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View Full Version : 2S 7.4V LiPo as Rx pack?


redgiki
05-11-2006, 02:10 PM
I'm interested in making my Great Planes Spirit (pure sailplane, not e-conversion) come in a little bit lighter. My current receiver battery pack is four NiMH batteries, and works fine, but is (obviously) quite heavy.

I have quite a few LiPo batteries from my other projects, including a number of 2S LiPos. Their capacity is 800mAh or greater, and their weight is way, way lighter than four NiMH batteries. I've been considering replacing the "ballast" of four NiMHs with just the one LiPo, and am wondering if anybody else has tried using a 7.4V LiPo for an extended period as a dedicated Rx battery?

My receiver is rated for the voltage, no problem at all, according to its specs. My question is the servos. I'm running two Futaba S3003 standard servos, one each for rudder and elevator (yeah, these are part of my weight...). I'm supposing they'll handle 7.4V nominal/8.4V peak reasonably well. They'll be a bit quicker than I'm used to, I'll wager :) I use a single Hitec HS-55 servo to drive the spoilers, and I'm fairly certain I may want to change that to a micro to handle the extra voltage.

Here are the interesting math bits:
4-cell NiCD Rx pack: 6v max, 5v average, 4.4v min
5-cell NiCD Rx pack: 7.5v max, 6v average, 5.5v min

2S LiPo peak voltage: 8.4v max, ~7.8v average, 6v min

Most servos are rated for both 5 and 6 volt packs. If I run with a dedicated LiPo Rx pack, I'm pushing the limit by about 1.5 to 2 volts average.

Do you think the servos will take it? Are there other types of servos which will handle higher voltage well?

As a side note, I'm aware of a number of giant-scale pilots who only run LiPos for their servos, with great success: more torque, more speed, etc. However, those servos are also MUCH larger with much bigger motors than the standard-size, micro, and sub-micro stuff I'm used to dealing with.

Anybody who has used a dedicated 2S LiPo Rx pack without a BEC between it and the receiver, chime in?

(I own several ParkBECs that I could use, of course. They just seem like an odd solution when there's no ESC involved, and the nominal voltage of a 2S LiPo so close to the normal operating voltage of most servos...)

I guess I could be the guinea pig... it's only a $70.00 plane...

Jason T
05-11-2006, 03:08 PM
I know some are using 2s lipo but with a voltage regulator to bring the voltage back down. But I do not know anyone using a 2s straight through like that. The servos may handle it but it also may cause them to fail sooner.

rcers
05-11-2006, 03:27 PM
It will likely cook the servos in short order. You need the regulator...I think the 55's get "funky" or jittery at 5cell (6v) packs...

Mike

griffin
05-11-2006, 05:59 PM
Advice from actual experience: Single cell lipo works perfectly my electron 6 receiver and 2 HS 55 servos in my hand launch. Hundreds of flights, never worry about battery duration and nice weight savings.

Advice from reading in sailplane forums: You need 2 cell and regulator (or a small and cheap ESC) for servos bigger than 55's.

good luck, Steve

redgiki
05-12-2006, 01:40 AM
I have looked at the following options now, and it's pretty clear what my choice should be:
Four non-rechargable Energizer E2 Lithium batteries in a plastic battery case. They work well, but their voltage (1.7V) seems a tad high and makes my single HS-55 jittery. I've flown on these for several hours already, and they work well. However, the non-rechargable aspect means I'm forking out $10.00 every month or so for new batteries. (Nice to keep a spare set with me when I'm flying though, just in case...) Weight: less than 1 ounce.
Four NiMH in the same plastic case. Weight: 7 ounces.
One 2S 800mAh Lithium battery and a ParkBEC voltage regulator. So what if I'm not using an ESC? It just has an unused connector. Weight: less than 1 ounce, and "feels" even slightly lighter than the Energizers. I have three of these laying around unused since the plane they are made for is grounded for some maintenance. I also have a 1300mAh that may go in instead, for slightly more weight but dramatically increased runtime.The NiMH are not going in unless I need ballast :)

I wonder if I should be concerned if I were to run two standard servos using an ESC? I have several brushed ESCs laying around which I no longer use. The HS-55 in my rig only gets used at landing (spoilers, how I love thee...), and usually ESCs are rated for "two servos"... which I assume means standard ones. Maybe I could get away without using the ParkBEC?

Even with the light packs, properly balanced my Great Planes Spirit is coming in at 34 ounces. I think I need to shed some weight from the tail. On the other hand, I've heard of many of these birds coming in at 37-39 ounces and flying just fine.

Anyway, thanks for the hints and advice! I may have to recommend the 1S LiPo route to a friend of mine looking for a way to ditch weight on his HLG. Right now it's a bit of a brick due to using a 4-cell AA NiCD battery pack. Appreciate the suggestions, all.

Now to figure out how to jam a 3S LiPo into my transmitter. Darn those flat packs!

pilotpete2
05-12-2006, 04:13 PM
Whoa, a Nimh Rx pack for your application weighs 1.94 oz. that's a 650mah hydrimax rx pack from Tower, not much of a difference from a 2S Lipo;).
A pair of HS-81's would be a good replacement for the standards servos.
my 2M Deimos I, impure:eek: electric comes in at 41.3 oz with a 3S 2000mah Lipo. Hey, if I hand crank a generator to charge my battery does that make my sailplane "pure":D LOL (lot's of lift);)
Regards,
Pete

redgiki
05-12-2006, 11:42 PM
Hmm, you clued me into the fact that my fishing scale isn't working right. A pack that I know weighs 7 oz showed up as 0 oz today, then it seemed to get "stuck".

Darnit. That's what I get for shopping at Wal-Mart. Grr! It seems to be fairly accurate at weights above a half pound or so, but below that, I need to find another solution.

pilotpete2
05-15-2006, 04:27 PM
Hmm, you clued me into the fact that my fishing scale isn't working right. A pack that I know weighs 7 oz showed up as 0 oz today, then it seemed to get "stuck".

Darnit. That's what I get for shopping at Wal-Mart. Grr! It seems to be fairly accurate at weights above a half pound or so, but below that, I need to find another solution.

Hey, do what I do, go "Postal" well actually that's what my wife does when I borrow her digital postal scale and don't put it back.:p
Glad I passed on the fish scale at Wally World:rolleyes:
Pete