Powering a 35Mhz Rx with Lipo Batteries

[Steve Hargreaves - Moderator]

Being very keen on the benefits of Lipo batteries (light weight, no self discharge etc) I wanted to convert my fleet to Lipos when replacing my NiMH Rx batteries.

 

I'm due to replace my batteries next year & decided to get myself an suitable 2S battery & regulator & see how it all worked out. I bought a 2100mAH 2S Lipo & a regulator (this one) & installed them in my hack.......(Futaba radio with a PCM Rx)

 

All worked fine on the  bench but when I got to the field & did a range check the radio went haywire feezing & locking me out when I was a few feet from the model. Raising the aerial a bit improved things until I got a few more feet away when the same thing happened. It seemed that the unit was radiating RF interference which soon overcame my Tx signal. Admittedly the UBEC was mounted pretty close to the Rx in this installation.....

 

Needless to say I didn't fly the model but replaced the LiPo set up with an NiMH battery & all was well.....normal service was resumed.

 

So, what to do? Are these (switching) regulators simply no good in conjunction with 35MHz? Would a ferrite ring on the input lead help? Wrap the thing in foil??

 

Has anyone had success in using LiPos with 35MHz? I know people will say "...switch to 2.4GHz..." & I agree that is probably the best solution but I have a lot of 35MHz gear that I have no intention of dumping just yet....am I doomed to 20th Century batteries to go with my 20th Century radio....

 

[ken anderson.]

hello steve-timbo has been using lipo's for rx's for a while i recall he has posted a few time's on the +/- of doing so......send up a flare he'll answer a fair few question's...or direct you to his previous post's.... 

 

        ken anderson...ne..1.

[Steve Hargreaves - Moderator]

Thanks Ken.....as a keen diciple of St Timbo (For His is The Way, The Truth & The Light) I know he is a great prophet of the Rx LiPo faith.....alas he is also one who follows the 2.4GHz path to righteousness leaving us 35MHz sinners to repent of our glitches & other RF plagues.

 

Yea & verily for it is written that we heretics are left to walk in the Valley of Death until Judgement Day (or until we can save up enough money for a shed load of Futaba Rxs!!!!)

 

[John Privett]

Steve, whilst ambling along the Valley of Death,  have you tried moving the regulator further away from the Rx and aerial to see if that improves things?

[Chris Bott - Moderator]

Steve quite a few people are saying great things about Sanyo Eneloop AA cells and also Vapextech Instants. Both of these are low self discharge NiMh technologies so they hold their charge for a long time even if they are not used. 

 

Still NiMhs so no weight saving, but then you don't need a regulator.

 

Try a Google for them. Quite a few model outlets are selling these as Tx and Rx packs now.

[Martin Harris - Moderator]

A couple of clubmates tried switching regulators on 35 MHz gear a couple of years ago - both experienced major glitching problems. I don't think either was able to resolve them before switching to 2.4 GHz...

Personally, I'm an advocate of keeping safety critical items simple - and a multitude of components in a switching regulator, efficient though they may be when all is running sweetly, is a minefield of components itching to release the magic smoke, fracture their spindly legs and joints waiting to go dry...

I just wish we could find a way to get hold of good quality, reasonably priced NiCds (maybe like the exemption for leaded solder in safety critical applications?) but it does seem that Eneloops are a possible "best of a poor bunch" solution. 

 

 

Has anyone taken note of the warning against the use of high capacity NiMHs in the latest edition of the BMFA handbook?  It makes interesting reading.

[Steve Hargreaves - Moderator]

Thanks Chris....the Vapex Instants & Sanyo Eneloop are my "Plan B"....I bought a load of Vapex Instant AA cells for powering the kids toys a while back & they have worked very well. You know how powered toy cars & things eat dry batteries...so I wanted to use rechargeables but then I thought they would go flat between "plays" meaning my boys would get the toy out & it wouldn't work because the batteries were flat....

 

As my wife frequently points out the only difference is that my toys are a bit bigger & cost a lot more.....!!

 

Martin....yeah I know what you mean but electronics are pretty reliable these days & SMT has got rid of "spindly legged components". I also fly a fair few electric models c/w ESCs which use switched mode BECs, again on 35MHz, with no glitches or other issues. I don't disagree with you though & maybe theres a lot to be said for the KISS principle.....

 

Anyone want to buy a 2S LiPo & regulator......hardly used & never flown....works great on 35MHz gear........

