Nimh Mythbusters test

[Myron Beaumont]

Peter

I do like your two batteries with two switches idea .So simple & you can check each easily from the charging socket with a meter as & when you want .Only a switch can go wrong which you'd pick up anyway .Also,where the batteries are buried somewhere .you can use your "reserve" without dismantling anything .

KISS indeed ,now why didn't I think of it ?

[FunnyFlyer]

Lee - did you ever manage to do the tests you were planning to do?

Edited By FunnyFlyer on 11/01/2012 08:49:47

[Steve Hargreaves - Moderator]

Give him a chance Funny Flyer....I think Lee is sneaking in after hours to do these tests....

 

PB are these the batteries you have in mind? I bought some for the kids toys recently & was very impressed by the capacity.....2900mAh would have been Sub C territory not that long ago....Vapex do a 2600mAh Rx pack here again not LSD. Their "Instant" LSD batteries have recently grown to 2300mAh (from 2100mAh) so should offer nearly 4 hours at Lee's 600mAh discharge.....yeah right...

 

I did read that these very high capacity AA cells are not as robust as the lower capacity units since the metal plates in them must necessarily be thinner to gain the extra surface area & hence higher capacity. I don't know if this is a problem or not in our application.

 

The 600mAh current draw is certainly interesting......way higher as an average than we are likely to see but I do wonder what sort of intermitent currents we might achieve in our models. Certainly a high current drain will drag the voltage of these AA cells right down. In most of my models I use 4 cell lsd packs & if I rapidly move all the controls I see the voltage monitor drop right down....often from the top green light to the bottom red light. The monitor goes back to top green when the stick stirring stops....people will say that, in real life, you would never see all servos moving full travel each way at the same time but those who have seen me fly might disagree...... The current drawn from the battery by all servos moving at once is obviously dragging the pack voltage right down briefly & my personal view is that this is where AA cells fall down....their inability to hold up the voltage under high transient currents; something larger NiMh cells & LiFe/LiPo batteres seem able to do.

 

I don't want to hijack Lees thread as this issue has been done to death in many other threads but thought it worth pointing out......

 

Watching with interest Lee.......

[Bob Cotsford]

I do wonder how much more current is drawn when the servos are subjected to flight loads, after all when the model is static the servos' load should just be the weight of the surface they are connected to. As Steve has seen, even this low load can be enough to drop the pack voltage enough to light the bottom scale of the monitor. Another reason why I'm converting to LiFe packs...

Edited By Bob Cotsford on 11/01/2012 10:12:11

[Myron Beaumont]

Steve

Just a thought ,but the fact that the voltage is dragged down with a high current flow doesn't mean that the current flow necessary to keep it all working is not actually there .Or does it?

I was thinking actually of a car battery (yes I know the chemistry is different- but )which drops to about 8 Volts when operating the starter but works OK & quickly recuperates.In other words I suppose -,Does our equipment depend on the voltage primarily or the current flow available regardless of capacity or maybe both . Maybe a stupid question but maybe you can put my mind at rest .I use only 4 cell Rx packs and have read the using 5 cells increases the speed of battery run down or am I wrong .All very confusing to me & maybe other old'uns.

[Steve Hargreaves - Moderator]

Myron, voltage & current are inextricably linked via Ohms law, V=IR. Basically everything conductive will have a certain amount of resistance & when a current flows there will be a certain voltage drop across it. The higher the current or the higher the resistance the greater this voltage drop.

 

All batteries have internal resistance so as current flows a certain amount of voltage drop occurs inside the battery which means we see a lower voltage outside the battery. This internal resistance is usually very small, however where large currents flow there can be a significant volt drop.....in your car battery example for instance the figure of 8 volts is probably reasonable & represents a 33% decrease in the battery voltage but this is caused by a huge current....several hundred amps.

 

Does it matter? Well for practical purposes probably not......the pack voltage will obviously drop under load & provided it doesn't drop too far then no real issues. However as we have seen with 2.4Ghz equipment the input voltage to the Rx can be critical....if it drops too far the Rx can & will shut down.....the so called "brown out". As the pack runs down then this becomes more likely as the voltage will get closer to the critical figure.....beware that "last flight of the day"

 

With regard to 5 cell packs running down quicker then yes they do.....!! You have increased the voltage which gives the current a bigger "push" so more current will flow. See Ohms law above.....R (resistance) is fixed (because you haven't changed the servos or Rx) so if you increase V (voltage) then I (current) must also increase. Taking more current out of a battery will flatten it quicker...simple as that really....

 

Sorry Lee...I'm doing what I said I didn't want to do & high jacking your thread....

