Lipos

[Mike Wyatt]

A comment from the last Outpost of the British Empire - namely, Durban, South Africa.

I've seldom seen so many words written about some relatively straight forward concepts - admittedly, the terminology can be a bit confusing, but one you've got your head around it, it's really quite simple.

Sure, the harder you use a LiPo battery, the less time it's going to last - a bit like the harder you drive your car, the more quickly it wears out.

A couple of points which I may have missed in the volumes of verbage -

- LiPo batteries vary tremendously from one manufacturer to another. I've had a couple which are marketed as being 15C, and are quite clearly not, but they make quite useful paperweights. I've now homed in on two brands which are excellent, but I'm not sure whether one is allowed to punt specific manufacturers in these columns !!

- The other thing is, what one is really interested in is the wattage that the power train is producing; specifically, one should be aiming for x number of watts per pound (x being dependent on the type of model). As Watts = Amps X Volts, if one increases the voltage by going from a 3s to a 4s or 5s pack, the Amps drawn will decrease for a given wattage requirement. This means that the LiPo pack is actually working less hard as it is being asked to produc less Amps.

The E-Flite web site (http://www.e-fliterc.com/) gives some informative guidelines for wattage requirements for different types of model.

[Tim Mackey]

Mike I couldnt agree more about the overcomplexity of the nature of this thread. I did however express, I thought, quite simply and plainly that very same fact RE: LiPO life will be reduced if pushed at max C rating a lot, and I think I also referrred to the point about not all LiPos being crerated equal !
I would just also point out, as this is a beginners section after all :-) that increasing the volts, will actually increase the amps drawn if other steps are not taken to reduce the load ( IE smaller prop ). I know that you have referred to wattage requirement, and of course your equation is correct,and it is indeed watts that we are after in our electrickery stuff, but I just wanted to flag up that increasing the volts will increase the amps, and beginners need to be aware of this, as they could end up releasing the magic smoke if their motor /esc is not rated for the current drawn.
Beginners note : It is AMPS ( current ) that kills motors and ESCs not volts - generally speaking...if thats OK to do of course, wouldnt want to go upsetting any Mr Johnnies again now would we.....

Quote extractede from a site linked to by another post -thanks :-)
"At 15c the EXTREME POWER 20c packs maintain 3.4v+ per cell for 80% of their discharge, and temperatures remain well under the 60‹c factory maximum ratings. We have data-logged flights using our EXTREME POWER 3s2200mah 20c pack in one of our ducted fan setups drawing 31 amps in the air, and it emulated the independent tests sitting at 10.2v during WOT runs with a 39‹c after flight temperature.

EXP-15c: We recommend keeping your maximum static amp draw to 12c and burst amps of 16-18c maximum for 5 seconds, this gives you an increased lifespan.

EXP-20c: We recommend keeping your maximum static amp draw to 17c for prop driven planes and 15c for EDF planes. Burst amps of 20-25c maximum should be observed for increased lifespan (Dependant on ventilation and cooling of the pack during flight).

Please be aware, manufacturers claims of 20c continuous and 30c maximum for any battery are quoted straight from the factory as the absolute maximum a new cell will perform at, this will result in a shortened lifespan and considerable risk of damaging the pack, not to mention most of these packs run over 60‹c at these discharge rates!

œ What is gCh rating? It is a factory term for the discharge rate a cell can supply. 10c means 10 times the capacity, so a 10c 2200mah battery would have a discharge rate of 22 amps maximum continuous.

œ We are considering labeling our packs with the discharge ratings we believe are sustainable, you may in the future see our packs labeled with continuous amp figures, they will be the same packs, but the rating will remove confusion and the need to calculate what you can actually draw from them and still maintain a long lifespan.

[Mike Wyatt]

Timbo - you are absolutely correct about the increase in volts allowing more amps to be available - I hang my head in shame for not mentioning this.

I also omitted to mention checking the voltage rating of the motor/ESC before whacking up the volts.

I am also inclined to overpower my models slightly so that the power train doesn't have to work so hard. I also find that it's the ESC rather than the LiPo pack that seems to take the strain in terms of getting too hot.

I've been flying electric for about a year, and the learning curve has veered from the vertical, but is still exceedingly steep !!

