Ask Sail: An Explosive Situation

When the engine is under load and moving the boat at a decent speed, the voltmeter is pegged past 16 volts, and after a little while the wires (which are appropriately sized) running to the battery switch from the engine get pretty hot. Am I hurting my batteries?
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Jonathan Kirby of Quincy, Massachusetts, asks:

I have a 1976 Hunter 30 with a Yanmar YSE12 diesel engine. The electrical system consists of two batteries wired in through a Guest battery selector. At idle speeds the DC voltmeter at the main panel reads 12.5 to 13 volts. When the engine is under load and moving the boat at a decent speed, the voltmeter is pegged past 16 volts, and after a little while the wires (which are appropriately sized) running to the battery switch from the engine get pretty hot. Am I hurting my batteries? Do I need a new alternator?

Nigel Calder replies:

You are not only hurting your batteries, you are at risk of having them explode! At this high charging voltage, they will be gassing vigorously as the water in the electrolyte is being broken down into hydrogen and oxygen (the batteries will sound like they are boiling), while generating a lot of heat. The batteries will be in a condition known as thermal runaway, with the alternator running at a high output (which is why the cables are hot). If there is an internal or external spark, the hydrogen can cause an explosion either inside or outside the batteries.

NigelCalder

The cause is almost certainly a faulty voltage regulator. The regulator is inside your alternator, so you need to take the alternator off and have it checked out. You should not run the system until the problem is sorted. I regret to say your batteries are also almost certainly toast. 

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