[Myron Beaumont]

Steve

Thanks for the explanation I should have worked it out for myself really ! (I mended TVs years ago by the way when it was worth it ). You mentioned that the voltage is more critical on 2.4 systems .I wonder if you could explain why & should we all be concerned about voltage drop as being a main factor regarding "brown outs"even though the current flow is really sufficient AND are the electronics up to the job regarding loss of signal based on voltage drop instead of the power -Watts being available at the lower voltage .Just what triggers the "brown out " ?Voltage drop or what ? Doesn't actually affect me as I havn't experienced it .Plenty of others have though from what I read .

Yes Lee ,I'm sorry if I have instigated a high jack of your thread .All good informative stuff though isn't it ?

[Steve Hargreaves - Moderator]

Well Myron, like you a brown out is not something I have experienced or indeed have much practical knowledge of. My understanding is that when the voltage to the Rx drops below a certain level for long enough (miliseconds?) the Rx will simply cease to function & shut down. This is unlike the older "analogue" (sorry PB I know you hate that term but it serves my purpose here) receivers which will work down to a very low voltage level.....

 

I'm not sure how much of an issue this is in reality...most 2.4GHz Rxs will reboot in a fraction of a second if this happens.....still, I think we could probably do without it if possible.....

[Myron Beaumont]

Steve

Yep -Amazing just what can happen in the proverbial "micro second " I suppose .What you are saying is that the voltage triggers a "brown out" even though there is no real problem with the power supply to the Rx .I don't understand a lot of this but maybe it's because I mostly uyse Futaba gear and a couple of cheapo 2.4 combo for what you might call park fliers in your towns .

Google up Langdale End YO 13 to see where the best flying field will be soon up North & travel a bit SE for a few hundred yards

(Must keep pushing the advertising )

[Tim Mackey]

Both brown outs, and servo current consumption ( in actual flight ) have been researched and results published here some time ago.

 

Servo current in flight.

 

Brown out voltage levels.

[Steve Hargreaves - Moderator]

Indeed Tim....I alluded to this earlier when I said it had been done to death..... but you know what Myrons like......

[Myron Beaumont]

I resemble the remarks

[Codename-John]

Posted by Steve Hargreaves on 11/01/2012 09:59:35:

Give him a chance Funny Flyer....I think Lee is sneaking in after hours to do these tests....

 

Going by the dates of Lee`s original post`s he`s already had over 12 months and still no results ! lol

 

 

[Peter Beeney]

Myron,
With regard to the ‘simple’ circuit, it’s perhaps the case that the more important a particular circuit is, the simpler it actually will be. On the basis that the less you include in it, the less there is to go wrong; and from my limited experience batteries are always at the end of the chain anyway when it comes to power, they are the last resort, again because they may be very reliable. At least, when they are given the correct TLC, that is! But, of course, they are often the most convenient items, too.

It seems to me that we sort of need an appropriate thread here, a general electrical discussion area that could wander about without having to consider too much about it stepping off the beaten track from time to time and thus not conforming to the forum rules. For instance, Steve’s post mentioning V = IR, Ohms Law, is fine, but perhaps we could add to that and say that’s only when R is actually a linear quantity. So then we could ask ourselves ‘Is a motor resistance, or impedance, (as in a servo motor) a linear quantity?’. Then we might have to consider slightly how a motor works. Following on from there, if we also think that a very big percentage of the power used by our airborne radio is actually driving the servo motors, does this now give us a clue as to exactly how much quicker the 6 volt battery will run down compared with the 4.8 volt one? If indeed it does run down faster? But this is now far removed from the O/P, so it might now be bad manners to take it further here.

Steve,
These are the very cells. Lee says these are not receiver batteries but I consider they can be used by the operator however he chooses. I think they will drive a model radio because the load is so light, but I’m afraid that if you wanted them to perform anywhere near the spec. you might be very disappointed. Surely the specification is there so that the purchaser can use them in what ever form he chooses. It’s a guide to the expected performance.
As I said before, I’m fairly confident these cell are very much going to be all the same, it would be a mighty big coincidence if I’d just happened to buy 4 in the same pack that were the only duff ones. This was my findings, just stating the facts as I see them, I’m not at all saying that they should be considered suspect in any way. The individual buyer should make up his own mind about this.

One last thing, Myron, I would certainly not be checking any nickel cells with a volt meter open circuit. I would not get any useful information from this. At the very least I need a load resistor as well, but there are some disadvantages with this system too. I would use an on board monitor, as I’ve said before, the GWS one is perfect for the job. These have certainly improved in more recent times; very accurate; if you use one for a while you really miss it when you don’t have it. It’s akin to not having a fuel gauge in your car.

PB

[Myron Beaumont]

Peter

It was the very inaccuracy mentioned about the monitors that has not led me down the route of buying and relying on them . However,since you mention the GWS one ,I shall certainly look into the matter on your recommendation .

