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To Dan Brown: (I sent this to Dan separately but thought
It might be of general interest.)
Hi Dan: Well, I enjoy a good debate as much as the next
guy but as far as the steel vs. copper pass thru is con-
serned, you are right if the only current that had to
be carried by the bolt was the alternator output. Since
the regulator will automatically boost the voltage to
make up for any losses, no big deal. I guess if you
never planned to start your engine a little resistance
wouldn't matter much. On the other hand, if one makes a
few reasonable assumptions regarding starters, batteries
and such, you might reconsider. First, let's assume that
the starter draws on a cold day (conservatively) maybe
100 amps. Depending on whether it's one of the new, high
efficiency units or a standard horse and buggy style -
then you could even be talking up to 200 amps cranking.
Then figure that on a good day, your 20-30 AH battery
output is gonna drop to around (conservatively again)
9-10 volts at that load due to internal resistance. Now
figure that the cables, with all the ring-lugs-to-bolt
interfaces probably represent about 5-10 milliohms. Lets
guess that the two pass thru steel bolts represent maybe
5 milliohms each. There's some amount of resistance in
the main and starter contactor's contact resistance,
maybe a total of 5 milliohms. If you add it all up, and
do a simple ohm's law calculation, you're looking at an
additional loss of 2.5 to 5.0 volts when you get to the
starter. So the starter gets, conservatively, 6.5 to 7.5
volts and more probably, 4.0 to 5.0 volts. It seems to me
that anything you can do to alleviate the loss, such as
making sure you have as low a resistance path to the
starter as possible, is important.
Regarding the silver plating, I guess you'll have to
argue with some engineers I used to work with who made
it work, on that one. I do know that the plating was only
called out at the point where bolted connections were
made.
Basically, I agree with all you who said "...forget the bolts
and just run the cables direct." By far the best way and the
failure modes are nil. And your starter motor will love you
for it (or conversely, you'll love the way your starter works
when your battery gets some time in service on it and the
temperature is low.)
We talked about battery types and their feeding earlier and
Brent's right, the recombinant gas (RG) types available today
are excellent and convenient. In our service, they're just about
totally maintenance free, don't care about attitude (the air-
plane's, not yours) don't outgas, if you don't abuse them, so
they don't need venting, and maintain low internal resistance
over a reasonable service life. All of these reasons and a few
more is why we've been retrofitting the B-1B with RG lead acid
batteries (including new chargers, of course) in place of NiCads.
All this with just one caveat: Sealed RG batteries need to be
charged at no more than 13.8 volts or you'll shorten their
life dramatically. Most fixed voltage regulators I've seen are
set around 14.6 or so and this'll eat an RG battery for lunch.
You need to use a regulator that can be ACCURATELY adjusted to
13.8 - 13.9 volts. You need to get someone who has an ACCURATE
voltmeter if you don't have one, and set the regulator after
installation in your plane. Incidentally, the charging info
came from a representative of Concorde after I had two of their
RG-25's go bad within one year. After getting smart, I checked
the specs on our new B-1 charger and the information was there
all the time if I'd only looked.
Finally, the Bosch relays are a great idea. I used a Potter &
Brumfield equivalent but are about the same rating. These are
neat little 1 inch cubes with push-on terminals and are rated at
70 amps. Also use them for other things which then can be
controlled with very small swithches. Keeps the panel from
getting crowded but does require explanation now and then.
Dan Schaefer
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