Battery Explosion

Report Text:

My yacht was lying alongside a marina, soon to depart on a voyage.  I had switched off the Genset which had been charging the batteries.  I reached out of the pilot house to start the engine, but could not quite reach the start key so came up the stairs, turned the key and immediately a large explosion happened.

I was dazed, looked into the boat to see the cockpit sole over the battery compartment had gone and smoke coming out of the battery area. I came back into the boat, turned off the battery isolators and grabbed the fire extinguisher and looked to see if there were any flames. I could see none, but a strong smell of battery gas. I opened the hatches to let air circulate and then left the boat.

Description of area and damage.  The battery bay is mounted under the pilot house sole in a plywood box which has been glassed inside.  Two catches secure a ½ inch ply top which runs in a rebate.  The batteries are a snug fit within the box, relying on the box for security in heavy seas.  A 1/2 inch vent pipe runs from the forward end..

Four lead acid batteries are mounted, connected into two banks of two feeding 4 isolators running domestic 1 and 2 and engine and generator start. The forward and aft most batteries are mounted under the lip of the boxes, such that the middle two batteries have to be removed to allow access to the fwd and aft electrolyte covers for maintenance. The electrolyte covers had blown off and the tops of the forward and aft most battery had blown in. There was evidence of the forward battery overheating.

The boat was built in 2004.  An extensive refit had been carried out March 2007 which included the fit of a generator.  I subsequently purchased the boat. No survey was required for insurance purposes.

According to the previous owner, the batteries had been replaced in 2007. We checked the electrolyte levels in the mid two batteries, but could not gain access to the fwd and aft batteries. I asked the previous owner  how he had checked and filled the others and he stated that they had drawn up how the batteries were wired, disconnected them and removed the mid two such that access could be gained to the forward and aft pair.

I had made the decision to run the batteries as fitted and when the electrolyte needed topping up in the mid two, to disconnect and check the fwd and aft batteries. I had noticed that the Battery Monitor showed charging currents of around 80 amps when the battery charger was first switched on. I had noticed a smell of battery gas when I had previously checked the area and had put this down to the high charge rates.

After the accident, an investigation was carried out. When the damaged wood was removed to allow access to the aft most battery positive post, there was evidence of arcing. It was clear that the terminal (secured with butterfly nuts) was loose. Corrosion around the post indicated the connection had been loose for some time. Although much of the electrolyte had been lost from batteries, residue remained in all, but the aft most battery, indicating that this had dried out. This battery was part of the bank which supplied the engine start circuit.


The explosion was caused by a combination of factors.

  • Batteries charging at a high rate causing build up of explosive gases in a very confined area.
  • Engine start being instigated within minutes of charging being carried out.
  • The small vent pipe was inadequate to clear explosive gases.
  • Poor access/poor maintenance resulted in the inadequate tightening/loosening of retaining butterfly nut. The resulting spark when the engine was started caused a spark which ignited the explosive mixture.

This was an accident waiting to happen.  The use of sophisticated charging systems, which can charge at high rates, coupled with compact battery stowage boxes will result in high levels of explosive gas until the vent clears the area.  The use of butterfly nuts in an area subject of vibration requires a high level of inspection to prevent such a spark.

Generally if lead acid batteries are fitted in spaces such as the galley or saloon then the access must be easy to allow checking of all securing bolts.  Positive ventilation should be considered.

CHIRP Comment:

We thank the owner for having shared the report and conclusions.  It illustrates the care that must be taken if fitting a high capacity generating set and, in particular, to the venting of the battery compartment.