Edited By Steve Hargreaves on 05/07/2010 17:13:54

[ken anderson.]

Posted by Steve Hargreaves on 05/07/2010 15:44:25:

Thanks Ken.....as a keen diciple of St Timbo (For His is The Way, The Truth & The Light) I know he is a great prophet of the Rx LiPo faith.....alas he is also one who follows the 2.4GHz path to righteousness leaving us 35MHz sinners to repent of our glitches & other RF plagues.

 

Yea & verily for it is written that we heretics are left to walk in the Valley of Death until Judgement Day (or until we can save up enough money for a shed load of Futaba Rxs!!!!)

 

 

steve-you have missed your vocation in life......for the above is EX... 'ssssss

 

    ken anderson...ne...1...

 

[Klippy]

Have you tried making a regulater Steve, an IC, some Vero board and 4 bits of wire. A 7805 should do the job.

[Tim Mackey]

I will second the Vapex instants /sanyo eneloops.

Now, Steve - have you considered the third and potentially excellent option?

2 x A123 cells in series - no regulator required and pretty much all the advantages of lipo.

A123 manufacturing do two sizes of these LiFe PO4 cells and I find the smaller 1100 mahr version is just fine for onboard radio gear.

If you  want a couple let me know .....I have several here.

Edited By Tim Mackey - Administrator on 05/07/2010 19:41:10

[Frank Skilbeck]

I'm using one of these off a 4s lipo with a MPX 35 MHz IPD Rx driving 6 servos and an opto ESC from a Cockpit Sx Tx, been flying this combo for over 12 months with not so much as a twitch. UBEC is approx 2" away from the RX but the aeriel runs down the fuselage away from the UBEC.

 

But as above the other alternative if you want low discharge batteries would be a 2s LiFe battery which has the same voltage as a 5 cell NimH pack.

[Chris Bott - Moderator]

A123's are a great solution, pretty much Nicad style bomb proof, as long as you have a suitable charger and that all your radio components will stand the 2x3.6V = 7.2V that they contain straight off the charger. Some items, (some servos) are not good with this voltage.

[Steve Hargreaves - Moderator]

I hadn't considered the Li-Ion route I must admit......one reason is that my charger doesn't have an option for charging these ....I am also a bit concerned that the voltage might be a tad high...I understand they are rated at 3.3 volts per cell....6.6volts sounds a little high to me...I know that a 4 cell NiMH fresh off the charger can show over 6 volts but it drops back to under 6 volts pretty quickly. That said lots of people use 5 cell packs so maybe it is a solution.

 

Also I think 1100mAH might be a bit low...especially given the higher voltage will increase the current drain.....could always use a 2S2P pack though I suppose.

 

Interestingly I tried John P's suggestion to move the regulator away from the Rx.....it made no difference I got about 6 feet (aerial down) away & the glitching started. "Right" I thought "I'll sort you out..." so I rounded up all the servo extension leads I could find & moved the battery about 6 feet from the model.......& do you know what? It made absolutely no difference whatsoever!!!!!

 

Being a bear of very little brain that means to me that the RF interference must be coming from the UBEC down the lead into the Rx.....methinks a ferrite ring or two might just make a difference (most of my ESCs with switched BECs have these on the Rx lead) so I've ordered a few from GC....more out of personal interest than anything else.

 

I'll post back & let you know how I get on...

[Tim Mackey]

The other cell they do is 2300 mahr.

Funny how w scoff these days at low capacity cells of "just" 1100mahr, when 20 years back we would fly most sunday hack type models all afternoon on a 600mah nicd.

Some servos may object to the higher initial voltage, but this does drop a little under load of course - I cant guarantee things of course - but can say that I have experienced no problems so far in the models I have used them in. A variety of servos including hitecs, spektrum digis, and a couple of ancient S148s in a sloper wing.

[Steve Hargreaves - Moderator]

You're absolutely right Timbo but things are comparative I suppose...at the side of a typical AA NiMH cell of around 2000mAh, 1100mAh doesn't seem very much.

 

Mind you the typical model we used to fly with a 600mAh battery was probably a 3 channel R/E model with control surfaces acting in a purely advisory role....with todays models fitted with 5 or more servos, much bigger control surfaces to bully the aeroplane around the sky & a pilot willing to put the sticks in the corners I doubt those 600mAh cells would last very long

[Tim Mackey]

Yes I accept the point about todays larger and more powerful models compared to yesteryear, however it never fails to amuse me how many flyers of similar sport models today such as the evergreen wot4 still wont fit anything less than a small nuclear powerplant onboard to power the thing. I owned and flew many a Wot4 and Acrowot on simple 600ma AA cells, and indeed, many considerably larger models too !

