After a week on the hard for a bottom job, bow thruster service, c drive service, salt and fresh water coolant flush to both engines, we launched Friday afternoon. Fifty feet from the dock, there was a loud noise (loud enough to cause the prairie dog effect on the dock) and a faint but unmistakable smell of something burning. I won't bore with the details between the event and the diagnosis, but the three hinges on the battery compartment were blown off and there were 9 inch or so cracks in the plywood at each of the points where the cover is screwed down.
I checked each of the cells with a battery tester and found no shorts. The vent hose was not obstructed when I checked it with a dinghy inflator. It is possible it was obstructed before the event and the force of the explosion blew it clear, but I doubt that based on the absence of detritus on deck under the vent. The next day, I ran both the 30 and 100 amp chargers and didn't see anything unusual on the Xantrex. The temperature of each of the batteries when charging were all within a couple tenths of a degree of one another (73F). The batteries are about 10 months old. The water level was checked and each cell topped off about 4 months ago. I checked all of the connections an found one that was loose and likely served as the source of ignition.
I can't explain why the concentration of hydrogen in the locker built up enough to allow combustion given the venting system.
I will install lock washers on the battery terminals to minimize the risk of loose connections. I am considering changing the venting system from passive to active by adding a small fan on one end or the other of the vent hose, but can't see why this should be necessary with the batteries and chargers working as they should.
Any thoughts or insight would be appreciated.
Lulu, SM 391
South Freeport, ME