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RC Radios, Transmitters, Receivers, Servos, gyros Discussion all about rc radios, transmitters, receivers, servos, etc.

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Old 03-16-2014, 01:20 PM   #1
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Default Question re running a separate receiver battery...

Does using a separate battery for the receiver override the ESC input? IOW, if I employ a 4.8V NiMH as power for the receiver, does that cause the receiver (Futaba 617) to ignore the six volts coming in from the ESC?

Or must I disable the BEC circuitry first?

The directions for the E-Flite 60A Pro-Switch-Mode say to cut the red wire on the receiver lead to disable the circuitry. They don't say anything about what this does other than disable the BEC. I'm new to electrics, so I don't have a clue.

Here's why I'm asking… my latest model uses older Futaba servos that are not rated for six volts. This didn't occur to me until I had them all mounted and hooked up. That model needs quite a bit of nose weight; so why not make it useful weight?

My goal here is to get 4.8 volts to the servos via a separate battery, while using a 4S 3900mAh TP main battery. These servos will run on six volts, but their operational life will be shortened.

Any information appreciated…

.
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Old 03-16-2014, 02:21 PM   #2
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Just disconnect the red lead from the connector that goes from the ESC to the throttle port on the receiver. You can use a pick or pin or point of an Xacto knife to raise the little tab on the red pin and pull it out of the connector. Tape it back out of the way so you can later reinsert it should you want to use the ESC without the external battery. Now, only the external battery (plugged into any unused channel on the receiver) will power the receiver and servos yet you still have control of the motor speed via the connector in the throttle channel. You do NOT want both the ESC and the external battery trying to power the servos at the same time. Breaking that red lead between the ESC and the receiver prevents this. Do put a switch between the external battery and the receiver though for convenience.
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Old 03-16-2014, 02:23 PM   #3
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Thanks much; exactly what I needed to know.

Oh yes, I'll use an EMS switch harness.

.
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Old 03-16-2014, 02:26 PM   #4
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If you power Rx from a separate battery ... ie a 4.8v NiMH ... then this is what you do :

Take a pin or thin screwdriver tip and lift the ;plastic tab holding the RED lead in the ESC Rx plug. Withdraw the red lead contact and use a bit of tape to insulate back out of way.
Plug the ESC Rx plug into Throttle channel.

Take your separate Power supply and I prefer to have via standard radio set flight switch harness - plug into any spare Rx channel slot. This then will power up the rx when switch is made. Usually the switch harness when OFF for Rx will allow you to charge the battery by the other plug.

You MUST disable one supply - you cannot have 2 supplies to the Rx unless you isolate channels etc. The same applies if you use two ESC's ... one ESC BEC must be disabled.

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Old 03-16-2014, 04:28 PM   #5
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I am building a larger model and will be using a buss tie circuit.

Main power will be off esc, but if the power is dropped below a separate RX battery, it switches over. You can do the same with with a additional BEC, that is connected to main battery, if you do not want to have the weight of another battery source.

Basically the higher voltage "wins" and is the output. if source BEC is 7 volts and back up is 6.6, the 7 is flowing, if it drops below that of that back up, it instantly switches over o the higher voltage.



for more BussTie using a Schottky diode

Shout out for dahawk for turning me on to it. takes about 15 minutes to bend the pins and solder up. Once you figure it out

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Old 03-16-2014, 07:31 PM   #6
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the use of the Rx battery to offset weight to gain cg is way better then the dead lead weight added. i often get away with not adding weight to my kits by locating the Rx battery anywhere it needs to be. for the RV 4-40 it way back by the rudder. when i build towerhobbies uproars the Rx battery was also in the back but not that far.
on the sig hog bipe,the 6v Rx battery was ziptied to the motor mount,can't get any more forward than that... no lead weight.

use a 6volt Rx battery,not sure if i read this somewhere, but 2.4 Rx's with 4 servos or more should have 6v due to brown out issues,but maybe thats no longer true with better tx/Rx's. especially if the servos are drawing a lot of power like digitals.

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Old 03-16-2014, 08:09 PM   #7
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Originally Posted by tobydogs View Post
use a 6volt Rx battery,not sure if i read this somewhere, but 2.4 Rx's with 4 servos or more should have 6v due to brown out issues,but maybe thats no longer true with better tx/Rx's. especially if the servos are drawing a lot of power like digitals.
I'm rolling the dice on this one; true, 6V is the smart way to go, but I KNOW I'll have a servo die prematurely if I run six volts through them. OTOH, I may not have any brown-out issue because I'm running analogs and not thrashing the model.

