I've done my due diligence of searching before posting this and haven't yet found a definitive answer to a question, so here goes:

I'm about to make my move to 2.4ghz for the first time (radio ordered not yet received), and I've come across various references to 2.4ghz receivers dropping out when they reach a low voltage point.
My experience with 72mhz receivers is that they might get twitchy at some low-voltage point, but you get a bit of a warning that way before you get into real trouble.

On the other hand, what I've read indicates that 2.4ghz receivers just shut down, and that's it.

So for those with experience, what's the deal, and if the 2.4ghz receivers are different in that way, what do you do differently and/or what precautions do you take when operating with them?

72MHz systems are analog. Continuously receiving signal albeit strong or weak then none. The stronger the signal the better the response.

2.4GHz systems are digital. On or Off. Signal continuously received and compared throughout the receiver battery voltage until battery too weak for the receiver to interpret signal. (Both FHSS and DHSS have caveats with metal & carbon fiber barriers)

That's it in a nutshell.

If your battery stays above 3.5v to 4.0v(depending on brand) the receiver will operate. If your system is not adequately charged or uses a battery that under load will fall into this range, there will be loss of reception until the battery voltage returns to above that level. Sloppy battery management will bite you whether using 72Mhz or 2.4Ghz.

The bottom line is to measure how much current your Rx and Servos draw when all the Servos are Stalled. This is the Worst Case Scenario. Be sure to use an Rx battery that has a Capacity well above this Threshold. If your Servos will tolerate it, use a 6v battery instead of a 4.8v battery.

An H9 Current Meter can be hooked up between an individual Servo and the Rx, or between the Rx Battery and the Rx. Look to see if there is a significant Voltage Drop when all the Servos are Stalled.

With Up to Date Firmware, if you have a Brownout, the Rx will only go out for 1/2 sec, but if you have sufficient Capacity, you will not see a Brownout at all.

Got it. I get the point about digital working or not with nothing in between. I learned that when I went to digital flat screen TV's at the house.
My main concern with the 2.4ghz receivers is that (as has been pointed out) they will drop out momentarily during a servo stall, or even during a hard manuever if the RX battery is anywhere near being on the low side.
It's not that I ever try to squeeze every last drop of voltage out of a charge, but that those momentary voltage drops that I never noticed with my 72mhz systems will become a major factor for losing aircraft when flown on 2.4ghz.

I've been using 6v RX batteries for years already, so I think my strategy will be to continue to do so.

As a side note, I have to wonder why a 2.4ghz "needs" a particular voltage to work anyway. What I mean is that if a 2.4ghz receiver were to be designed to work within a voltage range of, say, 3-6 volts, then that would guarantee that the servos would fail first, but gradually, and that would allow the plane to be landed while still under radio control of the RX that would still see 3+ volts and therefore not drop out.
Just thinkin' out loud...

Edit:
In the mean time, I ran across this: http://www.airlandseahobbiesinfo.com...erd-airplanes/
Based on that information, I think that my strategy will be to continue to use analog servos (all my servos are analog to this day... no digital) so that I can safely use 6V RX packs to ensure the integrity of the digital RX while at the same time not having to worry about burning up any digital servos.
This still brings me back to wondering why RX's can't be designed to operate with less voltage than they do now.

If you used a device to Isolate the Servos from the Rx, so that the Servos were powered Separately, the Rx would run fine at 3.5v. Some people use an Isolator or a Power Box to accomplish that.

it "needs" a minimum voltage to work because of the microprocessor inside that sorts out all of the digital bits - the processor will reboot if the voltage gets below a minimum point, because there are no guarantees it will continue to work properly below that voltage, so it starts back up in a known condition.

The voltage doesn't have to stay below that minimum point for very long to trigger a re-boot, so a dirty or intermittent switch or connector can cause it, even if your battery is overkill for the application - use good switches and keep connectors clean, too.

