Gee- I had overlooked the current draw for the injectors! It would
  seem that a 40 amp alternator is not sufficient for my application,
  and I have pretty minimal draw after accounting for fuel pump,
  injectors, and minimal avionics. This would explain why I'm getting
  good voltage on the ground at 1800 rpm, but not in the  air at 6400rpm.
  Anyone know what the injectors draw?

  There is a 45 amp alternator that fits my mounting application, but
  only bumping it up 5 amps is not enough. I might have to see about
  mounting a second alternator. Sort of a PITA. That Ammeter from
  Digikey looks good though.

  Brian Trubee

  Unplug one and use an ohmmeter to measure the coil resistance.

  The max current can be found by dividing the number of ohms found
  into the max voltage or 15V.

  Which injector are you using. Some of the older ones used peak and
  hold and the above scheme won't work. If it is around 10 or 12 ohms
  it is not peak and hold.

  If it is 12 ohms it is drawing 1.25 amps wide open max. However they
  never run wide open all the time so 80% of that is about right. Say
  1 amp. A larger alternator won't hurt.

  Paul Lamar

  Just ordered the ammeter and shunt. The bigger issue will be where
  to install the second alternator. Not an insurmountable problem.

  Brian Trubee

  Brian,
  Not a suggestion, but information.

  The standard alternator for the Renesis engine is  100A .

  The secondary alternator, gives you more backup, however.

  Rich

  I have the standard Mazda alternator on the standard location on the
  2nd gen.  My other alternator, a Kubuta, is mounted where the vacuum
  pump was originally mounted. The Kubuta is quite a bit smaller and I
  was able to use the vacuum mount as well with just a bit of
  modification to the alternator.  I have considered replacing the
  Mazda Alt due to weight, but it fits so nicely for now.

  Chris
  Houston

  As Brian is finding, light weight is not the only criteria to consider. You
  have to have the balls to get the job done. A small second backup alternator
  is a good idea.

  Dale Davies


  A couple of questions here, 1/  If the alternator wont operate without
 the battery as previously mentioned
 what happens if the battery fails in flight and has to be isolated. Do
 you still have  reliable alternator power?
 2/ If electrical power fails altogether for what ever reason, The EFI
 fuel pump stops along with the engine, the Wright bros fuel metering
 system might be a necessity ?
  Comments please!!!
  The outboard alternator and a normal alt seems a good Idea as belt
 failure is covered for redundancy. The PSRU alternator as designed
 seems a very good idea.
  Graeme Riley

 There is such a thing as a permanent magnet alternator that works without a battery.

 Most are rather small with modest output.

 Paul Lamar

Many alternators are self-exciting if they don't get stalled..
There's enough residual magnetism to generate field current.  ie, if
you take the battery out, the alternator will continue (and even start
on its own) to produce regulated voltage, though ripple and noise
performance may suffer.

If you stall the alternator by shorting its output (with a heavy
load), its ability to turn back on and start making power can be
compromised.

And, even a battery with a shorted cell - bringing the stack voltage
down to 11v or so is enough voltage to get a conventional alternator
going - even if the alternator isn't the self-exciting style.  Just
have to put enough current in the field to get things going.

Discussed way long ago: mount a PM alternator rotor on the back side
of the flex plate - stator in the end housing.  Harley (or outboard)
parts.  Eshaft speed ensures good output.  Parts aren't expensive.


Regards,

Matt-


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