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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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