4.8 v or 6 v receiver battery

[Ian Moody]

Jonathan, Don't worry about it . You will be fine with the pack you have as long as it is charged before flying which is a good routine to always do. There will me minimal risk of any failure. Another good thing will to check the battery with a battery checker before going flying for the day.

Have fun which is what it is all about.

[Martin Harris - Moderator]

Posted by Martin McIntosh on 10/02/2018 17:21:37:

Sorry, but some of you have just got the wrong end of the stick here. I have never heard of a cell failing open circuit, otherwise why would there still be 3.6V available? You can check as much as you like on the ground but if a cell is going to die you can bet your boots that it will happen in the air.

Good point. Examination of older packs often shows corrosion around the cell links so there's a reasonable argument that the more cells, the more chance of a catastrophic failure from deterioration or faulty welding...

My own tests on input voltages have confirmed that my servos stop moving well before the radio link is lost but not all systems are equal, so if yours is designed for greater than 5 volt supplies (ESC BECs and 5 cell packs rather than 4 cell NiXX packs) and won't handle a wide range of input voltages then go with an alternative power source.

The switch/wiring is probably the most likely point of failure though - and a large factor in my solution of choice for any larger or more valuable models which is to use paralleled batteries and switch harnesses.

[stu knowles]

My 4 and 5 cell low discharge AA packs, mainly Vapextec work OK and show a healthy voltage on most checkers. When checked with the Futaba battery checker which checks under a 1 amp load a battery showing 5.1 volt - no load drops to 3.3 under load.

When I eventually decided that it wasn't duff checker I stopped using AA size cells altogether.

Sub C cells are fine or I use LiFe depending on the space available and the need for ballast.

On the other hand, I am sure that there are hundreds of not thousands of models being flown with AA Eneloops.

You pay your money and makes your choice in this game.

[Jonathan M]

Posted by Ian Moody on 10/02/2018 19:56:05:

Jonathan, Don't worry about it . You will be fine with the pack you have as long as it is charged before flying which is a good routine to always do. There will me minimal risk of any failure. Another good thing will to check the battery with a battery checker before going flying for the day.

Have fun which is what it is all about.

Ian, I'll choose your answer!

Interesting thing about battery-checkers, of which I've got two. The first applies a 0.3A load and still shows fully-charged even after a some use. The second applies a 0.5A load which, at the identical battery state, will show a drop. Needless to say I only the second checker!

[brokenenglish]

Posted by Martin McIntosh on 10/02/2018 17:21:37:

People on here are still talking of NiCads. I believe that these have not even been manufactured for several years due to the toxicity so if you are still using them then it is high time for the (recycle) bin. Ever heard of a turbine or LMA type model flying with them?

Martin, this just shows the differences between us, and therefore the value of discussion.

I'm one of the people who've mentioned Nicads... So:

Firstly, Nicads are still available and, if you want them, you'll be able to obtain them (Tower Hobbies, for example).

Secondly, be aware that when we were forced to change from Nicads to NiMH, there are some applications that no longer worked acceptably (spark ignition systems). This taught me that NiMH are far "weaker" than NiCd. You just don't get the same power, and IMO this has been conveniently "forgotten". I therefore decided to stay with NiCd for my Rx power, for as long as possible. This "as long as possible" factor hasn't yet failed, i.e. NiCd are still available...

The reason that I was drawn into this discussion is that I wanted to learn whether some form of Lithium technology would offer a more powerful alternative to NiMH. If that's the case, I'm interested... If not, I'll stay with NiCd in preference to NiMH.

[Martin McIntosh]

Vapextec? Never heard of them but they sound like a high capacity and very low discharge current type to me, similar to those 2600`s, which incidentally do not like a charge rate of more than 100mA otherwise they will give a false peak. A wall wart charger is fine with them.

JM, you are probably fine with those until a cell fails. I used 4.8V for years until 2.4 came along. On 35mHz you would only notice the servos slowing down because the Rx`s were not so fussy about voltage but these days you need to have a rethink about the power system.

I have learned what I have said the hard way.

[Martin McIntosh]

brokenenglish, I see your point in that NiMh does not deliver the power of NiCad but an ignition system does not demand very much. I find NiMh`s to be OK on mine.

