Spektrum 2.4 4.8v RX battery?

[martin collins 1]

I have been flying Spektrum 2.4 since the transmitters first came out in this country and have always believed that the receivers required a 6v battery to safely power them, i recently bought a couple of used models that were ready to go with the rx and battery fitted, both models had 4.8 nimh packs fitted. Have i misunderstood for all these years or was the previous owner flirting with danger?

[Engine Doctor]

JMO but should be OK providing the batteries have enough capacity to maintain the voltage while under load. AA Nimh cells up to 2000 mah are usually ok but go higher and as the load increases the internal resistance rises and voltage drops . Not noticed much on 35 Meg as servos would just get slow but on 2.4 when the voltage drops below the threshold then it huts down until the voltage recovers by which time your model is probably park in a subterranean spot 😉.

If batteries are old or poor quality just reoplace them for peace of mind as its not worth the risk and if there is room sub C cells are a better option.

[Philip Lewis 3]

Back in the old DSM days it was well known that Spectrum receivers had a much higher voltage at which point they would "brownout" (shut down) than other receivers hence the caution, they would brownout at over three volts whereas other receivers continued to function down to way less voltage than that.

 

However that was a long time ago and I beleive was fixed some time ago now. You can probably find some old video's on U Tube about it if you search.

[Phil Green]

Early Spekky receivers used a linear 3.3 volt regulator with a relatively high dropout voltage which necessitated a low ESR battery that wouldnt sag under flight loads.

This sparked the whole ridiculous 5-cell pack thing which was a bad solution then and defo isnt necessary using todays Spekky gear.

Additionally early receivers had a slow recovery from brown-out as the processor took its time to reboot & re-link.

Both of these early problems have been resolved and an adequately-specced 4.8v pack is fine, as it always was  😉

 

As a comparison, all the Frsky receivers I've tested work fine down to 2.5v where the servos have long since given up!

 

 

 

[Andy Stephenson]

I was always dubious about the 5cell battery thing because as far as I could see Specky sets were suplied with JR servos in the early days very few of which had a 6 volt rating.

[Phil Green]

Technically it was (is) a ridiculous solution Andy 😉 

You dont fix a power supply that is sagging under load by raising the voltage its dropping from, you increase its ability to maintain voltage at higher currents by reducing its ESR.  Actually using 5 cells raises the overall pack ESR by 25% so the amount of sag for a given load is proportionally worse.

 

Edited August 2 by Phil Green

[GaryW]

I've been flying on Spektrum since the original DX7 came out back when I was working in the model trade and I've always rune my Spektrum Rx's on 4.8v NimH Packs and never had an issue .... Only time I've ever used 6v ( 5 cell ) pack is when running High Torque Servos or Digital Servos in Helicopters... This whole " you must rune 6v ( cell ) pack thing thats being going around since Spetrum was first released in the UK IMO is total BS.... It's more personal choice or preference of an RC Modeller what Rx Batteries they use in their models which coincidently I now run my Heli Rx's on 2 cell Lipos but thats only because I also use the Align 2 in 1 Regulators to run both the Rx & Glow Power.... Even my lattest aquisition being an 102" DB Centurion weighing in at 6.5kg all up is only running on 4.8v Rx Power although there are 2 RX Pack running in parallel as the servos are slightly higher torque than say a futaba 3001 ( pics below )

 

 

If you think back to the days of the original DX7 they had 9.6v ( 8 cell ) Tx packs then as time went on they went to basically around 2000 mah 4.8v ( 4 cell ) packs like iwhat's in the DX7S. 8's & 9's etc... now with my current NX8 and also with the NX6 & 10.. the standard battery in those are single cell 3.6v Li-Ion which also shows 2.4ghz radio gear uses far less power than what we all remember of the old 35mhz days... only thing that hasn't changed as far as I can tell over the years really is the voltage of servos

 

In my own experience my view is this whole 4.8v or 6v Rx Pack debate/argument is a pointless one

 

 

Edited August 2 by GaryW

[leccyflyer]

4 hours ago, martin collins 1 said:

I have been flying Spektrum 2.4 since the transmitters first came out in this country and have always believed that the receivers required a 6v battery to safely power them, i recently bought a couple of used models that were ready to go with the rx and battery fitted, both models had 4.8 nimh packs fitted. Have i misunderstood for all these years or was the previous owner flirting with danger?

