External Power
As you can imagine, this airplane needs lots of electricity. Because the
avionics were home made and needed a lot of debugging/testing, my airplane
spent many hours in the hanger powered up. Obviously, I need a way to easily
get lots of power into my airplane as well as an efficient way to create
this power.
Above is a photo showing the external power connector. Quite a bit of
thought went into this. I needed to get at least 15 amps into the airplane
to run the computers and avionics etc. It seemed prudent to make the
capacity even larger to accommodate rapid battery charging in the event I
ran the battery dead by leaving the master switch on etc. Installing a
typical connector with 300 amps capacity to allow starting the engine didn't
seem like such a great idea. In addition to be very heavy, there needed to
be heavy gauge wiring associated with the connector. Jump starting an
airplane with a dead battery is always risky. After starting, the alternator
is turned on at which point it will produce 100 percent power until the
battery is somewhat charged. This puts a huge strain on the alternator and
causes significant heating inside the battery as it jammed full with up to
100 amps of charging current. These are not good things to take into flight.
They are avoided by taking a little time to charge the battery up some, then
start up and go.
With this system , I can charge a completely dead battery in 55 minutes. I
can start the engine after just 10 minutes of charging. So I can stand a ten
minute delay to think about turning off the master next time!
The details:
12 pin .095 Molex connector. The pins are rated 8 amps each, the connector
is good for 48 amps. The location was very important. At first it seems like
the best location would be a small door in the fuselage side near the
battery. This was a difficult choice as it required making a hole, door and
structure to hold the connector firmly. As it would be connected directly to
the battery, it also needed a 50 amp fuse and other stuff. Seemed like too
much work. In it's current location, the connector is wired to the "B"
terminal on the alternator. This was a stroke of luck, as it only required
6" of wire and attached to a low impedance route right to the battery bus.
There is already a fuse between the alternator and the battery bus, so any
short circuit in the power supply would be protected from a battery assisted
meltdown. Further more, it can power the battery bus (and all other systems)
without a battery installed, or I can turn the master switch on to charge
the battery.
The power supply is a computer switching type. I primarily use the one on
the left. It is rated at 35 amps continuous. I chose this unit as it had an
adjustable output using a small trimmer potentiometer. It is rated at 12
volts, but trims up to 15.0 volts easily (I set this one to 14.0 volts).
There is an endless supply of surplus computer type power supplies out
there. Sitting to the right of the enclosed box power supply, is a typical
open frame switcher. It's also rated at 12 volts, adjustable and puts out 12
amps. I bought a couple of these for $10 each. Soon I will build a traveling
power supply to carry with me when I fly to far away lands. Should be able
to build the 12 amp unit to weigh less than 1 pound.
At left, a 35 AMP 12 volt switcher
adjusted up to 14 volts. On the right, a 8 amp open frame switcher. The
latter cost $10 new surplus
Here I have finally built a traveling
external power supply. This is an open frame switching 12 volt unit. I
adjusted the output voltage up to 13.9 volts. After load testing it, I
found it would make 11 amps at 13.9 volts if I put a fan on it to keep it
cool. The output wires ( 16 GA.) are 6 feet long so the unit sits on the
ground beneath the engine. A short pigtail power cord assumes an extension
cord will be available. I left it uncovered to save weight. The small size
allows the unit to fit in my "travel kit" along with tools and small spare
parts.
Total weight: 1.4 lbs.
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