I would change back but suspect that any available now would be very old stock.

On a side note, I have a NiCad battery powered Dremel tool purchased in the US twenty years ago and the pack still delivers full voltage and capacity after all that time, despite being severely abused and overcharged by about three weeks since I tend to forget that it is on the charger!

[Brian Spearing]

To clear up any doubts about Rx battery voltage in flight, surely it makes sense to log the voltage and apply suitable voice warnings.

And for Hitec users, take a direct line from the Rx battery to the SPC socket (if that’s its name) on the Rx. And set a low voltage alarm too.

[onetenor]

I got some models from the widow of a deceased modeller's widow which had been hanging in disintegrating bin bags for 8 yrs. The batteries a were mainly NiCads .When checked they were still holding a fair charge. I drained them and recharged. Perfect charge and holding. And still charging to perfect another 5 yrs later Nimhs too. The boat's batteries were sub "C" for power AA for radio. The planes were AA and AAA packs

[Peter Christy]

You need to be a bit careful when measuring voltages on Rx and ignition packs, as there can be transient "spikes" of current draw that may pull the voltage down momentarily - too quickly to measure on a voltmeter.

To illustrate: a couple of years back, I replaced the ignition system on an old gasser helicopter of mine with an RxCel systems. The original system relied on a mechanical ignition advance and had not proved to be very reliable. Once the new ignition system was in place, I no longer had enough power to lift off! Same battery pack as before, only the ignition system had changed.

It took a lot of head scratching before I discovered the cause - the internal resistance of the NiMh battery pack!

Although the average current draw of the new system was quite low, it was drawing very short "peaks", which the battery couldn't cope with. I think the RxCel may be a capacitor discharge type, and the internal resistance of the battery was not allowing the capacitor to fully charge between firings. Substituting a sub-C pack for the original AAs provided a complete cure.

Lesson learned: Small NiMhs DO have a higher internal resistance, which may cause voltage drops on high current "spikes". It is not enough to consider the capacity of the battery in mAH to determine its suitability. You need to be sure it can deliver its rated voltage under load.

As I said in an earlier post, if the voltage of a 4-cell pack is dropping below 3.5 volts, it ain't suitable for the job! And that means transients, not just steady current!

Someone mentioned Vapextech cells. These appear to be similar to Eneloops, and have a very low self-discharge property. However, all these small AA or AAA cells are really only suitable for relatively constant current applications. They are fine in transmitters, and models using analogue or small digital servos. However, if you are using a lot of high power digital servos (think helicopters or complex scale models), then use sub-Cs.

It is not simply a matter of battery capacity!

--

Pete

[Bob Cotsford]

Posted by Brian Spearing on 10/02/2018 22:45:19:

To clear up any doubts about Rx battery voltage in flight, surely it makes sense to log the voltage and apply suitable voice warnings.

And for Hitec users, take a direct line from the Rx battery to the SPC socket (if that’s its name) on the Rx. And set a low voltage alarm too.

Unfortunately the vast majority of club modellers are likely to be flying basic radios without telemetry - at least that's the situation at my club! Half of those with telemetry capable radios don't know how to use it.

[Cuban8]

Posted by stu knowles on 10/02/2018 20:26:37:

My 4 and 5 cell low discharge AA packs, mainly Vapextec work OK and show a healthy voltage on most checkers. When checked with the Futaba battery checker which checks under a 1 amp load a battery showing 5.1 volt - no load drops to 3.3 under load.

 

Which bears out my worry about the pathetic C rating of some AA packs.

Unfortunately, this remains a much misunderstood subject and continues to get people into trouble. Not helped by the fact that nimh RX packs in general have no indication as to how much current they are able to safely source at their nominal rated voltage.. Heavy duty, high power, professional quality, and all the other meaningless titles that are used by manufacturers to describe their products, just adds to the confusion.

How much current is enough? Again not a simple question. Some time ago, I tested a fully stalled Futaba 3001 and compared it to a fully stalled three quid cheapy mini servo. IIRC at 4.8 V (from an adjustable current limited PSU) the 3001's stalled current was about 400mA. The cheapy took over twice the current despite being physically much smaller with nothing like the torque of the 3001. The cheap servo's running current was also higher than the 3001 with considerable current spikes, pulling the volts down at each change of direction.