Pick your poison, eh?

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Old 03-17-2014, 03:20 AM   #8
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Originally Posted by LSP972 View Post
I'm rolling the dice on this one; true, 6V is the smart way to go, but I KNOW I'll have a servo die prematurely if I run six volts through them. OTOH, I may not have any brown-out issue because I'm running analogs and not thrashing the model.

Pick your poison, eh?

.
Budget servos seem split 50-50 on whether they handle 6v ... some do, some don't.

4.8v was the normal for decades for all models before todays E models appeared. The only difference in packs was mAh capacity .... I had models with 250mAh packs up to 1200mAh in the 80's. I never had any issues of power loss because of pack size. I accept that they were NiCD which have a potential delivery capability greater than NiMH. But NiMH have a greater capability than some would lead you to believe. I have a scar on my left leg from a shorted AA NiMH cell ... it discharged high enough amps to burn a hole in my jeans and a scar on my leg.

Today people get obsessed with it ... and I sit back and think - OK - whatever floats their boat.

As long as you use a pack size and form that can supply demand and not sag too far during it - you are fine.
Some people like the quicker action of the 6v vs the 4.8v .... well all I can say is I fly some pretty darned fast models that demand fast response and I have never needed 6v. There are plenty of servos out there with fast timings on 4.8v. Even the cheapoTg9e from HK is faster than many.

I'm not trying to convince anyone to NOT use 6v ... I'm just balancing the posts on it.

I use ESC BEC on 99% of my models, uBEC on a few, 4.8v NiMH on my non-electric models.

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Old 03-17-2014, 02:51 AM   #9
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Originally Posted by waytooslow View Post
I am building a larger model and will be using a buss tie circuit.

Main power will be off esc, but if the power is dropped below a separate RX battery, it switches over. You can do the same with with a additional BEC, that is connected to main battery, if you do not want to have the weight of another battery source.

Basically the higher voltage "wins" and is the output. if source BEC is 7 volts and back up is 6.6, the 7 is flowing, if it drops below that of that back up, it instantly switches over o the higher voltage.



for more BussTie using a Schottky diode

Shout out for dahawk for turning me on to it. takes about 15 minutes to bend the pins and solder up. Once you figure it out
Yeah
I'm one who is very much in favor of using dual battery power supplies to giant scale models. There is just to many $$$$ involved to risk receiver power issues.

Here is a thread on a combo with an uBEC and a two cell A123 (Or LiFe) battery pack.
http://www.wattflyer.com/forums/showthread.php?t=63794

Also, find attached a drawing for a pair of A123 packs, using Shottky diodes. I've used the transistor type cases in the previous posting, but IMHO, those cases do not have adequate strength. At least compared to a dedicated 9 Amp axial Shottky diode. Bend those transistor type leads back and forth three or four times, and they will break off. The 9 Amp Shottky diodes listed in the attached drawings have very heavy duty wire leads, nearly requiring a needle nose pliars to bend them.


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Old 03-16-2014, 09:01 PM   #10
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Why not just use the BEC? In any case, if the servos are rated for 6s (which almost all are these days) they will be fine running at 6v. I run all my servos at 6v and never have had any problems, in fact the last servo i had fail was running on 5v.

The problem with a battery is voltage varies. A 4.8V NiXx battery when it's part discharged and under load will be around 4.4v or less. Add some resistance in the wiring and connectors and you will be down under 4V at the receiver, and getting too close to comfort to brown-out territory.

The ESC's own (switching) BEC or, if you need more juice, a stand alone BEC will give far more stable voltage.
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Old 03-16-2014, 11:49 PM   #11
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IMHO,if your 4.8v rx pack shows 4.4v under load,it's time to charge,or bin it.Under normal use conditions,your rx pack should not be below 4.8v.A fully charged pack should show around 5.5v.
Btw,if buying new packs,I would go with the lsd batteries.Even the cheap ones from hobbyking far outlast the normal nihms.I use them in all my tv,video,etc. remotes.My camera will run for a couple of months on them.
My$0.02.
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Old 03-17-2014, 12:27 AM   #12
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Originally Posted by DHC Beaver View Post
IMHO,if your 4.8v rx pack shows 4.4v under load,it's time to charge,or bin it.
4.8v is the 'resting' voltage. Just like any other battery once you put it under some load and draw current from it the voltage drops significantly, very significantly in the case on NiMh batteries which cant tollerate high discharge rates.

