Big battery banks and high capacity alternators- A cautionary tale...

[2ndwish]

This weekend, during a long cruise, a friend of mine radioed that he was having concerns about his engine (Universal M35B) and something didn't smell right. When I arrived he was running his engine and there was a clear burning smell. We removed the engine cover and found sparks flying from the alternator output. He shut down the engine, turned off the "perko" and another friend joined us in the diagnosis.

The battery bank is about 400 AH and I believe the alt was more than 80A with an external Balmar regulator (equipped for alt temp sensing but not installed). What happened was that the alternator output nut had worked loose (diesels are known to vibrate) and the joint had become resistive. That joint, driven by alternator and in part by the external alt regulator, got so hot that it melted nearby wires and the alternator output stud isolating standoff. The stud in turn, able to move, was intermittently shorting to the case. A post mortem revealed that the stud was white with oxidation suggesting it was red hot at some point. Furthermore, since the battery cable system was not fused at the battery (per Mainesail and ABYC recommendations), the short took out the "Perko" switch and melted adjacent wires. It was fortunate that our other friend has an awesome spares kit (and some fantastic wiring and diagnostic skills to boot) and actually had a switch that fit the panel. This was all caused by a wire in a high capacity alternator system that had worked loose. We disconnected the alternator, he got his batteries charged with a generator and he made it safely home without incident.

The cautionary message: check your connections regularly on a high current system and fuse your batteries. Our friend was very fortunate to not have a fire on-board. Hope this information helps the next person.

[Clay Greene]

That's a really good post - thanks.  I like the Blue Sea Systems battery terminal fuses - very simple installation.  You do need a couple of inches above your batteries because the posts for the fuse mounts are higher than the battery posts.  Maine Sail has good photos of the terminal fuse installation on his website. 

[Ron Hill]

Guys : Posted this "Tip" in the Mainsheet tech notes many years ago :

"On the alternator output stud use a double nut (jam nut) or a self locking nut - as diesels do vibrate and single nuts tend to back off"

Great stuff in those old Mainsheet notes!!

[Footloose]

I do not believe that fusing would have prevented this problem.  Fuses provide over current protection, which is often caused by wires or devices shorting.  Arcing does not cause over current, but does create a great deal of heat.  Arcing of worn outlets is one of the top causes of house fires.  An device is plugged into an old outlet, which makes a less than ideal connection, causing an arc and heat.  Curtains catch on fire.  Ken's picture of the burned twist lock connectors are another example.  Breakers and fuses don't blow in these conditions.

[2ndwish]

Footloose- You're right, it would not have prevented the problem, but it would have saved the"perko" switch and the wires around it. When the stud of the alternator contacted the housing, it shorted the batteries. The switch was the weak link in the circuit so it went first- luckily (it acted as a somewhat expensive fuse).

[Noah]

Also,  the "big battery bank" had nothing to do with the reported problem. Nothing wrong with a well installed/maintained battery bank, regardless of size.

[2ndwish]

I never meant to suggest there is something wrong with big battery banks. It is just that deeply discharged banks and the high output alternators (and chargers) that charge larger banks, carry large currents.  Currents are what causes the heating at resistive connections. The heat generated in a resistor goes as the square of the current (twice the current, 4 times the heat). The post was intended convey the message that if you have such a system (and many of us do), check your connections regularly because they are more prone to this particular problem. The connections that should be checked are on virtually every heavy duty cable in the system and the alternator output. The alternator output (and the starter lug which is often the other end of it)is particularly prone to loosening because the engine vibrates and gets hot and cold.