To be clear, the Futaba FASST system is not voltage sensitive. It will remain linked and functioning until the voltage is too low for the servos to work correctly. Using digital servos that point is 3 volts, which is the minimum voltage most digitals need. Analog servos will go lower. There have been reports (I cannot find the link right now) showing the FASST RX remaining linked as low as 1 volt.

Futaba recommnds using a battery that will deliver sufficient voltage under the load you will use.

That's good to know for the Futaba guys, but the radio I have on the way is an Aurora 9 so I don't know if that info applies.

Any receiver will drop out of the voltage is dragged too low.
Use an adaquate power source and it won't be an issue

Sorry, you did not share that information in your first post so it seemed as if you were asking a general question and not one specific to the Aurora 9. Why not shoot them an email and ask them??

The question for each manufacturer how low is "too low". "Too Low" is higher for some than for others.

It's pretty unrealistic to suggest that going down to 3v on a pack is in any way viable.

Of course. A 4.8v pack that reads 3v is toast. And I know it's not likely that a healthy 4.8v pack would be drawn down to 3v by a temporary servo load either.

What I'm getting at is that I'd simply like to see a situation where the receiver is the last to "check out" in a low voltage situation.
So to correct my off-the-cuff low voltage point, it would probably be more practical if I were to hope for a 4.0v voltage seen by the 2.4ghz receiver before it drops.
For all I know, the Hitec RX's might work that low, but I haven't done enough research to know that, or whatever their lowest voltage variation is.

It is not the voltage of the pack, it is the voltage at the receiver bus that counts. Even with a perfect battery, if you have sufficient impedance in the leads and switch, the voltage can fall at the receiver bus when a pulse of current is demanded by the servos. This voltage drop, even if it last for only a few microseconds, can cause the receiver to reboot. In short, the quality of the switch and wiring between the battery and the receiver is far more important than the size (MAH capacity) of the battery. You can not measure this with a meter, you need a device that will sense and record this drop when it lasts for only a few microseconds. An oscilloscope works well. The Voltwatch is also a pretty good indicator of this potential problem. If you have one plugged directly into the receiver bus (not at the end of a Y with something else) and you see the red LED flicker as you cycle the sticks, you have a potential problem. The Voltwatch is a much better indicator than a loaded voltmeter for checking out any 2.4GHz system.

Why even let that situation occur, though? I do my best not to plan for failure, but to ensure that failure does not occur. With today's high capacity batteries and inexpensive voltage monitors, a person really doesn't have much excuse for running out of battery power.

That 600mah pack that was standard in a 3-servo analog flight pack twenty years ago just isn't safe to use today. We all know this, right? Some people have learned the hard way, I suppose. Nobody would put a much larger engine on a model without adding more fuel, and then scratch their head when they can't get the same flight time.

The point at which today's 2.4ghz receivers stop working just isn't a huge issue, to me. Like every other aspect of this hobby it's something to be aware of and to plan around. If a person allows the situation to develop, they've likely already lost the model due to the rest of the flight pack failing as well. Is there a meaningful difference between a receiver becoming intermittent and a set of servos not having enough juice to adequately move the control surfaces?

Please pardon me for perhaps thinking faster than I'm writing, but there may be an advantage to a receiver becoming intermittent first. I think the situation where the system voltage drops due to increased load from the servos would be first noticeable on a Spektrum that browns out at 3.5v, right? Assuming it's just a temporary load increase and the system reconnects, at least the pilot will have that moment of awareness that he didn't have control. That would provide an opportunity to recognize the problem and save the model, I suppose. How that compares to increasingly unresponsive controls I'm not sure. In either case, I don't know that too many of us would have the quick thinking to recognize the issue. After all, if we were that bright we'd not have let the issue come about.

And yet it has been shown to happen to JR/Spektrum systems. Which is one reason they came out with a plug in capacitor to smooth out voltage bumps and now advertise how fast their system re-binds when it has lost binding due to low voltage and it blinks the LED to show you that it went into a low voltage re-bind. So this is not at all unlikely.