If you take a typical full size AA cell NiMh battery of 1500mAh, a 5A discharge current as you might get with four standard size servos, is over '3C' discharge rate (3.33c to be exact).

Take a look at this discharge curve, you will see that at a 3c discharge even a fully charged brand new NiMh very quickly drops to 1.1v per cell, so that's 4.4v for a 4 cell pack.


At 5A you get at the very least half a volt drop over the connector at the bettery and at the Rx, so by the time you get to the Rx that's already down below 4v even for a fully charged pack. Things would be much worse if you used a smaller light weight pack of lower capacity.

NiMh packs are really bad at holding voltage under load. Old NiCd were much better.
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Old 03-17-2014, 03:29 AM   #13
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Originally Posted by DHC Beaver View Post
IMHO,if your 4.8v rx pack shows 4.4v under load,it's time to charge,or bin it.Under normal use conditions,your rx pack should not be below 4.8v.A fully charged pack should show around 5.5v.
Btw,if buying new packs,I would go with the lsd batteries.Even the cheap ones from hobbyking far outlast the normal nihms.I use them in all my tv,video,etc. remotes.My camera will run for a couple of months on them.
My$0.02.
Your post is only true for NO-LOAD situation ... but I agree that 4.4V is actually lower than I see in use.

Charged pack will have each cell at 1.3+v once settled .... 5.5V total is just of charger and before the pack has settled. 5.2 - 5.3v is more like it.

With serious load a NiMH will drop to about 1.1+ to 1.2v ... but will hold it well ... unlike Dry-Cells that basically are terrible under load. It takes a really serious load to push a NiMH to below 1.1v ... which is still well above most Rx's minimum requirement.

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Old 03-17-2014, 03:34 AM   #14
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Originally Posted by JetPlaneFlyer
if the servos are rated for 6s (which almost all are these days) they will be fine running at 6v.
The key words there are "almost all are".

These are not.


May I suggest there may be a typing error there JPF ? 6S ??

I see servos rated at 6V ... but not 6S.

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Old 03-17-2014, 03:45 AM   #15
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There are battery back-up devices available over counter that two packs are connected and it switches over in even of failure of main pack.
They were around even in the 80's more for the large model guys.

I still have one .... and it is fast enough on switch over for Rx to stay locked on.

Denny and I have had chats about the use of NiMH many times and I have to say that a load of 14A indicates to me ... a lot of high power digital servos and serious control surfaces loading them up.
The average model out there will never even approach a fraction of such load. If they did - then the usual BEC would be useless as many of them are 2A at designed voltage.
My moderately sized heavy biplane with 61 Glow power has 40gr servos ... big surfaces .... and the Rx is powered by a 4 cell NiMH pack of 2300mAh. I've measured the load when 'waggling sticks' ... and even holding a surface, I have not been able to force it past 2A ... The pack will last me all weekend flying .. but I am able to connect it to my flight box for a quick top-up if necessary.

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Old 03-17-2014, 02:02 AM   #16
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Originally Posted by JetPlaneFlyer View Post
if the servos are rated for 6s (which almost all are these days) they will be fine running at 6v.
The key words there are "almost all are".

These are not.

.
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Old 03-17-2014, 02:32 AM   #17
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Originally Posted by LSP972 View Post
Does using a separate battery for the receiver override the ESC input? IOW, if I employ a 4.8V NiMH as power for the receiver, does that cause the receiver (Futaba 617) to ignore the six volts coming in from the ESC?

Or must I disable the BEC circuitry first?

The directions for the E-Flite 60A Pro-Switch-Mode say to cut the red wire on the receiver lead to disable the circuitry. They don't say anything about what this does other than disable the BEC. I'm new to electrics, so I don't have a clue.

Here's why I'm asking… my latest model uses older Futaba servos that are not rated for six volts. This didn't occur to me until I had them all mounted and hooked up. That model needs quite a bit of nose weight; so why not make it useful weight?

My goal here is to get 4.8 volts to the servos via a separate battery, while using a 4S 3900mAh TP main battery. These servos will run on six volts, but their operational life will be shortened.

Any information appreciated…

.
What I would do is just install one of those switching type Battery Elimination Circuits such as the Castle Creations 10 Amp uBEC. These things will put out 10 Amps to your receiver and its servos (when pulling lots of current), while holding 5 Volts DC at its output. That 5 Volts is about what a 4 cell Nih battery pack operates at. These uBEC's weigh 0.4 ounces, and cost less than a decent 4 cell Nih battery pack. I'm using about 10 of those CC uBEC's in my various models, from a few hundred watts up front to my giant scale 3000 Watt model with a Hacker A50 motor up front. Absolutely zero problems with any of those CC uBEC's.