I believe it was a bodge to address an issue that was being experienced by early adopters of Spektrum gear, with their receivers suffering from brown out. As Phil said, probably exacerbated by those receivers taking longer than usual to reboot, by which time your model might well have been stuffed. For years I didn't experience any issues whatsoever, most models being electric powered with BEC supplying the receivers, then we suffered our first brown out and true to form the model was destroyed, where it had been flying perfectly prior to that. I'm not sure whether HH ever actually put it in writing to use a 5 cell pack to deal with the issue, but they certainly replaced receivers with a note - "Tested OK but replaced FOC for Customer's Peace of Mind".

 

Also, for a time some model shops - no names, no pack drill - would sell a single capacitor on a servo lead, which you plugged into a spare servo channel and which was supposed to provide support to the RX should the pack voltage drop momentarily. I'm informed that it was pure snake oil, but treated it in the manner of one of those Tiger Guns that you carry on Safari in the Highlands, Have never seen a Tiger once, since carrying one.

 

Have still got plenty of models with 4 cell receiver packs, though thinking on, most of those are gliders which would be on Frsky anyway, I'm sure there are some Spectrum ones with 4 cell pack though.

[Paul Marsh]

All 2.4 I use 5 cell 6v batteries now as the 2.4rx are using a computer not a amplifier, where low power  35mhz would still work, but a brownout on 2.4 would cause the computer to shut down and hopefully if enough power to restart - providing there is enough power, when probably there isn't, then you get a loss of control and crash.

Only time i use a 4.8 pack is on older JR servos, but use a dual battery, one to power the flight servos (especially a Hitec 75 retract servo where 6v has burnt them out as 4.8 v only) and a 6v pack for a rx only.

most 2.4 rx stop working around 3.0 to 3.3v and 4.8v doesn't give much headroom for a high load such as a high speed loop...

[Phil Green]

Paul I'm afraid I disagree with most of that  🙂

The fact is that the receiver draws very little current, an insignificant amount compared to the servos.  The servo load is what causes a sub-optimal battery to sag, the receiver doesnt contribute to that, but does suffer the consequences.  This has nothing whatsoever to do with the band be it 2.4, 35 or 27 and everything to do with esr and wiring losses.  The use of separate packs is fine but there is absolutely no reason to run the receiver alone from 6 volts, if it has the usual linear LDO then all that excess voltage is converted to heat.  In typical use a receiver will never cause a battery to sag - it draws only a few milliamps which keeps the dropout very low and is constant, regardless of servo load.

All of todays 2.4 receivers run on 3.3v and with a typical low dropout regulator a 4v input gives ample headroom. A 4.8v pack gives headroom to spare. Powering a receiver alone with 5 cells makes no sense. But going back to single packs, if during a 'high-speed loop' your servos cause your voltage to sag then the supply has too low a current rating, not too low a starting voltage.  A power supply having insufficient current capacity for the load should be fixed properly at source, not propped up with an extra cell, which actually increases the amount of sag for a given load 🙂

Cheers

Phil

 

 

 

[Mike Blandford]

The processor chip that is used in some receivers (by FrSky now and, I believe, Spektrum now) is specified to operate down to 1.9V (RF section, actual processor 1.8V).

Add on whatever the LDO voltage is (likely less than 0.5V) and the Rx should continue operating down to less than 2.5V.

As a receiver, it will only take a small current, but if sending telemetry the current draw will be higher, up to 100mA.

 

Mike

[MattyB]

On 02/08/2024 at 11:37, Philip Lewis 3 said:

Back in the old DSM days it was well known that Spectrum receivers had a much higher voltage at which point they would "brownout" (shut down) than other receivers hence the caution, they would brownout at over three volts whereas other receivers continued to function down to way less voltage than that.

 

However that was a long time ago and I beleive was fixed some time ago now. You can probably find some old video's on U Tube about it if you search.

Correct, some early Spek receivers did suffer from very  high brownout ( reboot) voltages so any dodgy RX pack could cause issues. That issue is long since gone now though, all those RXs have been replaced in the s  ok we’rek line up a long time ago

[Cuban8]

A four cell rx pack of good quality and able to source adequate current without  significant voltage sag for the servo demands in a particular model will be perfectly OK......whether four or five or n cells, if they can't deliver the current expect trouble - with any brand of gear btw.