Which is exactly what happens with a Futaba FASST system. Run the voltage as low as is possible for any servo movement to happen and the radio remains linked to the TX.

Futaba on most radios also has a low voltage failsafe. If the RX senses a voltage of around 3.8 volts it moves the throttle to the low position. Cycle the throttle stick and it resets the failsafe and lets you land. It does not effect the other channels. Nor will it lose link, which I think is undesirable. But it will let you know when your RX voltage is dangerously low, but keep working while doing so.

Futaba definitely goes well below that. JR/Spektrum now can go to 3 volts, but they will lose link and need to rebind, and I have no idea what Hitec does under low voltage. Again, why not contact Hitec, or Mike Mayberry in the Aurora 9 thread, and ask him.

Or this... http://www.xtremepowersystems.net/pr...hp?prod=XPS-TT

These posts may clarify some concerns regarding the Hitec 2.4GHz system
<font color="#22229c"><u>Optima RX - SPC Cable and use</u></font>
<font color="#22229c"><u>Optima RX - Multiple Batteries may be installed</u></font> Dual batteries for redundancy or even one for each servo.
and more under
<u><font color="#00265e">Aurora A9, Spectra Pro &amp; Optima - FAQ &amp; Undocumented Features</font></u> - Mixes, Setups, Tips.

Regards
Alan T.
<u><font color="#00265e">Alan's Hobby, Model &amp; RC FAQ Web Links</font></u>

So the answer to Hitec RX drop-out voltage is "3.5v".
Since that could happen very easily not so much by a weak battery, but by a much more common occurance of servo load, it appears that it's sort of "official" that if you go with the Hitec gear, it is absolutely essential to move up to 5-cell receiver packs as "standard issue".

That way, you don't have to be concerned about a low battery or servo load so much, but now you have to make sure that your servos can handle the extra voltage.
I understand that some digital servos can't, but I'll address that issue when the time comes.
First, I'm off to order some all new 5-cell flight packs.

With just a very few exceptions almost all current servos can handle the voltage of a fully charged 5-cell Nicad or NiMH pack. There are a couple of helicopter tailrotor servos and some micro servos that cannot, but almost everything else out there will work just fine.

NO BS Batteries is a great source of battery packs: http://www.hangtimes.com

Hi, Highflight -RCU,

The issues you refer to were blown up by guys who liked ....WHATEVER...brand better than Spektrum and started some hype on it. The receiver IS the last to check out. The servos are made to work on no voltage lower than 3.8, so at that point they're already not working right. The first thing that fails is always the pilot. The pilot blaming the receiver for blacking out because he failed to properly monitor and charge his batteries.....it doesn't hold up. The Spektrums "reboot" when the voltage drops below 3.5, a full half-volt below 4.0, which is already WAY-WAY-WAY too low to be flying. Obviously, already unsafe. I do not know what the Aurora has for its reset voltage. We've had field chargers for 20 years now. There are no excuses for run-down batteries being up in the air these days.

Your inclination to use 6-Volt packs is the best idea to start with. I use nothing smaller than 2700 in NiMh, I get them reasonably priced from RadicalRC.com. Keeping your system modest and using low-draw servos, always a good, prudent move. If your system design increases to where the draws are high, then you could look at bigger systems. Rather than that, my next step up is to add a another battery into a "Y" in the rudder or elevator servo lines so that servo can use the extra pack for direct amperage draw. Simple and effective, keeps good voltage going at the Receiver. Beyond that, then I would look at LiPos and regulators and all that stuff. But to be honest, that's where I draw the line, I just don't have a need for a plane with a system as aggressive as that. If I want to fly something too complicated to enjoy any recreational benefits I'll just buy a few helicopters!

Use your common sense, you seem to be doing fine already. I incorporate weight issues into mine. Need a little more weight up front? Great, I order in a beefy 5-Cell NiMh 6-volt pack and tuck 'er in there!

~ Jim ~