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If you dispense with the diode, everything gets simplified. B+ / Alt grounds, Batt blows fuse, problem is totally isolated. Battery expends 100 amps for very short time and goes on about its business (of getting you home). What actual, rational purpose is served by trying to avoid using battery power to blow a fuse? Why not go ahead and let it happen and get on with things? I have to wonder why we need such complexity.
This is the first time I've heard of such a scheme as a solution ... Jim S.
Lehanover@aol.com wrote:
Could you install a diode in the line so that no energy from the battery could be used by the alternator, along with a fuse? Lynn E. Hanover
That would defeat the purpose of the fuse. If battery current can't flow toward the shorted B+ or grounded alt field winding such as Paul had, what is your power source for blowing the fuse? A diode would result in say a grounded B+ to result in the alternator thrashing its brains out pumping all the current it was capable of straight to ground. Fuse exceeds alt output so it wouldn't blow, and you'd be on battery power while the alt tries valiantly, but in vain, to blow the fuse by itself.
What would be the benefit from that? ... Jim S.
I see a number of situations where the rather high value of the proposed
fuses might come back to haunt you.
Any failure the produces a load that is less than the fuse value, can still be a big load on the battery. If you believe that the fuse is designed to protect the wire and the aircraft from a fire hazard, I would think that the fuse would be mounted on or very close by the alternator. The diode would be at the other end mounted on or close by the battery terminal. Should wire to the battery be damaged or short to ground, the battery cannot "see" it. No energy from the battery is expended, nothing that the battery is supporting gets damaged or even shuts down for a second.The wire from the short to the alternator is protected by the fuse. Fuse blows, idiot light comes on, you go home.
An alternator failure that results in any voltage lower than the battery voltage has no affect other than loss of charging voltage. Idiot light comes on, you go home. Battery suffers internal short from over charging. Bad smell, idiot light comes on. Alternator continues to charge (now a 10 volt battery) with 14.2 volts, smell gets worse, as battery gets hotter. The 100 amp fuse gets hot, but hangs in there. You flip marine battery management switch to backup battery, you head for airport, any airport. You drop GPS, it goes under the seat, damn......... Alternator turns primary battery into hydrogen bomb, you cross the fence way too fast trying to remember how to forward slip, and wondering why there is a 100 amp fuse in that system. I can weld with 100 amps. You can too.
I suspect that sizing the fuze or better yet the breaker, requires more thinking. I suspect that a small drycell backup to run just the all electronic engine is a must have piece. If you cruise at 5,500 RPM instead of the 1,900 RPM in the street car, I would think about slowing the alternator speed with a bigger pulley. The alternator should not be functional at cranking speeds, and should not be a factor in the voltage reduction due to cranking. I think any switch that moves engine systems to the backup battery should not only disconnect the alternator but also cut the field circuit. Lynn E. Hanover
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