REFLECTOR: 24 volt electrical system

[Al Gietzen]

Andy;

 

Yes, 12v system with backup battery.  Both batteries power engine critical
bus directly via isolation diodes.  The batteries are charged in parallel,
but with battery 2 contactor off, it charges through a Schottky diode to
maintain isolation.  I generally start on both batteries.

 

I may have bought the first one 5-6 years ago direct from Hawker - about
$90.  The second one a got at a great price off e-bay (new) a couple years
ago, I think $55.  I changed out the first one recently with one I bought
BatteryWeb.com in Florida - $78 plus $18 shipping.  Do a search on PC-680
and see where the best deal is.

 

All the best

 

Al

Al,

 

I have been looking at the Odyssey PC-680 or the PC-625 batteries also, are
you charging these batteries in parallel or are you using another system
such as Batterylink ACR to charge your batteries from the alternator? I also
assume by your statements that you are running a 12 volt system with the
second battery as a backup or standby system. Where did you buy your
batteries, AS or another dealer?

 

Andy

----- Original Message ----- 

From: Al Gietzen <mailto:ALVentures at cox.net>  

To: 'Velocity Aircraft <mailto:reflector at tvbf.org>  Owners and Builders
list' 

Sent: Wednesday, September 27, 2006 6:20 PM

Subject: Re: REFLECTOR: 24 volt electrical system

 

 

Since a given load draws half the current at 24 volts that it does at 12 you
can use smaller wire with the same results. you can use wire that's only 1/4
the size, which is a pretty big deal when you're are running #18 and
everyone else is running #14. Most small planes have nearly 50 pounds of
wire in them. How would you like to save 10 pounds of dead weight?

I'm sure 24V system has some advantages; but the 50 lb. number is way too
big.  The vast majority of wires in my all electric airplane are #22.  Wires
smaller than that are more difficult to work with and do not make as
reliable a crimp; so even with 24v you may find that you'll want to use a
least #24 or larger.  A few big ones are #18, and, of course there is the
one big #2 running from contactors to starter.

 

Engine start will drop a 12-volt system to around 9 volts causing EFIS
systems to reboot, radios to loose presets and fuel totalizers to restart. A
24 volt system will only drop to about 18 volts during engine start, which
is well above the 10.5 volt minimum for modern, digital avionics. No backup
batteries, no switching, no relays, and no fiddly, complex systems to solve
a non-problem.

If my (2) PC-680 batteries are a bit low to begin with, the EFIS sometimes
reboots after engine start. Otherwise no other effects.

 

FWIW,

 

Al


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