Peter Beeney

  Myron,      I thought that as you had invited comments on your post I’d have a little bash at trying to answer. Particularly as this morning (Sunday) I was given 4 battery packs to test and report on. I’ve been testing batteries for about the last many, many years!     Taking your points one at the time:          1)    Unfortunately your information is a bit sparse. Is it a 4 or 5 cell pack?  Is the 250mA a guess or a measured current? What is the capacity on the label? Is it  the 600mAh that you mention later? Regarding the 70mA charger output, this can vary. I’ve tested a few in the past, but they are becoming a bit out of vogue now. The earlier ones, 50mA, always seemed to be fairly accurate but the later ones can be unreliable. They are supposed to be constant current devices, when the battery is flat they will supply better than 70mA but as soon as the battery voltage comes up, which it does very quickly, the current can drop off to as low as 40 or even 30mA. Can I suggest the only way to determine this is to measure it carefully.            2)    Here you seemed to have flipped to the transmitter pack. However, it does appear from the figures you may have been undercharging. 4 hours sounds a nice likely number. Just to recap, lithium batteries apart, if you charge at 0.1C for 10 hours plus 40%, another 4 hours, (the telecoms outfit that I worked for insisted on a 60% overcharge), then you will have guaranteed to have put the requisite charge in but of course that doesn't mean you will get 1C out. If you do, within some well defined parameters, then the battery is in good nick. You certainly would not do any harm to try 24 hours at that low rate.          3)    1.5 hours discharge time sounds slightly iffy. BUT, if you are discharging at 400mA, using a constant power, (constant current), discharge load, on a 600mAh pack then you are spot on. We need more info. please.            4)     I’m not familiar with the ‘Wattage meter’ or even watt meters (for modellers) in general. I am tempted to buy one though, I have some vague questions about these. In particular, the inline impedance interests me                                                                                                                                                                                                                    The figures, as you say, don’t add up, but without more information on the terms of discharge any other comment would be just speculation.  To try and establish the in-flight current drain, why not give the model a fair number of flights, using a good, known capacity pack, clock everything so that you know the total time elapsed, then discharge the battery, noting the remaining number of milliampere hours. Don’t recharge, that could give you a false reading. Subtract this number from the full capacity, and divide that figure by the time. That will give you a basic average drain.                                                                 An example. You have a 1000mAh battery. You fly for a total of 30 minutes, or 0.5 hour. Then your subsequent discharge is 500mAh measured, so 500mAh used. 500 divided by 0.5 equals 1000mAh, or 1Ah. Thus your average discharge rate is 1 Amp per hour. In reality, I think it would be a lot less than this, rx packs in ordinary fixed-wing models generally have the Life of Old Riley! Permanent holiday!             Regarding the 3y old Hitec packs, I have a colleague with a JR transmitter that still uses the original Yuasa pack, 27 years on. It still passes my rigid test. Also once upon a time I bought a scrap string of 20 4Ah D size Saft NiCads at a boot fair. For 50 pence. The lot. They were ex. security use. I gave most away, keeping 4. I used those for around 15 years as glow drivers, it’s only just very recently that I’ve finally pensioned the last one off. At one point I was getting seriously worried they would see me off!

And again, please.

My entry, please.

Another Way.  In the dim and distant past, long before the likes of things like UBECs and Opto isolators, I, too, asked myself this question, why should I cut the red wire? I thought about it for a short time and then decided I didn’t need to. So I never have. The reasons and explanations for this procedure have always been very thin on the ground, and with the greatest respect for the writers of the one’s that I have seen, I have never been very convinced.  Just this very weekend, I’ve purchased four small electric motors and standard speed controllers for a model and I shall not have any problems in simply connecting them all straight to the receiver. In fact, I shall consider it to be beneficial to the set-up. The above appears to be a very casual statement, but it’s not entirely without some proof of evidence. At least, to me personally. If anyone’s interested, I could write a slightly more in-depth explanation, which would not be very technical, because I’m not very technical. I just poke and hope and I’m a great believer in the keeping it simple ideology.  As always, this is my lateral thinking again, and should not be considered to be any advice or instructions of any sort.    Pete.

 Ivor, Perhaps you should take the reciprocal of the last few words and say “The more I know about this caper, the fewer problems I shall have.” Although it’s best to probably not bank on that too much! At least, not in the beginning anyway. I don’t think it’s very educational, it’s not really relevant to any thing very much. I believe there are threads on this forum aimed at beginners, etc. I’m sure someone can point you in the right direction. Good Luck! Pete.

Is this Real, or what? Gentlemen, now I’m confused again.   At great personal risk, can we go back to the impedance bit, the expression impedance = resistance + reactance. I would say that we can satisfactorily conclude that this expression relates to an AC circuit, containing resistance, capacitance, inductance, and is also influenced by the applied frequency.  I think the reciprocal can be shown to be admittance = conductance + susceptance, but not by using the product over sum formula. The impedance expression is, I think, a complex quality, the resistance part is a real number, with the reactance part containing a j-operator, which makes it an imaginary number. So we cannot multiply the resistance by the reactance. The admittance expression is also likewise a complex number, with the same j-operator in the susceptance part. So, likewise we cannot multiply the conductance by the susceptance.  Also both the resistance part and the reactance part can be shown, (and conductance) + (susceptance), at least theoretically, to be equal to zero, so that again gives us problems. I would consider too, that the product over sum only holds good for two parallel resistors, after that it simply becomes 1 / 1/r1 + 1/r2 + 1/r3…... 1/rN I’m still hoping the drift here is again also imaginary, Ohm’s Law must still pop up in here somewhere, surely?  Or is that Hall’s Effect? Or even shades of Aeromodellers Nightmare perhaps? Perplexed Pete.   PS. Please don’t let this sort of rubbish put anyone off electric flight. This is all throwaway stuff! However, perhaps the more you know about the basic general principles, the easier it becomes to sort out the many problems.  

