Mike Wills used the 2 motor cycle battery trick for the RV4.

  The situation is you use both batteries for starting
  and then, after a short while for charging purposes, you switch
  one off for flying.

  The other is kept in reserve in case of alternator or battery
  failures. This probably puts a glitch on the main power bus when the
  back up battery is switch in or out.

  I will add a choke and large capacitor on the main bus to smooth
  the glitch.

  Paul


  Just a suggestion,

  Look into Aeroelectric Connection ---Robert Nucholls. Google for his
  website. He is an aircraft electric Maven, involved with electronic
  design for major manufacturers.

  He has a constantly updated and updatable book which goes into most of
  the concepts and problems of aircraft electrical systems. It includes a
  multitude of schematics (circuit diagrams) ranging from the ridiculous
  to the sublime, depending on what your desires are. His website also has
  a plethora of references as well as blogs, bulletin boards etc.... easy
  to get lost in.

  A definitely recommended and perhaps required book.

  Using an engine that is totally electrically dependent behooves us to be
  more sophisticated in our electrical approach---- unless we have some
  glider training.

  Rich

  Does he talk about chokes on the main avionics buss?

  Two of these fit Mike's battery box. I am still not sure these
  are the one's he used as the batteries where missing when I picked
  up the airplane.

  There are a bunch of equivalents out there yet to be researched.


  Paul Lamar


Just jump on the bandwagon, I too highly recommend Bob Knuckles website and book
as must haves. I have been to both the EAA and Bob's electrical seminars, both
are good, but Bob's written material is MUCH more in depth. He was a electrical
design engineer for the likes of Piper, Cessna, Beech, Ratheon among others
(hmmm, maybe he can't hold a job ;-)). Seriously, he is very versed in "owner
built and maintained aircraft".

I am using his Z-14 schematic which consist OD dual alternators and dual
batteries. Both systems are used for cranking and both systems can run
completely independently OR together for a nice belt/suspended system worthy of
a rotary install.

Well worth your time and his book is often updated for free.and I still think is
only about $40.00 plus a load of additional and supporting information on his
sight. He even provides professional consultation, HOWEVER, I have yet to see a
question post on his list serve that he didn't happily and authoritivly answer.

Chris Barber

That is certainly the best set up for most airplanes.

I don't have room for dual alternators. Two small batteries are the next best
thing as I have plenty of room for them. The other thing is 2 alternators would
add to the RV4 CG problem while the batteries can trim the CG. One must have an
  alternator (each one ) large enough and powerful enough to supply the needed
current. Fuel pumps, EFI, Ignition and avionics.

Paul Lamar

I understand about the two alternators.  I was using two standard Mazda alternators, one mounted where the A/C compressor mounted using the Pineapple racing alternator bracket and one in the standard location.  I never liked the Pineapple Racing bracket much and it seemed to be a bit wobbly and off track.

However, I removed the Pineapple Racing bracket when a good friend, who owns a company that installs A/C A/C's in many light aircraft provided me an A/C unit for my bird.  I installed the compressor in the standard A/C compressor location and located my second alternator using the standard location and bracket for the Mazda air pump.  However, now, instead of using a second full size Mazda alternator, my second alternator is a smaller 40 amp Kubuto alternator. Knuckles has articles explaining why using different size alternators and even batteries is not a legitimate concern. I have had no problems it running all my systems when I disengage the primary electrical system.  The smaller unit does not seem to extend away from the engine much at all and weighs less than half of the full size unit at about six lbs.

Chris