Having four of those cheap servos in a 'plane with a low C, AA RX pack and you'd be asking for trouble IMHO even with four or five 'good'(!) cells.

 

Edited By Cuban8 on 11/02/2018 10:57:08

[Martin McIntosh]

You have got me thinking about the CDI on one of my models now Pete. It is a 50cc 1:5 Hurricane which I have had for many years but am now afraid to fly because of a possible power problem. On its 7th flight the motor quit a couple of seconds after take off but I got away with it. Tried to restart but it would only fire once each flick. I was using a 6V Eneloop pack which could have damaged the CDI so I bought a RcXel unit and fitted a 4.8V pack instead. Fine for quite a while and flew it at Greenacres which is several hundred feet above sea level. Shortly afterwards back at my local club site which is only about 110ft ASL it lacked power so much that I had to fly it almost out of sight before I had enough height to turn it. Checked the motor recently with fresh petrol and although it ran and sounded good the tacho was only showing 6500 on a 23x8 wood. I could almost swear that it previously did nearer 7500. Shall try again with a more powerful power supply of some sort. I could use my 18A variable mains driven one for testing. I have a storage `scope which will show up any large voltage drops.

[Peter Christy]

Hi, Martin - I'll bet you find that if you replace the eneloops with sub-Cs, all the performance will come back. My model was an old (very!) Hirobo Bell 47, with a 20cc "Echo" engine. The original Hirobo ignition used a magnet on the flywheel, and a pick-up coil on a moving arm that was driven by the throttle servo. It also had a separate HT coil in the cooling shroud. I never did think much of that mechanical advance mechanism, and one day, the LT wire to the coil (hidden from sight, so not checkable) fractured. This resulted in the engine stopping instantly and a fairly heavy landing!

I think the original system was just "transistor assisted" contacts - similar to an old-fashioned car or motor bike, but with the contact breaker replaced with a transistor. Whatever, it provided adequate power on four eneloops, but with the RxCel, I couldn't even get it off the ground! As you say, it sounded OK, and I spent hours fiddling with the timing trying to fix it. It was Steve Roberts from MacGregors who suggested a bigger battery pack, and sure enough, that provided an instant cure!

I think RxCel have now modified their ignitions to accept higher voltages (2S LiPos or LiFes) but mine is an early one and strictly 4-cell NiXX only!

If you don't have room for sub-Cs, try and find an old (but good!) 4-cell NiCad pack. NiCads have a lower internal resistance, and should hold up better.

And yes, a 'scope is the only way to accurately analyze the problem!

I'll be interested to know how you get on!

--

Pete

[Jonathan M]

Can I throw a question into the mix?

Everything else being equal with a 4.8v NiMh pack of 4xAA cells, if its capacity is 1000mAh will it have a lower internal resistance to an identical pack with a capacity of 2000mAh, and will it be more 'reliable'?

[Denis Watkins]

Posted by Jonathan M on 11/02/2018 16:39:11:

Can I throw a question into the mix?

Everything else being equal with a 4.8v NiMh pack of 4xAA cells, if its capacity is 1000mAh will it have a lower internal resistance to an identical pack with a capacity of 2000mAh, and will it be more 'reliable'?

Every part of chemical construction gives the chance of varied resistance readings of neither being a stronger bet.

It is just that, heads or tails, a mystery, if your pack may be higher internal resistance than your neighbours.

The 1000mah pack is mechanically stronger internally than the 2000mah

Due to its internal components being twice as thick, fitting inside the same space as the thinner 2000mah cells internal components and rolled up sheets

Edited By Denis Watkins on 11/02/2018 16:50:14

[Steve Hargreaves - Moderator]

The problem with AA cells is that the manufacturers have got involved in a capacity war & the mAh capacity of these cells has increased dramatically...from 500mAh in ye olden dayes to more than 2000mAh even up towards 3000mAh today. The only way they can do this is to pack more "surface area" into each cell. Clearly the battery itself hasn't got any bigger so to manage this they have made the plates thinner & the trade off here is a higher internal resistance for the cell. As others have observed these cells are fine in a constant low current drain application but ask them to deliver a larger current even for a short period of time & the terminal voltage collapses.

