Message-ID: <19980921183330.23399.qmail@www04.netaddress.usa.net>
Date: Mon, 21 Sep 1998 18:33:30
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: firewall pass thru, relays, batteries, gross wt
Mime-Version: 1.0

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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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