Ask Sail  Ask Sail: An Explosive Situation  Bookmark and Share
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