So, in answer to Jonathan M then yes a lower capacity cell should have lower internal resistance & be able to support higher currents if that is what you mean by "more reliable"....

[Peter Christy]

Hmm! That's a tricky one! The problem is with that phrase "everything else being equal"!

In my experience, lower capacity generally indicates lower internal resistance. Its been the race for more and more capacity in a given cell size (primarily for non-RC applications) that has resulted in the increased internal resistance.

However, battery technology marches ever onwards, and lower capacity cells may well be a) older and b) as a consequence of "a", of an earlier technology.

Eneloops (and the equivalent Vapextech) cells seem generally to have lower internal resistance than similar cells from other manufacturers, but I suspect that if you could get AA eneloops of 1500 mAH capacity (say), they would have a lower internal resistance than 2000s. My personal belief is that it is a mistake to go for the cells with the greatest capacity. I use sub-C cells in many applications not because I need the capacity, but because they offer lower internal resistance.

The cells are made by winding "plates" of material into a spiral to fit them into a cylindrical container. The capacity is a function of the surface area of the plates. If you can make the distance between the plates thinner, you can get more "winds" in, and hence greater capacity. But making them thinner makes them more susceptible to shorts, if physically damaged, and also appears to increase the internal resistance.

In principle, the answer to your question is "yes"! But life is never that simple, and everything else is rarely equal!

--

Pete

[Solly]

Like everyone else on this forum I can only speak from personal experience. I have used Spektrum radio since it's introduction into the UK with 4.8 volt rx batteries without any problems at all. Including in models with retracts and flaps. I check the batteries for capacity before each flying session and if I find one below par I try recycling it, then if that doesn't work I bin it. So I really don't see any need to change to 6 volts.

[john stones 1 - Moderator]

One simple reason is servos perform better, whether that improves our flying or enjoyment is another story.

[Jonathan M]

Thanks for the replies to my "everything else being equal" question.

There's an analogy with 12v lead-acid batteries used in small boats, where there are normally two independent circuits:

The engine starting battery would have a relatively limited capacity of say 70Ah but having thick plates can deliver high cold-cranking Amps for very short periods - much like a car.

The domestic battery bank would have a capacity of say 220Ah (two 110Ah batteries in parallel so still 12v) but each of these batteries would be constructed of many more, much thinner plates. Except in an emergency, it would be foolish to regularly use a domestic battery to start a cold diesel engine as the sudden colossal demands would distort the thin plates and shorten the battery's life. (Being lead-acid, the other thing to be wary of is regularly discharging below 50% capacity, as this also shortens life.)

[cymaz]

 

 

 

Sub c for ignition, though watch the voltage. I did read some reports that the Rxcel icu throws a fit now and again.

 

Edited By cymaz on 11/02/2018 17:48:52

[Geoff S]

Posted by Peter Christy on 11/02/2018 17:05:28:

The cells are made by winding "plates" of material into a spiral to fit them into a cylindrical container. The capacity is a function of the surface area of the plates. If you can make the distance between the plates thinner, you can get more "winds" in, and hence greater capacity. But making them thinner makes them more susceptible to shorts, if physically damaged, and also appears to increase the internal resistance.

In principle, the answer to your question is "yes"! But life is never that simple, and everything else is rarely equal!

--

Pete

In electrolytic capacitors (both aluminium and tantalum) capacity was increased by etching the plates to increase effective surface area. Perhaps a similar technique has been used for cells. How that affects the internal resistance, I wouldn't know but first thoughts would be that increased area should reduce resistance.

However, I agree with Peter that improved battery reliability is better served by using sub C's rather than using more AAs in series to increase the voltage and hence the tolerance for sag when extra current is drawn.

Geoff

[Tomcat999]

My Spektrum 4.8v Rx battery has 3 wires. The only replacement I found is 2 wire. Will 2 wire work okay, or do I need 3 wire???

[Mike Mc]

6V eneloops for the past 15 years. They last a very long time and are low self-discharge. Use to buy them but lately I have been making packs myself. Get the white ones not the black ones. 

 

Old and new Futaba radios. Futaba and Hitec servos.

 

Edited November 14, 2023 by Mike Mc