 1997 Evinrude 90=HP engine. The boat's battery is new in a new battery box that I have mounted to the deck in stern on the port side. There is a spreading black stain on the deck around the battery box and a strong sulfur "rotten egg" smell in the boat when we get in it. The boat is running fine. What is source of black stain and smell?

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Re: Black stain around battery and sulfur smell

Postby Wweez » Mon Aug 01, 2016 11:14 pm


The engine is overcharging the battery.
How you fix it is beyond me, except to take it to a good mechanic.

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Joined: Tue Oct 20, 2015 8:46 am

Re: Black stain around battery and sulfur smell

Postby Acseatsri » Tue Aug 02, 2016 9:46 am

Or the battery has a cracked case.

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Re: Black stain around battery and sulfur smell

Postby kwik_wurk » Tue Aug 02, 2016 11:49 am

As per the two prior comments, you very likely have either over load, over charge charge, or a short causing this problem. You could have a cracked case. That is easy to check for and see.

If it's a conventional lead-acid battery, pop the tops off and check water levels. Use a cheapo battery tester to check fluid quality. If something is way off, return the battery. You'll want to have distilled water handy to top off each cell.

Anything that vents from the battery is going to be a heavy gas, so it will sit in the boat if there is no airflow.

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Re: Black Stain Around Battery and Sulfur Smell

Postby richv » Mon Aug 15, 2016 8:03 am

This is usually a result of overcharging. Buy yourself a decent digital voltmeter (DVM) if you don't already own one. Start up the engine and measure the charging voltage. Let the engine run until the battery voltage levels out. If it exceeds 14.0 volts by much, the battery is being overcharged.

The "rotten egg" smell is caused by hydrogen sulfide being outgassed by overcharging. The black stain is a chemical reaction. You need to address this quickly, since overcharging will destroy the battery, and many battery places will be able to identify overcharging as the cause of the destruction and will void the warranty, assuming this is a consideration.

Even if it isn't, the fumes will eventually damage the wiring, resulting in very expensive rewiring. Also, while it's rare, overcharging can result in catastrophic overheating of batteries. While I've never known it to cause a fire with lead-acid batteries (as it can with Lithium, Ni-Cd, and NiMh batteries), I've seen cases melt or split and spill acid everywhere.