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Old Aug 25, 2008, 04:21 PM
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Batteries

Hey guys, im in the process of converting an electric to glow, so I need a flight pack for my receiver like a standard glow bird. What kind of batteries do you guys use and where could I get one?

I am about to use a 2/3 AAA 4 cell (4.8v) 300mah NiMh I made and have been using in gliders for a while. It weighs 1.2 ounces (33grams) which is a little porky IMO. My smallest lipo is 7.4v 350mah and is .9 ounces but the voltage is too high (Still in the stone age with my 72mhz RX ) So I would have to use my 1.1 ounce BEC to step the voltage from 7.4 to 6 volts..I end up with a 2 ounce battery system. I do have a 1 gram GWS 2amp ESC and a 6 gram jeti 6 ESC, but they arnt designed to handle 5 servos!

I have a bunch of 110mah Nicd cells but they weigh pretty much the same as my NiMh pack but with much less capacity. So maybe a 150mah NiMh would be adequate for 2-3 flights on 5 micro servos?
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Old Aug 25, 2008, 08:15 PM
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Remember that the batteries have a lot less work to do in a fuel powered airplane. The current drain is going to be very small, especially in the mini and micro servo sizes that are used in 1/2 A airplanes.

I have used 200 ma nicads in my airplanes for years with no problems. And I am talking here about standard sized servos in .25-.40 airframes.

So go for a mini-receiver/servo package, and run it off the smallest physical sized battery pack with around 200mah or better, and you should be able to easily get a full flight session in with your 1/2 A airplanes.

Of course, others might have different ideas.

Happy skies! Jim
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Old Aug 25, 2008, 08:58 PM
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Thanks for the reply Jim, but I am going into no mans land..I need to save every gram possible. Shooting for a 3D 1/2A plane.

I know the strain on the battery in a fuel powered plane is almost insignificant, probably .5 amps vs 20-25 amps the electric version would need.
NICD pack would weigh about 35 grams and yield 110mah @ 4.8v... my NIMH is 33grams/ 300mah @4.8v. 300mah is probably enough to fly a .25 size plane with standard servos for 3-4 maybe 5 flights. But thats a guess. (My much larger os1.20 with all digital servos seem to take 150-200mah per flight) So if 300mah is plenty enough for standard plane, then I might be able to get by on 150mah for my 3D plane and save 15 ever so precious grams.
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Old Aug 27, 2008, 04:42 AM
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I you want to save every gram, I think the lightest you can get is to get a DC/DC converter and use one Lipo. If you charged (topped up) after every flight, a Fullriver 130mah lipo (or a something better) would probably be sufficient. This setup would only be about 5 grams!

See this thread for info on where to get a light DC/DC converter: http://www.rcgroups.com/forums/showthread.php?t=642057

If you don't want to live so close to the ragged edge, you can use a larger battery and 5V step down regulator and up the battery size until you're comfortable. E.g. a simple setup would be a ~200mah (more than plenty) 2S lipo and use an old GWS ISC-50 ESC (or CC Pixie 7P) for the BEC only. A setup like this would be about 20 grams.

BTW, a 3D 1/2A plane sounds interesting, make sure to post updates if you proceed. I'm interested in seeing how you make out.
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Old Aug 27, 2008, 07:53 AM
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Groundfx, I think thats very dangerous IMHO. Im sure it works but Im not going to chance it.

Yes, lipo's and liion would be the the absolute lightest, and I did explore this route. My lightest battery is a 7.4v 350mah kokam, weighs about 25 grams or so. But my lightest converter is 2 grams. Sounds too good to be true right? My regulator is a GWS ESC. Since it is so small, I highly doubt the built in BEC will be able to deliver enough juice to 5 servos. Next step up is a JETI 6amp ESC, but this suffers the same limitation. The most powerful reg. I have is a Align BEC but this weighs 1 ounce. So the high voltage lipo battery must be converted down to a lower voltage. Lithiums greatest strenght becomes is greatest flaw in this size range. In a larger plane, say .30 and up, lithium becomes a viable choice. But this small, the high voltage means you have to introduce a (constant) amount of weight. Where as Nicd/mh, you just hack off a cell.

EDIT: For got to mention..
I got the plane is smashed condition. There is no dihedral in this plane, notice you can clearly see dihedral in the picture! UPS did a horrendous job this time The wing, spar and everything on the right half is unrepairable, ailerons, rudder and fuse are repairable but I have to rip up all of the covering. I knew this would happen to me.. Trying to resolve the issue with AW right now.

Good news is that the plane weighs under 10 ounces, most likely on the nail at 8 ounces because I did not take the heavy "protective" plastic off. And the OS.15 does barely fit on the hard wood rails, with no MODs to the frame! Even greater news is that there are two pockets in the wing for me to put the fuel tank in (no, UPS didnt do these!)
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Old Aug 30, 2008, 09:32 PM
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I also thought you were talking about lightly loaded servos on a true 1/2a (e.g. .049), with a OS .15 ... then no way, that would be dangerous.

In that case a switching BEC would be more powerful. Like this one: http://www.dimensionengineering.com/ParkBEC.htm ... it can drive up to 6 micro servos.

FYI, in the old days, the 1/2a (i.e. Cox TD 049) pylon guys (2 servos) would run tiny 4 x 1/3AA (110mah) NiCd's and then charge after every flight (which takes less than 15 minutes.) It was perfectly safe as you could not drain the batteries in the time you had power via the small tank.
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Old Aug 30, 2008, 11:10 PM
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Quote:
I also thought you were talking about lightly loaded servos on a true 1/2a (e.g. .049), with a OS .15 ... then no way, that would be dangerous.
No prob, I should have been a little more specific.


I just got a 4.8v 170mah NIMH , weighs 20 grams! Not that many MAH's but its light.
Ill play with it a little see what works best, then consider that BEC..they have very reasonable prices but I work by "If its not broke, dont fix it" Thanks for the link..going in my book marks
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