To fuse or not to fuse.. that is the question!

[Tom Daly]

disconnect

With respect to Steve D and others,
It's hard to fuse between big engine cranking amps and a "dead short".
There's no such thing as a dead short when you compare to a nominal
starter cranking a big diesel. (of course, battery cable must have a clean
and safe lead to the starter but even if it didn't, how would it chafe enough
to draw more current than the 800A diesels starter?)
You can propose a "just right" fuse ,but
you will have to deal with the "I can almost start it", vs an an almost
perfect short. Not gonna happen. The bigger issue, if you want to find one here, is if the starter solenoid contacts weld and you are drawing 800-1000A continuously. How will you quickly disconnect? Can that Perko switch be
rotated away from welding contacts? (I have not had the unfortunate

situation to test, but I do have big cable cutters at hand for the worst case.)

 

[MYTraveler]

It's hard to fuse between big engine cranking amps and a "dead short".

Isn't that true only when the instantaneous capacity of the starting battery bank, measured in amps, commonly referred to as CCA, is not much greater than the load imposed by the starter. When a starting bank has ample capacity, as mine does because I know that as the batteries age and wear their "CCA" will deteriorate, isn't there significant additional amperage that will flow from a dead short? And, if so, wouldn't a fuse provide significant protection?

 

[Steve DAntonio]

Based on measurements done by Blue Sea Systems' recently deceased Wayne Kelso (he will be sorely missed, he was a great asset), the dead short current of an AGM battery is 3-4 times the CCA.

 

[Bacchus]

Isn't that true only when the instantaneous capacity of the starting battery bank, measured in amps, commonly referred to as CCA, is not much greater than the load imposed by the starter. When a starting bank has ample capacity, as mine does because I know that as the batteries age and wear their "CCA" will deteriorate, isn't there significant additional amperage that will flow from a dead short? And, if so, wouldn't a fuse provide significant protection?

Yes if a type T fuse but others that might seem to be safe can weld vs openning. As I understand it that's the point of ABYC "large bank" fuse reqmts. They don't require it for starting as long as other conditions are met... but even (maybe especially) a large house bank does need proper protection that in many cases only a type T satisfies. I believe MRBF will satisfy up to a point but only type T beyond that. I don't recall the exact limits so hesitate to quote bank A #s w/o looking them up.

 

[motion30]

I never gave this much thought. But after seeing several boats burn to the water line in my Anchorage and installing a large lithium Bank I have started to pay attention. Have installed class T fuses on my lithium batteries and protective sleeves on my starting battery cables

 

[Action]

Having covered connectors helps too should some stray piece of metal (tool) move.

 

[Steve DAntonio]

Generally, more than 2 8D batteries in parallel will exceed the ampere interrupt capacity of an ANL fuse, in these cases a class T fuse, whose AIC is 20k amps, should be used. There's no harm in using a class T if in doubt.

 

[diver dave]

…..and protective sleeves on my starting battery cables

Check their flame rating.