Catastrophic Battery Failure

[Snider]

I was in the boat on the trailer in the yard updating my chart plotter software, and as I was buttoning everything up I glanced at the Pro sport 20 onboard charger. It was showing a fault in battery 1, I went to the battery selector switch and really without thinking decided to switch it to battery 2, when I did this I heard buzzing from the battery compartment and was unable to pass the switch through the "all" selector to 2, it was stuck on ALL. I backed up and proceeded to watch battery 1 spark, smoke and almost catch fire, quickly I went back to the switch and turned it off, then unplugged the power cord to the boat and grabbed the fire extinguisher, watching this happen next to 10gal of gasoline wasn't very fun.

I unhooked and removed the batteries, battery 1 now has a large golfball sized hole in the top at the positive terminal and battery 2 has a little melting also at the positive post. I used the boat last week no problems. My initial thought was that battery one for some reason was discharged and when I went to "all" battery 2 violently discharged into battery 1 causing the fire. However when I measured the voltage after I removed the batteries, battery 1 was at 12.7v and battery 2 was at 12.6v. Both are about 2 years old, duracell 92ah AGM's. I leave them on the pro charger when not in use, the boat sits in the yard in the trailer when not in use.

I'm not really sure what happened, any ideas? Obviously I want to prevent that from happening again. I ordered a BlueSea battery selector that only has position 1, position 2 and off, no need for "all" as far as I can tell. Should I have had fuses at each batteries positive post? The boat didn't have them and I've been debating on adding them.

I'm planning on going back with similar batteries and the new switch. I really just want to understand what caused this so I can mitigate it in the future.

[Snider]

[Doug n]

That is really scary. I put blue sea fuses on the positive posts of both my batteries. I'm thinking of these fuses as protection from catastrophic failure, like what you had, and not protection from a normal load getting close to the fuse rating. My 75 hp yamaha starts well with 150 amp fuses. (100A is too light). I keep 200A fuses as spare as I don't know how close I am to blowing the 150A fuses. I have found it very hard to getting info on fusing for starting current on outboards. The fuses are oversized for the size of my battery wiring, even after installing 4 gauge wiring, but are a lot better than not having any at all as is usually found on a lot of boats.
Doug

[ssobol]

Fuses won't help if there is an internal short of the battery. Or a direct short to the positive post and ground.

If there is an external short somewhere that is sufficient to do this sort of damage to the battery I would expect to see evidence in the boat wiring somewhere.

It is possible that there was a high current load and a bad connection to the positive post. The connection became highly resistive and got very hot. This current could be below the fuse threshold but sufficient to generate a lot of heat over time. After all, the battery case is just plastic and probably melts at a around 300 deg F. A battery post that is close to red hot would have no problem melting out of the case or shifting enough to cause an internal short of the battery.

[Snider]

I’ve been reading my Boatowners Mechanical and Electrical Manual and all I can come up with is along your same line of thinking. Wonder if I had a poor connection at the positive terminal of battery 1, because of this maybe battery # 1 wasn’t charging and had a lower charge than battery 2. Then when I switched the selector to “all” the batteries equalizing caused heat at the bad connection which generated enough heat to melt the battery and cause an internal
short? The only confounding variable was battery 1’s voltage was higher than battery 2’s at 12.7v instead of 12.6, is it possible battery 2 just hadn’t recovered from the quick discharge? I’ll go measure their voltage again tomorrow.
Thanks for the reply.

[garyf]

I'm not seeing any mention of what your positive battery cable looks like... I assume it's damaged of course, but to what extent? Was it actively involved in the problem or just damaged from the heat below?

Mark this one down as a "win" - it obviously could have been much worse - but you're 100% right in trying to get to the root cause to avoid in the future.

[thataway]

Agree with photos of battery cable. A short somewhere--but the voltages of 12.6 and 12.7 doesn't really go along with that.

Redo the whole electrical system. I would be suspect of the charger, putting in a high voltage/amperage for a period of time. The AGM's don't tolerate serious overcharging (which would boil off a flooded lead acid battery).

I probably would get a new "smart charger". I have the Noco, which works well and is not super expensive. Blue Seas, Master Volt. ProMariner- pronautic series, Victron are all top of the line. I probably missed a couple.

Normally a fuse is not required for the starting circuit. For the house circuits ABYC standards call for a fuse or breaker within 7" of the battery terminal. The battery charger needs to be fused on both ends.

[ssobol]

It's also possible that the positive battery cable had corrosion in the wire strands where the lug is clamped on (not uncommon). This could have been a resistive load that got hot and heated the lug which heated the post. The positive battery cable should show evidence of this.

[Snider]

Here are the cables. I'm pretty confident it was a lose connection at battery 1 and battery 2's positive terminal, more so at battery 1. Both of these terminals had wing nuts on them which I should have changed out to nuts with lock washers and properly torqued them. The loose connection generated heat when the batteries tried to equalize. Both in Calder's book ( gave this specific example of a loose battery post nut which lead to fire) and the guy at the battery shop validate this theory. The guy at the battery shop said they see this a good bit with golf carts as the connections rattle loose then gets hot 2/2 high resistance and it melts the battery top. I don't believe the charger was over charging because it was showing a fault with battery 1, probably 2/2 the loose connection. Had I turned the selector to off instead of going through "all" in order to get to battery 2 and then checked the connection I could have prevented this.

I'm replacing the selector switch with a BlueSea version and replacing all of the battery cables as well as two new batteries. The rings on the cables at the battery were ⅜ while the battery studs were 5/16. I'm going to size all of this properly and start new.

You can see in the picture that the wing nut is still on the terminal which melted.

[Snider]

[Snider]

I carefully inspected the cables, I did not see any visible corrosion. It appears that the highest amount of heat was at the battery terminal itself.

[thataway]

Certainly the wrong size ring terminals, poor contacts contribute. Too bad the batteries are even sold with wing nuts. For those who don't understand, the wing nut does not allow a tight enough connection when "finger tight". There would have to have been a lot of differential voltage--yet the voltages you recored were for all purposes the same......

[Snider]

[Snider]

I’m open to other possible causes and solutions of course, I really just want to prevent it from happening again. When I get everything hooked back up I will check the voltage output from the battery charger. I don’t think it was charging battery one though, because it was showing a fault with it.

[Foggy]

Oh yea.

The mandate for battery connections: dry, tight, clean.

Aye.