We all know about C ratings for our lipos but hardly ever consider the C of rx packs. Unhelpful that the manufacturers AFAIA don't mark their batteries accordingly. My own experiments have shown AA nimhs to be around 2C or 3C if you're lucky - higher capacity cells being not so good. Easy enough to check with most chargers - you may well be surprised at what you find.

My preference for 5 cell packs is that I've come across weak individual cells within the battery and to lose capacity down to 3 cells whilst flying is far worse than losing one and running on four especially after a long day with many flights.

Regular capacity checks are needed to root out suspect packs and I've found several 'time bombs' waiting to go off over the years.

[Christopher Wolfe]

5 minutes ago, MattyB said:

Correct, some early Spek receivers did suffer from very  high brownout ( reboot) voltages so any dodgy RX pack could cause issues. That issue is long since gone now though, all those RXs have been replaced in the s  ok we’rek line up a long time ago

A friend in the U.K. told me about ten years ago  "Don't take a Spekky to Robot Wars" (he was a competitor) 🙃

[Cuban8]

Not going to resurrect all the anti Spektrum nonsense of years ago - but the fact that many folks were caught out by using "dodgy RX packs" didn't help the 'problem'. 

I clearly recall many club mates proudly showing off their ten years+ old nicads/nimhs and saying they were very reliable and had never given any bother.......🤪

 

[Kim Taylor]

I'm not sure if the following adds to the discussion, but I have copied my reply to a similar thread over on RCGroups.

 

I had a similar issue recently, resulting in the total loss of my 63"Gangster (the Brits will know what that is, for USA modellers it's a bit like a Kaos).
Anyway, on the second flight of the day, I suddenly had loss of pitch control, although throttle and ailerons still worked OK. Long story short, I was unable to land and the model hit the ground hard.
After investigation I found that the 2300 5 cell NiMh rx pack, whilst showing good voltage, was unable to output enough current to power the (digital) rudder and elevator servos, which promptly stopped working. The aileron servos were standard Futaba analogue type, which seem to have a lower voltage / current requirement which is why I had roll control.
The upshot is that I have thrown out all of my NiMh rx packs, at least for airborne use, and replaced them with LiFePo4 2 cell packs. The difference in speed of the servos is noticeable (on the ground, anyway).

 

Really, just thought that it might prevent someone 'losing' a model unnecessarily. I'd never considered that a digital servo would freeze in the way that these did when the voltage dropped.

Kim

[RottenRow]

What was the voltage range of the digital servos?

If they were ‘normal’ 4.8 to 6V servos I am surprised that they would have stopped working completely whilst the others (and the receiver) carried on apparently normally.

Some digital servos are rated at higher voltages (7.2V and above) and don’t work at 4.8 or 6V, or might do until the ‘edge’ has gone off of the battery charge. Some manufacturers make versions of some of their servos in both voltage ranges.

 

Brian.

[Paul Marsh]

18 hours ago, Phil Green said:

Paul I'm afraid I disagree with most of that  🙂

 

Phil

Well, I'll do it my way,  nothing wrong with having extra and servos are quicker and digital servos run best on 6v if they are designed to. 

Also I use Opto switches in some models - especially larger or more power hungry and have to use 6v or 7.4 v Life/Lipo anyway as the Powerbox one I have if using 4.8v sees it as a flat battery - the lights are red.

 

 

 

[2.4g Shaun]

28 minutes ago, Paul Marsh said:

Hi Paul,

Reading through what Phil said was, there isn't any advantage running an Rx on a separate 5 cell pack not you shouldn't  use 5 cells. When using a powerbox or 6v - 7.2 V servos it's a different scenario. The point being made is an Rx contributes two tenths of diddly squat to the draw on a flight pack;it's virtually all down to the servos.

 

[Kim Taylor]

2 hours ago, RottenRow said:

What was the voltage range of the digital servos?

If they were ‘normal’ 4.8 to 6V servos I am surprised that they would have stopped working completely whilst the others (and the receiver) carried on apparently normally.

Some digital servos are rated at higher voltages (7.2V and above) and don’t work at 4.8 or 6V, or might do until the ‘edge’ has gone off of the battery charge. Some manufacturers make versions of some of their servos in both voltage ranges.

 

Brian.

They were 'standard' servos, not hv types.

They were obviously just particularly badly affected by low voltage under load. I seem to recall that the voltage was dropping as low as 3.5V on bench test immediately after the crash, when I was giving the sticks a good stir.

 

Eta because of the problem on that plane, I tested another which had a similar power supply, and found the same issue, with digital servos from a different manufacturer (admittedly before charging the battery).

