>This comes up often, usually by someone who has gotten some great "deals"
>on avionics and who tries to make it all work harmoniously. Every attempt
>I know of to mix and match avionics of different voltages has, sooner or
>later, been abandoned by those who tried.
>
>Were you living in Montana or some other cold spot, I'd suggest 28 volts
>to compensate for the effects of low temps on batteries. As an added
>aside, you could then use that voltage to advantage and reduce the weight
>of your plane by a few pounds by running lighter gauge wire.
A common misconception about batteries . . . the goal when
cranking an engine is to maximize the energy transfer from
the battery's chemistry to the starter motor. The biggest
impediment to this transfer is a combination of resistances
in battery, wire, contactors and terminals.
When GA (most notibly Cessna) went all 28 v it was mostly for
reasons of economy . . . one size alternator/battery fits all
from C-150 to C-210 . . . yes, there was some notible improvement
in cranking at cold weather with 28v . . .
In retrospect, this was mostly due to poor attention paid to
reducing system impedances. A little clean up work with choice
of wire sizes and hardware location and upgrading the starter
which had roots in the 1938 6v Jeep, there would have been
no detectable difference in cranking performance of the two
systems.
Today, the modern RG battery is head and shoulders above
the flooded batteries upon which many of us base our perceptions
of battery and system performance.
When I was helping B&C test batteries to gain STC on their
RG products for TC aircraft, we put two brand new batteries
in the freezer overnight. One was a Concord flooded product,
the other was a B&C RG. In the morning, we loaded each battery
in turn with 300 amps . . . about half again more current than
it takes to crank an engine.
The flooded battery started out at 8 volts and slid downhill
from there. At the end of 30 seconds of loading, the RG batttery
had not yet fallen to 8 volts!
This can be attributed to one and only one attribute of the
RG technology . . . very low internal resistance compared
to flooded batteries at ALL temperatures.
>But you don't. You live in Florida. Go with the 12 volt system.
>Everything else being equal, 12 volt equipment is more plentiful and
>often cheaper.
. . . not often, ALWAYS. You need to consider beyond the
cost of acquisition. Cost of ownership for a 28 v system are
real cash and time suckers compared to 14 v.
>Second suggestion. Wait until a few months before the airframe is
>finished, painted and the engine in place BEFORE buying avionics. I've
>brand new Loran and ADF units that are virtually boat anchors. Good
>deals when I bought them, probably just like you, expecting I'd have the
>thing built and flying "in no time"
Excellent advice. Electronics advances faster than any other
commodity . . . I wouldn't buy a battery or a radio until just
before you're ready to install them. Use jumper cables to a
car/boat/tractor battery or an alternator simulator to test
the airplane's systems. Buy a fresh new battery to install
before first light under the wheels.
Bob . . .
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( Knowing about a thing is different than )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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