I’m beginning to think that I’m understanding some of this, nearly. Re: the drift, I think that one of my lad-about-town friends summed it up very succinctly in a recent conversation at the field. He remarked how he occasionally enjoyed a pleasant evening with some sparkling wine, some scintillating conversation in convivial company and then a nice early night to bed. ‘Cos he had to get up early in the morning. “Oh’ said I “ You get up early and go to work?” “Oh no,” he replied “ I gets up early an’ goes Ohm!” Apologies for the very old joke and I think I’ve drifted so far I’m right off tune. It’s stopped raining so I’m off to the field. Pete. 

Thank you gentlemen, I can now sleep easy. I’ve never been into speakers, r/c has been my downfall. The way I imagine it, the DC resistance should be as low as possible. The swings as high as possible, 24v in today’s mega-watt cacophony? If so, and 24 volts DC was applied without any current limit, then I would tend to think that Rob Cope would be on the button. That should have Jonnie Ray, not Jonnine. (I think!)  Sorry. But it was the 1950’s and that’s my excuse. And it’s not stopped Raining since. Hopefully this post is still obeying Ohm’s Law? Pete.   

Yet another 6d worth I seem to remember little diagrams of amplifier circuits with a capacitor in series with the speaker? To block the DC component? And the ohmic value was derived from the impedance at a 1kHz  test frequency? This was a long time ago, plus it was only toy stuff!  Jonnine Ray was  Just Walkin’ in the Rain then. Perhaps things are much different now, I shall very pleased to find out how much. Pete.

Peter, Many thanks for that.   I will give it some thought.  Just not quite sure when. Seriously though, I think the speed controller must be quite a hard working little piece of kit, and clever too, at least when it was first designed. I think that what you are saying in there somewhere is that my theory about pulse width is sort of correct. The term ratio ‘on’ to ‘off’ sounds familiar. Perhaps not entirely convinced about the RMS values. As I remember, the RMS value is the effective DC equivalent value of a sine wave, can’t remember other wave forms. Although of course, if we are getting the equivalent DC power out it must be the same, sorry I spoke.  It just might not be 0.707Vmax though? I will (and read that as a may!) try and find some reading matter on the subject. Again, thanks. Pete.  

Peter Rieden, Thank you for your elegant explanations. Largely over my head I’m afraid but interesting none the less. I, too, have a question. I’ve always vaguely known that these motors use chopped DC (I’ve always called it pulsed) to create a revolving magnetic field which carries the rotor around with it. Also that the controller senses the transient emf’s to work out the position of the field in relation to the rotor. But I’ve also always assumed that the speed is controlled by the width of these pulses, i.e. the amplitude stays constant but the duration of the pulse varies. If, as you say, the voltage controls the speed does the amplitude vary and the width remain constant? I’m intrigued!   Pete.  

Steve, The trick with the cheesy ribs is to leave the rind on. (Does Gorgonzola have rind?) Then use a hot-glue gun with cheese sticks. When I use cheese the mice come in and eat the middle out, thus adding lightness. Very convenient. I have an old HEME AC/DC clamp meter, a relic from work, a long time ago. Apart from reading amps, it also reads volts, hertz, VAs, watts and gives you the phase angle. And you can display it on a ‘scope. If I ever get sufficiently motivated, I might just have a play around with this. I might get, in my case anyway, some sort of highly misleading clue as to what is actually going on. I have thought about this before, in relation to other slight doubts that I’ve had but as I don’t want to start any more frenetic debates I’ll say no more. But that is a big if!  I’ve only just bought this laptop and sometimes I have great difficulty in getting to grips with this, let alone a USB oscilloscope! Pete.

Sorry about the power factor letter folks, it was only intended as a flippant throwaway. I, too, hadn't read the letter from Kevin Annells at that point but now I've had a ganders it does seem a trifle misleading. If it is related to some sort of constant-power application it's a bit difficult to see what it is, particularly when applied to model aeroplanes, and as far as I'm concerned, unnecessary when I'm sorting out power requirements . If not, then it would certainly appear to stand Ohm's Law on it's head. Would it not be possible to invite Kevin to expand his original letter, and answer the comments, perhaps on the forum even. Then we might be able to see where he is coming from. Or not. Pete.

 David, I haven't seen the original letter yet, but perhaps there is something missing? Back in the time of 'Galloping Ghosts', one of my friends and work colleagues always considered that Ohms Law always occurred at around ten to five in the afternoon. Regular. Even a shade earlier if we were lucky. Could we not we insert this variable into the equation, on the 'watt if'  principle, just to see what happens?   Pete.

Another 6d worth. I've always assumed watts to be derived from V times A times p.f. Where p. f. = Power Factor. I would think that generally within a DC circuit the p.f. is very close to one, and thus is insignificant.  However, as soon as we talk about electric motors we start to introduce an element of inductance and capacitance. Now we're talking VI's, (volt amps) as opposed to watts. Does this unbalance the applecart at all? Just thought I'd mention it.   Pete.

   My entry, please.

   My entry here too!

Please count me in, too!

My entry please. 

Please count me in, too!