This convinced me to  change the battery chemistry going forward. 

 

Edited August 3 by Kim Taylor

[Phil Green]

Sorry to harp on but can I try another way to put this? 🙂  lets take Kims post above as an example, a nominal 4.8v pack drops which to 3.5v under load, a sag of 1.3 volts or .325v per cell.  Assuming the switch and wiring is adequate, and the battery isnt flat, this is entirely caused by a high pack ESR.

How to fix that sag?

By adding an identical fifth cell, for the same load that sag becomes 1.625 volts, which from a nominal 6v leaves 4.375 volts. 

Now whilst 4.375 volts is ample for the receiver, and, compared to 3.5v may appear to be a 'cure' - its not, far from it! 🙂.

Keeping to NiMh for the sake of the 4/5 cell discussion, we have two proposals:

A 4.8v nominal, adequately specced, low ESR pack which will hold its voltage under load with little variation

or

A 6v nominal pack with a demonstrably higher ESR which constantly shoots up & down throughout the flight between 6v nominal to 4.375 volts, never presenting a constant voltage, and which at its lowest point is way lower than the 4.8v pack. 

Further, this voltage variation happens at all frequencies from DC to the servo motor PWM rate and appears as 1.625v of electrical noise on the receiver bus, a Bad Thing 🙂

 

Ok, its a much-simplified example - some parts of the flight electronics (resistive) take more current if fed a higher voltage (ie going  4.8  -->  6v) and some parts (switchmode, PWM) take the same power therefore less current at the higher voltage. For the sake of the example, lets say these cancel and the overall current stays about the same.  We've also assumed the 4 & 5 cell packs SoC are at their nominal voltage - but hopefully it shows how a power supply problem would be approached professionally, ie by increasing its current capability by reducing the source ESR 🙂

Again, sorry to harp on.

Cheers

Phil

[RottenRow]

I agree with all of the above Phil, except that Kim’s example was for a 5 cell pack already (6V nominal but voltage should have been somewhat above that).

This means for a 5 cell pack to drop to 3.5V on the load of four or five servos shows that they must have either been of particularly high ESR, almost discharged, or the switch (or possibly connectors) had a high resistance (which would have had the same effect as a high ESR battery pack).

 

Brian.

[Nigel R]

On 02/08/2024 at 14:44, GaryW said:

Even my lattest aquisition being an 102" DB Centurion weighing in at 6.5kg all up is only running on 4.8v Rx Power although there are 2 RX Pack running in parallel as the servos are slightly higher torque than say a futaba 3001 ( pics below )

 

I'd recommend you don't put two nimhs in parallel - the chemistry / characteristics are not properly suitable for it. If you need the capacity / current capability, use a good sub-C pack, or move over to a lithium pack of some sort.

 

Specifically on the Centurian, a slow flying thing like that, large though it is at 14lb, isn't likely to put huge demands on the servos... how many servos does it use on loaded flight controls? It only has rudder/elevator IIRC.

[Kim Taylor]

5 hours ago, RottenRow said:

I agree with all of the above Phil, except that Kim’s example was for a 5 cell pack already (6V nominal but voltage should have been somewhat above that).

This means for a 5 cell pack to drop to 3.5V on the load of four or five servos shows that they must have either been of particularly high ESR, almost discharged, or the switch (or possibly connectors) had a high resistance (which would have had the same effect as a high ESR battery pack).

 

Brian.

Yes, Brian you're right on points 1 and 2.

The pack was getting on a bit, but I thought I'd charged it up fully the previous evening. What I now suspect is that my charger sensed a false peak and shut off well before the pack was fully charged. On reflection at the time, alarm bells should have rung as the charging time was unusually short. This was the point where I could (should) have twigged that something was wrong. 

I realise that my issue was not as per the OP, but just wanted to point out that it's not always a rx brown out that brings a model down.

Sorry if I've derailed the discussion. 

Kim 

[Nigel R]

Still talking batteries, don't think it's been derailed Kim.

 

We've highlighted a key point, batteries can and do degrade over time, and with age become weaker and weaker, to the extent even that a regular charging regime cannot even fill them up.

 

Reinforces the point - the RX battery is a critical component - check its health regularly. It is easily done nowadays with our hobby multi-chargers, they can run a discharge/charge cycle in an hour or thereabouts, and most of all - tell you how much went back in, the key indicator of cell health.