>Want to jump or charge your 24 volt battery - consider a
>charger that is 6, 12 & 24 or a 24 volt only. If you need to jump
>start it it takes two, care and really long cables or you need to
>have the proper high $$$ adapter to plug into a power unit.
Ground power is a strong driving issue . . . you can jump
start your 14 v airplane from a vehicle.
>I don't find the worry about using converters powering gauges
>such as fuel pressure since a 24volt battery is still using less
>amperage, it should last longer.
24 v batteries are two 12 v batteries in series with smaller
cells. For the same ENERGY storage, you need the same number
of pounds of lead and acid . . . but a 24 volt battery has
more plastic in it and is probably heavier/larger by some
small amount than the equivalent 12 v battery. Given the
smaller plate area/chemistry per cell, I've noted that the
24 v battery is less tolerant to deep discharge cycles
than the 12 v with fatter cells . . . all other things being equal,
I think the 12 v battery will outlast the 24 in similar service.
>Light bulbs will cost more (24 V) unless there is more of a
>quantity built for 24 volt than 12 volt.
12 v lamps are made in tens of millions . . . further, you
have more options for some REALLY nice exterior lighting.
Taked for example the lamp you can see at
http://www.aeroelectric.com/4352.JPG
is about 2" tall, 5" wide, puts out lots of light on 55
watts (about 4.5 amps drain on system) and fits very
nicely in the leading edge of a wing. Further, it's
a modern, automotive halogen that will probably last
for the lifetime of your airplane.
>I do like the more watts offered by 24 volt comm's example;
>KX-155 or 165 7.5 watts (12 volt) broadcast verses 10 watts (24 volt).
>KX-196 or 197 - the 12 volt is 10 watts vs 16 watts for 24 volt.
>The advantage is - usually - range and clarity.
. . . take a peek at:
http://www.decibelproducts.com/mrktng-eng/scripts/freespace.cfm
This is a free space path loss calculator that can tell you
what the theroetical talk distance is between your radio
and somebody elses.
Assume you have 1 watt of effective radiated power (this is
30 dbmw in engineer-speak). Assume the guy you want to talk
to has a receiver capable of hearing a 5 MICROVOLT signal
(not difficult to do) . . . this signal would have an energy
level of -123 dbmw for a total allowable path loss of
153 dbmw
Go to that calculator and enter distances using 120 Mhz as
the frequency of interest and you'll find a freespace
talking distance of 6000 miles.
Now, there ARE other factors that ADD to your losses including
coax and antenna efficiency. Also local noise at the other
end competes with your arriving signal. Sooo . . . lets assume
that you're at 15,000 feet and talking with somebody who is
50 miles away (not over the horizon for you) . . . and he can
just read your signal when you use your 1 watt transmitter.
Going back to the calculator we find that the path loss over
50 miles is 112 db. Let us say you switch over to a 2 watt
transmitter . . .this means your signal at the other end gets
a 3 db boost. Now you can tolerate a 115 db loss between you
an the other guy and still be heard with the same clarity.
Back at the calculator we find that 70 miles is the range
for doubling your power output. Further, 50-70 miles is SO
small compared to the free-space range that one must conclude
that other factors have a strong effect on range of communciations.
Curvature of the earth, noise at receiving end and poor
selection of antennas and/or feedlines all stack up to
attenuate your signal.
It's much easier to talk further with system efficiency
cleanup than to boost transmitter power output.
>
>Two 5 amp 14.7 volt converters weigh less then 3 lbs. so,
>if a back-up is needed it's no big deal.
This is true . . . with reservations. There are electronic
components out there that let you build very light down-converters
that are also very efficient. However, they are strong
oscillators (read transmitters) that can interfere
with other systems on board . . . most notably receivers.
Unless the converter has been tested for aircraft applications,
approach with caution . . . I'm not saying don't try it
but do enough testing of your finished installation to make
sure there are no adverse effects from a relatively
unknown product.
>With all the newer call for 12/14 volt stuff, it's a trade.
>I did it because my engine came so equipped and knew
>of some of the advantages of 24/28 systems.
>One of them is how long you can crank the engine.
>However, I have always felt that if the engine doesn't
>start up in 1 to 4 blades you might have a problem that's
>being overlooked. But I want that power if I ever need a restart
>in the air.
Excellent point. I used to be able to push-start
my 6-cyl Chevy out in the street by myself. I could
just get it rolling, jump in and pop the critter into
low and it would fire off on the first cylinder that
rolled over. Keeping an engine finely tuned and understanding
a particular engine's idiosyncrasies can make a BIG
difference in starter wear, battery life, etc.
>All in all it's a personal choice but the 24volt system might
>cost as much as $500 if done correctly (that includes a 24 volt
>battery charger.) And weight savings on a Long is 16.5
>pounds by my old calculations (1983).
I'm really interested in this weight savings number. Starters
in the ol' Prestolite pig don't get any lighter at 28V, batteries
with the same ENGERY (12V/32 a.h. versus 24V/16 a.h.) are within
a few percent of each other for weight. The alternators share the
same frames so in spite of the fact that they put out 2x energy,
they weigh within ounces of each other. All things being
equal except voltage, only drops in wire size contribute greatly
to weight reduction.
Now, if you make a swap to B&C equipment from certified junk,
AND go to 28 v, the weight savings can be spectacular . . .
but it didn't happed because of system voltage change . . .
Bob . . .
--------------------------------------------
( 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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