Down the road, if you ever want to run your receiver and its servos at a higher voltage, the CC uBEC can be programmed with a $20 something PC USB dongle to any voltage between 4.8 and 9 Volts DC.

As for me, I'd NEVER use a 4 cell Nih pack on any 2.4 Ghz receiver. Especially if the model is over perhaps 1KW power level or so. Yup, Futaba doesn't have the "brownout" reputation of the Spektrum receivers. Doesn't mean that it could not happen with Futaba.

My giant scale model's servos have been measured to pull a peak current of 14 Amps while just twiddling the transmitter sticks round and round. (With a $350 Fluke 87V digital meter) Pulling 14 Amps on a 4 cell Nih "AA" pack will pull its voltage down below the point where your receiver can reboot due to under voltage.

Ref:
http://www.castlecreations.com/products/ccbec.html

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Old 03-17-2014, 03:10 AM   #18
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Originally Posted by kyleservicetech View Post

Yup, Futaba doesn't have the "brownout" reputation of the Spektrum receivers. Doesn't mean that it could not happen with Futaba.

]
Again... I understand all of that and am fully aware of the risk I'm taking. I'd be taking the same risk by running six volts, in the guise of one or more fried servos.

.
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Old 03-17-2014, 04:08 AM   #19
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Reality and RX voltage/current supply.

Before 2.4 ghz we got away with a lot of stuff that really should have crashed a LOT more models. 72 mhz RXs (AM or FM PPM... many PCM's would go to "failsafe" at about... 3.8v) didn't drop offline until voltage was well below where the servos simply could not even move.
A large percentage of "glitches" and "I been hit!" incidents were more likely to have been low voltage causing strange results then any kind of interference.

Futaba's 2.4 ghz will still work until voltage is so low the servos can't even move (based on the Futaba 300X series and equivalents)

Spektrum dropping offline at appx 3.8v ( which is below where the servos are only delivering 50% of rated 4.8v torque) would never be a problem with 4 cell NiCd as the RX power source if the battery and switch harness are up to delivering the current demanded in flight. But with modern RC aircraft and the old "standard" 500 mah 4 cell NiCd packs your RX power was marginal even for a .40 class TRAINER powered by a .46!

I tried to stick with NiMh to power the RX of a larger model, using 2 X 5 cell 3000 mah Sub C packs each with its own switch harness and just "stirring the sticks" with the prop stopped I could bring voltage down to 4.0v at the RX by DX-18 telemetry... JR heavy duty switch harnesses could not deliver the current demanded.
I switched to 2S (6.6v nominal) LiFe 1250 mah (X2 with home made heavier duty switch harnesses) and suddenly the voltage doesn't drop more than 0.5 v in the same ground check.

Don't blame the brown-outs on the RX... its your RX's power supply that is wrong. The Spektrum RXs will never brown out if the power supply is adequate.

Inadequate servo torque rating can make the situation even worse... I tested a model with 60 in-oz rated servos and it demanded >15 amps (max my logger I used would read) in flight. I changed to 90 in-oz servos in the same model, being flown the same way and because I wasn't stalling the servos max demand was down to 7 amps.
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Old 03-17-2014, 04:52 AM   #20
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I think one of the factors that has affected our use of Rx power today - is the Binding and Lock-in function.

Previously in the days of 500 mAh NiCD packs - we used various radios ... 27, 35, 40, 42, 72 MhZ etc. depending on country and model type.

IF your Rx power dropped low enough to cause loss of Rx capability - it didn't need re-boot to gain lock again. It was literally instantaneous on signal pick up .. so those 'glitches' we used to have MAY have been power sags. BUT Rx's in those days also tolerated far lower voltage sag than todays ... AND believe it or not servos today AND then worked down to very low levels - the speed and torque suffered.

I agree that today because of todays technology - we may be seeing what was 'hidden' before.

The other factor of course is that yesterdays NiCD's were lower capacity but literally potential bombs in output capability when pushed. Todays high capacity NiMH can never approach such high discharge rates as the old NiCD could. People have to be careful not to mix the two in discussions.

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Old 03-17-2014, 07:53 AM   #21
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Originally Posted by fhhuber View Post
I tried to stick with NiMh to power the RX of a larger model, using 2 X 5 cell 3000 mah Sub C packs each with its own switch harness and just "stirring the sticks" with the prop stopped I could bring voltage down to 4.0v at the RX by DX-18 telemetry...

