Larry Driskill

Hey, Good Evening,

The inside back cover of the November issue of Flying Models has an FMA ad touting their M5 receiver and noting that it will operate with a single 3.7 volt lithium polymer cell (3.7V).

I'm still getting my ideas together for an .010 powered LiteHawk and want to build it very light.

The lightest battery I currently have is a 50mAh Nicad. I'm not sure just how small and light Li-Po cells can be found, but I only need enough battery capacity for a couple of short flights to be happy.

So . . . will micro servos operate satisfactorily on only 3.7 volts?

DICKEYBIRD

Never done it myself Larry but I've read about many of the r/c HLG guys using 3 cell 50 MAH nicad packs to save weight. I think servos work well on low voltage other than the fact that that they move slower and have less torque. Slow servos on a little hot rod airplane? Might be a problem.

Larry Driskill

Here are some weights I found:

50 mAh Nicad 14 grams
80 mAh Li-Po 2.2 grams

I suppose I would have to buy a 3.7 volt Li-Po cell and see how quickly it was able to move the servos.

But it would be a large outlay for 11.8 grams - Li-Po cell, FMA receiver, Li-Po charger.

For now I think to more rational decision may be to build the plane with the 50 mAh battery.

DICKEYBIRD

Methinks that's the wise choice for now. Maybe there's another battery type available out there somewhere that would provide the power & voltage needed for .010 planes at a very low weight....we just need to find it!

Hey, I know, let's ask the MicroFlite guy....I hear he's just about to market some NiUN cells (Nickel-Unobtanium) as soon as the patent clears.

mclintock

My Feedback: (3)

You could do what cox/estes does in their cheap rtf rudder only fomies-
use LR44 button cells. They are not re-chargeable, but hey, in bulk from an online place they are real cheap.

ZAGNUT

My Feedback: (1)

a very good DIY lipo charger doesn't cost more than a couple dollars to build [link=http://shdesigns.org/lionchg.html]this circuit[/link] works great and can be built for any number of cells and charge current.

also, a 4 cell 160ma nimh weighs only 14 grams so maybe there are some smaller cells with this chemistry that can save some weight


dave

BMatthews

I believe that FMA or someone else has a super light airborne pack that is modified in design for the servos and reciever to specifically run from a single Li-poly or Li-ion (the same cell chemistrys but in different cases of course)

BuzzBomber

My Feedback: (4)

Perhaps you were thinking of this: [link=http://www.hobbypeople.net/gallery/443413.asp]cirrus micro joule flight pack[/link]?

Dukester

My Feedback: (31)

Larry,

Though I don't do it primarily for weight, I have used a combination of a 2s1p kokam 1500 with a GWS 2 amp speed control for my smaller planes. The GWS esc has a 5 volt 1 amp esc built in and without the motor wires and short battery wires, it's about 1/8 of an ounce. The whole thing is about 1.5 ounces with the 2s kokam pack. With smaller lipolys I would think you could get a nice 5v pack with 200 or 300 mah capacity for well under 1/2 ounce. ]

Duke

Larry Driskill

I think I have the idea . . . I would need to use a 7.4 volt Li-Po and the speed control would drop me down to a usable voltage?

Larry Driskill

Ouch! Now you have reminded me of the only painful thread I have seen on this forum. Some of many things I like about the group of folks that post here is their civility, helpful demeanor, good humor, and depth of knowledge. Maybe that thread fell a bit short of the usual standards.

jboy381-RCU

larry,
batteries america has what you need...they have the lipo cells and a voltage reducer for like 40.00...a 7.2 pack weight 1/4 oz...

john

Larry Driskill

John, thank you.

Could you find what the voltage reducer weighs? I could not.

I bet the Dickiebird is right about not wanting slow servos on a quick wing, but the 6 volt set up you have found should take me to faster servo response and 145mAh or 190mAH should fly a plane with two HS-55s all morning.

jboy381-RCU

larry, i'll send them an e-mail to try and find out...i want to know myself because i'm wanting to use this set up for my plane also...they have a lipo charger too i think...which one are you wanting to use the 5.4 or 6 volt???

john

mclintock

My Feedback: (3)

don't forget this thread about reducing voltage from 2s lipo for rx;

[link=http://www.rcuniverse.com/forum/m_1989712/anchors_1991377/mpage_1/key_hand%252Ccad/anchor/tm.htm#1991377]'not just for electric thread''[/link]

Larry Driskill

Well, I see there is some good news and some bad news in the great posts you folks have made in efforts to help me save a few grams.

First the good news:

There are paths that lead toward shaving those grams.

And the bad news:

I would need a little more hand holding to actually "build" a 7.4 to 6.0 (or 4.8) volt system with specific parts numbers and simple "solder the red wire to the plus tab" instructions before I get involved in making my own bits. I can get out of my depth real quick with electronics . . . even in what you guys would think of as the shallow end of the pool.

I like the batteries and regulator on the Batteries America site. That I think I could handle.

And the ESC looks like a good fix.

Nice to know that the saving are there to be bought or fabricated.

Japanman

Hi Larry, how are you? It`s getting cooler here in Japan.
I made a glider with about a 2foot wingspan with a hs80 and a futaba micro servo, running off 3 nicad 110mah cells. I had many good flights, but on one ocassion the servos froze midflight and it went in at speed. After I repaired it I found that the problem was the battery getting low, and the hs80 drawing too much current, which pulled the battery voltage down enough to make the recevier (a futaba twochannel) blackout. After that I put in another cell to raise the pack voltage to stop it happening.

In my limited experience from the gear I own, my futaba and JR servos have a much lower peak current than my hitech and cirrus ones. I test this quite crudely by plugging in a near flat battery pack in the REC and an onboard voltage meter. Then I plug in and try different servos- I stall them and look at the voltage meter. With a hs80 it will pull my near flat 270mah nicad pack right into the red/ beyond.
With my futaba (same size as the hs80..s3203?? i forget) or JR 9gm servos the voltage drops very much less- Listening to the different servo sounds the cirrus tiny 5.4g servos and the hs80`s make a very high pitched sound that makes me suspect that the motors are somewhat 'hot and high revving' for the voltage they are run at and that is why they pull down the battery voltage at stall.
The JR / Futaba servos on the other hand sound lazy.

If it was me I`d probably just try what rec`s you have on a 3.6v NiMH pack and be carefull about which servos you use- you don`t want the blackout effect happening of course but I don`t think you will if you don`t use a 'hot' servo.
That way you don`t have to spend big $$ on a battery pack or charger. I got my my 2/3AAA cells from the battery station for $1 each!

I flew my 1 channel 020 powered plane o.o.s with no problems off 3.6 volts and a futaba rec and servo so I can`t see that it could be a problem.

Stefan

Bipe Flyer

I like to use NiMH for 1/2A RX packs. The 280mAh pack weighs 28g and the 120mAh weighs 14g.

DICKEYBIRD

Yup, I tried to stay completely out of that one but I couldn't help poking a little fun though. You're very right about the 1/2A forum, a great bunch of folks with a breadth of knowledge and experience that's unrivaled.