Yes,when checking voltage ,I have

several car bulbs on short leads to attach to measure the loaded voltage of packs.

[Lee Smalley]

unfortunately (or fortunately depending on how you look at it) work has gone mad busy and the chamber and my time that i had to do this test has gone right out of the window, i would love to do the test but i can't see it happening now, maybe i can do a smaller test that will take less time, lets see what i can do over the comming months

[Steve Hargreaves - Moderator]

Posted by Codename-John on 11/01/2012 21:37:20:

Posted by Steve Hargreaves on 11/01/2012 09:59:35:

Give him a chance Funny Flyer....I think Lee is sneaking in after hours to do these tests....

 

Going by the dates of Lee`s original post`s he`s already had over 12 months and still no results ! lol

 

 

Ooops......I forgot last year was 2011.......I thought Lee had only jusy posted......

 

Come on Lee....get a bloomin' move on.....

[Steve Hargreaves - Moderator]

I take your point about the capacity of the cells Peter....they must have achieved the figure somehow though.....perhaps though a 0.01C discharge rate at exactly 20 degrees C. Exactly the same conditions that we would use them in......I don't think!!!!

 

I guess its a bit like EU fuel comsumption figures for cars.....totally unachieveable in the real world.

 

I take your point about the impedence of our circuits but surely this is just making it far too complicated to be meaningful.....adding power factors & the like to our simple calculations would make for interesting debate but would be of little practical value.....

 

Regarding battery monitors then I always make a point of checking mine immediately after a flight......whilst "resting" in the pits batteries will recover slightly & give a higher reading the next time you switch on....for a minute or so anyway.....obviously they are not perfect but come under the "better than nothing" heading.....

[Bob Cotsford]

good point about checking monitors after a flight Steve, also this highlights one good use for the telemetry that's appearing now. I reckon over half my models are now equipped with battery sensors that will give me an audible warning if the rx voltage starts to drop or if LiPo voltage is getting near the magic 3.3v/cell (courtesy of FrSky)

[Peter Beeney]

Steve - I’ve never been convinced that these cells are anything more than what they appear. I’m sure that it would be impossible to achieve the capacity on the label, under any circumstances. All the other tests failed too, and they couldn’t be doctored. Mobile phone companies have had some problems with batteries, they even get past the people that are checking them! And look what they have to lose! For these consumable cells, 999 folks in a 1000 would not be fully aware of this, they just buy more replacements; and when I mentioned it to the supplier he simply ignored it. What else could he do?

Whenever the question about the 5 cell packs running down quicker than 4 cell crops up, I always get the impression that this always reads as though it’s going to be a lot quicker. I think this not always necessarily so. For starters it’s only 1.2V difference anyway; and I wasn’t actually talking about power factor, far from it, that’s something else, I was just trying to make it rather more simple, and consider the mechanical aspect, in terms of actual energy used over time. Thus for one or two reasons, in model planes, the 5 cell might last just as long, in terms of flying time.

As regards the power factor angle, sorry about that, but, as Erfolg might say, ‘ohh, I do like to slip them in’, this has always been dismissed as not applying to model aeroplanes, so therefore not very important, again something which I’ve never under stood. In a perfect world, an electric motor, such as used to power a model plane, with a power factor of 0.5 would use twice the amount of current as a motor with a factor of 1, for the same amount of power, in watts, produced at the prop. I’m not sure that I’d just want to ignore that. Although this never seems to get a mention anywhere, it’s possible it’s lurking in there somewhere, to a greater or lesser degree. I’m sometimes puzzled by what seems to be a rather high current when given for the no load revolution figures. Why is this. But again, this is perhaps not really the right place to dive into this.

I think I did say somewhere that a good place to fix the monitor is near the switch so that you eye catches it as you switch off. So great minds, as always, think alike! I did buy some bits to make an on-board audio low volt alarm to supplement the monitor, but at the moment it’s one of those I’venotgotarondtoityet jobs, I need more time. The GWS monitor is much better than "better than nothing", you can use all the battery power in perfect safety. I’m sure I’ve listed the discharge figures before in another thread, but I could soon do it again if it’s worth it. It only takes a couple of minutes with the right kit.
And although I certainly agree the voltage will recover, I don’t think it’s that much. One led perhaps, hardly likely to be the full range. I’m sure that once the leds have started to indicate that the battery is beginning to discharge you tend to concentrate on it a little bit harder. For me, that’s the whole idea.

We won’t even go near the brown out issue, I also have some problems with this, too…….

PB

Edited By Peter Beeney on 13/01/2012 15:17:08

[Keith Simmons]

I found this from Giant Cod. Link it looks that Lipo batteries cannot maintain a discharge what it says on the label above 35C without suffering damage and some of the connectors supplied are not fit for purpose