Exactly the point i was making. The 'problem' is that most people don't measure NiMh Rx battery voltage under load, and even if they do they don't measure it with something that can identify the transient dips. So they don't know how low it can go during momentary high demand situations.

And Nigel, yes sorry, fat fingers, i did of course mean 6v not 6s


PS.. to the OP... ok, your servos are only good for 5v... I still don't see any reason not to use a BEC. Most have default output in the 5-5.5v range which will be perfectly ok for your servos. As noted above, under load you drop some voltage in the wiring anyway.
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Old 03-17-2014, 08:12 AM   #22
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You don't need a meter in fact.

Connect it all up and then 'stir' the sticks like crazy ... see if rx blinks or drops out.

Next simple weights added to a couple of surfaces to imitate extreme flight loads - stir sticks again ... see what happens with Rx ..

I do this with each model and yet to find any that fail.

Some people think that flight loads on surfaces are constant or near constant .. that's not actually the case. They are if airframe is fixed and cannot move in connection with the surfaces forces. But in flight, as surface deflects, exerts a momentary force on the airframe which then reacts by moving in the direction commanded ... Pitch up / down, roll etc. The command is continued but force now is far less as the airframe sustains it change of direction / attitude. The next significant force is momentary when you command to stop or return to level flight etc.
The power source, gearing, control connections have to be suitable to carry those momentary forces ... and that is what is the killer.
My simple test routine imitates that and would soon tell you if you have a problem.
I would also suggest it is done NOT with full just off charger power pack - but a semi depleted one to further imitate real life.

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Old 03-17-2014, 06:47 PM   #23
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Originally Posted by JetPlaneFlyer View Post
Exactly the point i was making. The 'problem' is that most people don't measure NiMh Rx battery voltage under load, and even if they do they don't measure it with something that can identify the transient dips. So they don't know how low it can go during momentary high demand situations.
That is the point, exactly. The current pulled by some of these servos can hit 2 Amps each when going full "forward" then instantly going full "reverse". In a giant scale model with 7 servos, I've measured peak currents of 14 Amps on my $350 Fluke 87V multimeter with its peak recording function. Also have used my oscilloscope to watch the voltage of a 4 Cell 2700 Mah "AA" type Nih pack while powering my giant scale model with its seven Hitec 645MG servos.

Just spinning the transmitter sticks round and round while watching the battery voltage on my oscilloscope was enough to convince me that a 4 cell "AA" type Nih pack was not a good idea on that giant scale model.

On the other hand, a fellow club member uses nothing but five cell Nih batteries for receiver power in his giant scale models. BUT, he is using those "Sub C" type batteries, NOT the "AA" batteries. Those Sub C type Nih batteries can easily handle the power requirements of just about any giant scale model with a wingspan of perhaps less than 110 inches. Those Sub C type Nih and Nicad batteries have been around for decades in countless battery operated power tools. They work and work well. For the giant scale models with 50 cc gassers up front, the weight of a five cell Sub C battery will hardly be noticed.

If you like to build your own Sub C Nih battery packs, here is a good source for the individual cells.
http://www.voltmanbatteries.com/serv...BATTERY/Detail

Or if you'd like the metal jacketed A123 cells with welded solder tabs, Voltmanbatteries again: FYI, between myself and various club members, we've purchased over 100 A123's from Voltmanbatteries. Nice place to do business.
http://www.voltmanbatteries.com/serv...1-dsh-A/Detail

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Old 03-17-2014, 06:38 PM   #24
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Originally Posted by fhhuber View Post

Spektrum dropping offline at appx 3.8v ( which is below where the servos are only delivering 50% of rated 4.8v torque) would never be a problem with 4 cell NiCd as the RX power source if the battery and switch harness are up to delivering the current demanded in flight.
I've checked my variety of Spektrum receivers on my adjustable voltage DC power supply. Those receivers in my models all drop out at about 3.2 Volts DC, plus minus a few percent.

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Old 03-17-2014, 10:10 AM   #25
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The loads in flight can be very high...

http://www.radiocontrolinfo.com/RCca...culator.php#WL
Simplistic calculator for estimating expected servo loads... deflecting the servos from level flight.
You can potentially double that calculator's result for potential load on ailerons in a sustained max G (that the aircraft can hold without stalling or breaking) pull such as a tight loop.
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