PS I am as guilty as the rest of them BTW 

However this has much to do with the fact that most of todays flight batteries are widely available in comparatively huge capacities....and cost far less in real terms than the old 600s !

Anyhoooooo....I will be self moderating for topic drift if I dont watch out

Edited By Tim Mackey - Administrator on 05/07/2010 23:11:22

[Martin Harris - Moderator]

A minimal risk solution to dropping the voltage slightly on A123s (or any type) is to run a diode in series which will drop 0.7V - only introduces 3 potential points of failure (one component and two joints,,,

 

Of course, as "larger and more powerful models"have been mentioned, it's worth considering running a second pack in parallel.

 

Steve, it looks like you're considering a 2000mA/h AA sized NiMH as the norm - this is what the BMFA have to say in the latest handbook - and bears out my personal experience with NiMHs:

 

(e) A noticeable feature of Ni-Mh technology has
been the increasing capacity of the cells for any
given cell size. For instance, the early AA pencells
were rated at around 700 mAH but you now see
capacities of around 2000 mAH for the same cell
size.
The only way this extra capacity can be achieved is
by increasing the surface area of the active
components within the cell and, for a given size of
casing, this can only be done by making these
components thinner. The problems that this will give
you are increased internal resistance (the cell won’t
give it’s energy up as easily and may get hot) and
increased fragility of the cell. Thinner materials can
be damaged more easily, both electrically when
charging or discharging and mechanically, for
instance, due to overheating when soldering or
being over-stressed in a crash.
These problems may not be apparent in your
transmitter pack but you should think carefully about
using very high capacity Ni-Mh cells in airborne
packs where the demand on the batteries will
fluctuate and can be much higher than in a
transmitter. You can easily get into a situation where
a high capacity pack is unable to supply the voltage
required by some hard working servos simply
because the internal resistance of the cells will not
let the energy stored in them be released quickly

enough.

 

On top of this, I would also consider damage due to vibration to be an increased possibility.

 

 

Edited By Martin Harris on 06/07/2010 09:37:35

[Steve Hargreaves - Moderator]

Thanks Martin ....yes I've heard this sort of thing before. Battery manufacturers get into a sort of capacity war where "Mine's bigger than yours" seems to be the goal. As you say the only way to achieve this is through greater surface area which can only mean that the components have to become thinner if they are to fit in the same size package. This inevitably means they are less robust & may become susceptible to the vibration inherent in our models. That said, finding an AA cell with much less than 2000mAh capacity isn't easy!!! According to Vapex their 2100mAh "Instant" cell is rated at 6.3A max discharge so 3C.......I wonder what the voltage will be at that current !!!!

 

With regard to model size then I'm really a 40-60 sized man at heart...my shed isn't really large enough for anything much bigger. My biggest/most powerful model is an enlarged Wot 4 (Wot 5?) with a 90 2 stroke up front......it is this model the LiPo & regulator were destined for as the regulator is rated to 5A which should be more than enough....even in "full attack" mode !!!!

 

I take the point though & anything larger than this would get a 4/5 C cell pack installed as these will naturally flow more current.

 

I do like the idea of fitting a forward diode to an A123 pack though.....voltage drop & polarity protection in one easy package....

[Former Member]

[This posting has been removed]

[Peter Beeney]

