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Battery Compartment Explosion

Mark Isaac
 

Hello All,

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.

Mark Isaac
Lulu, SM 391
South Freeport, ME
greatketch@...
 

Mark,

Sorry to hear about your excitement, but glad the only damage was to the boat.

I assume you have standard flooded lead acid batteries--if not--then things are strange... 

Just from normal charging, hydrogen is being generated by the batteries.  It would be a bad assumption that a passive vent is sufficient to ALWAYS keep the battery box hydrogen concentration below the level that will ignite--given a spark. The problem is that hydrogen has such a very low flammable limit in air, between 2% and 6% depending on how you define it.  Certainly local concentrations of H2 in the box could get that high in normal operation.

And a loose battery cable certainly can create the needed spark...  

Moral of the story...  battery cables need to be kept tight, and any kind of equipment added inside the battery box need to be ignition protected--or better yet mounted outside the box.

Bill Kinney
SM160, Harmonie
Annapolis, MD, USA
Mark Isaac
 

Thanks to the person who linked my post to the appropriate conversation.

Yes, they are marine deep cycle flooded lead acid batteries.

Bill, thanks for what I am taking home from this: no need to look further for problems with the cells or system and keep the terminals tight.

Mark Isaac
SM391, Lulu
South Freeport, Maine
Ralph Caruso
 

I used to work in a field where water was disassociated into hydrogen and oxygen, and I can tell you that any accumulation of this mixture (called  a "stoichiometric mixture) can explode at the very slightest application of energy.  We had several experiences where this mix accumulated inside steel pipes, with no ignition source available whatsoever, and detonations occurred.  The ONLY way to avoid a detonation is to vent the space, either by forced ventilation or with natural ventilation pathways  that continuously slope upward, with NO voids that could accumulate the gas. I remember that the amount of energy that was required to cause a detonation was described as "the amount of energy that a mosquito uses to do a pushup" - somewhere around 0.5 erg, if I remember correctly - so all sorts of things can set it off.

I am not an Amel owner - I lurk here for good ideas that I can use on my Jeanneau - but I have 4 golf cart batteries, one engine starting battery, and one genset starting battery under the berth in my aft stateroom, and I have installed those catalytic combiner battery caps on the golf cart batteries, while the starting batteries are "sealed" lead acid.  I also have a small fan that exhausts the battery compartment into the boat whenever it senses voltage on the batteries above 13v - ie, charging is occurring.  I would prefer to vent overboard, but can't figure out a good piping solution that would also keep the water out while ensuring that it NEVER slopes downward.  I figure that the risk of water coming into the boat from a bad vent design would be higher than the risk of a hydrogen accumulation inside the boat - it is not very airtight, with the main hatch almost directly above  the batteries.

Interesting to hear about hydrogen explosions in boats...

Ralph Caruso
s/v Petillant
Jeanneau SO43DS
Alan Leslie
 

Seems to me that a lot of these issues could be solved / eliminated by using sealed batteries...like AGM
If all the regs are set correctly there should be no gassing form VRLA  (AGM) batteries and this whole explosion hazard is minimized, not to mention the PITA  regular maintenance of keeping wet cell batteries topped up. 
Cheers
Alan
Elyse SM437

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