   Steve.
        When the very first lipos appeared on the scene, as single cells, 650 or 850 mAh as I recall, I used a couple to make a 8V pack and ran the radio with a 1A, 6V linear regulator. This was in a 5 servo SPAD, (Simple Plastic Aeroplane Design), with a single conversion PPM rx. I really gave it some wellie, both on the ground first and then in the air, carefully monitoring the temperature of the reg., the critical bit, and it was absolutely fine, no problems at all. The secret is the low voltage differential, at best you are probably only ever going to dissipate a couple of watts across the device. This was actually a test of the lipos, and as it so happened one suddenly completely and inextricably failed, fortunately overnight, which didn’t impress me much. But, as I said, they were some of the first, I think they’ve improved greatly since then!
   I simply wired mine in the switch harness, but as Tony is rightly implying, a small piece of Veroboard would certainly make a more robust and probably a neater job.
   I would have no hesitation in doing this again, even with a larger model. The range of 5V regulators is pretty comprehensive, I‘m not quite so sure about larger capacity 6V types, 2A and upwards say, but they must be around.
   I was also checking the ability of the cells in terms of flying time per charge and so on but it soon became apparent that I would tire of flying the model before the cells would tire of supplying the power. Also I was concerned about accidentally leaving it switched on, and thus flattening the battery, so I wired in a 1Hz flashing led, fixed in the top of the fuz, which came on when when I switched on. It wasn’t very bright in the sunlight, but it did work in anger once, when I spotted it ticking away in the back of the car in the dusk.
   I have tried to literally ruin regulators, and other such components, just to see what they will take, but short of giving them a sharp tap with a small club hammer they refuse to give in. However, they do shut down, to prevent self-destruction, which is not good news for the model. But that’s a long way down the road, I’ve never been able to get anywhere near it in practical model tests, such as when I’ve been trying to prove severe volt drops for brownouts etc. Something I’ve never managed to do, either, but that’s another story!

   Hope this is of some interest, at least. PB

[Brian Parker]

Just for interest.
 An easy and cheap way to reduce voltages ( at reasonably high current levels) is to use bridge rectifiers.

  15 and 20 amp bridge rectifier (15x15mm and 20x20mm) about £1 each.

 

  Connect in series to obtain the voltage required. ie. 5 in series will drop a 12volt (13.2volt) lead acid battery down to 6 volts. Bolt to a heatsink if using at max. rated current.

 

[Steve Hargreaves - Moderator]

Well my ferrite rings arrived from GC (cheers Rob!!) so I rushed to the workshop to fit them. I decided that since one turn through the ring is good then several turns must be better  I managed to get three turns of cable through & secured them to the ferrite ring with a spot of CA to keep everything nice & tight.

 

With trembling hands I plugged the regulator into the Rx & switched on........it made absolutely no difference  !!!! Well that's not totally true actually.....I did manage to get slightly further away before the glitching started.....

 

Just to complete the experiment I swapped the (PCM) Rx out for a Dual Conversion PPM unit.......that made it even worse!!!

 

Finally & again out of interest to see how the interference was being caused I tried connecting the Rx to a standard NiMH battery for power & laid the regulator (still connected to the 2S Lipo) on top of the Rx. My thinking here was to see if the interference was being radiated (in which case the above situation should still have caused problems) or if it was being fed down the wires from the regulator to the Rx. The radio worked perfectly in this case so I can only conclude that the regulator is suppling a steady 5VDC with a horrendous amount of noise over the top of it!!! Oh for a 'scope to see what is going on!!!!

 

Ah well........Eneloops it is then!!!!

[Brian Parker]

Steve,
How deep are the ferrite chokes,

GC only list the inner and outer diameter.
The critical factor is the depth.

 Inductance is proportional to the length of the turns through the ring (ie the depth of the ring) and also the square of the number of turns.

 I suggest (respectfully) that you need to increase the number of turns and perhaps also use larger rings.

 The aim is to increase the series impedance and thusblock the unwanted common mode currents.

 Basically the more turns the better, you can also stack the rings to increase depth.

[Steve Hargreaves - Moderator]

Interesting thought Brian, thanks for that. I thought that just stacking the rings would make very little difference because I wouldn't be able to bond the inductors electrically hence each would just behave as a separate ring.

 

I will try what you suggest & use two or three rings.

 

They are about 5mm deep by the way....

[Peter Beeney]

  Steve,
      A couple of small points, if I may. Are you saying that the interference was actually causing the servos connected to the PCM rx to glitch? In other words, to move about randomly? If this is the case, it’s certainly very unusual, it’s not the standard PCM operating procedure. Or is it pointing to the interference being induced on the servo wires, perhaps? And that seems to be as equally baffling!
   Also your little check with the regulator lying on top of the rx, was it’s output at that point open circuit? If so, would it be an idea to put varying loads on it, from say half an amp up to three amps? Otherwise I’d have thought that it was virtually shut down. It seems likely to me that any radiation emitted might be somewhat proportional to load.
   I’d have thought the interference would enter the rx via the aerial. I’ve always thought that the power input on 35Mhz is pretty secure. I’ve never seen or heard of any power supply incidents. And the little coil in the switching regulator might be acting as a little very short range transmitter. If this is the case, then with a reasonable load on it, I’m thinking that putting the regulator on top of the rx, or very near to the aerial, might get you a result.     PB