[Tim Mackey]

I apologise for not mentioning that it was actually Alistair T who linked the site from where I attached the quotation above re: IT IS NOT GOOD PRACTICE TO USE PACKS AT THE FULL STATED C RATE CONTINUOUSLY.
Sorry for shouting..just wasn't sure whether I had stressed this point earlier and just in case you missed it...here it is again.....
Please be aware, manufacturers claims of 20c continuous and 30c maximum for any battery are quoted straight from the factory as the absolute maximum a new cell will perform at, this will result in a shortened lifespan and considerable risk of damaging the pack, not to mention most of these packs run over 60‹c at these discharge rates!
LOL
yes Mike it is a steep learning curve, but I hope you are enjoying it - do you have a good flying facility out there ??

[Johnnie]

Timbo
Sorry to see the blood pressure is still way up there.

Why dont you explain if not for me but for the benefit of the other beginners in the forum why the power requirements of your model increase if you increase the voltage. i.e. if you increase voltage the current drawn increases.

[Alistair Taylor]

I = [V-(Kb x kRPM)]/Rm

simple really

[Tim Mackey]

Johnnie. I do not suffer from high blood pressure, low actually...which is perhaps worse at my age! I am happy to explain the reason for the increase in current, but am now suspicious that you are being somewhat sarcastic, I apologise if that is not so, and in the interest of good relations - which I hope we can muster, here goes.... Strictly speaking the power requirements of the model do NOT increase as you infer, the requirements are a matter of sufficient power (watts) for a given airframe and required performance, something which should have already been established at the time of choosing the powertraain components. The Amps consumed by said powertrain will however increase with increasing voltage due to our old friend ohms law - but i suspect you knew that :-)
Incidentally moderators...where are the "smilies" etc that I and others have been asking for since day 1 of this forums birth...so much easier to convey one's "mood and tone" with a few well chosen icons. Anyway back to ohms law.
I ( current ) = V ( volts) / R ( resistance ). Now although an electric motor does not have a truly fixed resistance in use, it is still the R in our equation, and therefore it can be seen that by increasing the V, and keeping the motor resistance / inductance / load (R)the same, current (I) will increase.
That is why when using for instance a 3s pack ( 11.1V nominal ) on a powertrain that was using a 2s ( 7.4V nominal ) it often results in too high a current for the motor /ESC. The simple solution is usually to reduce the prop size, which reduces the motor load, and therefore effectively presents a lower resistance to the current flow, and therefore current (I) reduces back to a safer level.
I genuinely hope that this simplified explanation meets with your approval, as I do NOT like "fighting" on forums...or indeed real life :-).
I do apologise for my rather aggressive tone of earlier posts.

Yours sincerely...Timbo :-)

[Alistair Taylor]

Sorry

I was being flippant.

The equation comes from here;

http://www.aveox.com/technical/dc.html

It confirms what Timbo says - Amps (I) increase as volts (V) increase, provided motor load/resistance (Rm) stays constant.

Johnnie - what planes do you fly at the moment? and what are you hoping to put the lipos in?


AlistairT

[Johnnie]

Timbo
Wouldnt a lower resistance in fact increase the current flow. However, looking at Alistairs version of I=E/R - reducing the prop size would increase the RPM which in turn would increase the back EMF term in the equation making the effective voltage smaller and so reducing the current. All assuming of course that the terminal resistance stays the same.

Apologies and smily faces.

Johnnie

[jimbo]

li po's still have me running scared,now they are talking about flame proof bags,so you dont set your car on fire on the way home.what hapened to the saphion batteries that were comming our way.

[Alistair Taylor]

Jimbo
Saphions are available from overlander, and others. I guess lipos have just been taken up by more people (you got a betamax video by any chance?).

Johnnie

Now my brain really hurts!

Didn't EMF use to sing "you're unbelievable"?

Yes, decreasing load would increase RPM and so increase back EMF, for a DC motor;

http://www.girr.org/girr/tips/tips5/motor_tips.html

In modellers terms, if you just add a cell to a battery pack without changing the prop (or the throttle!), the RPM and current both go up.

I'd love to know whether this is still the case for brushless motors.

Wowsers - if you google this stuff you get all manner of technical jiggery pokery.

A ha - Wikipedia say - Model airplane brushless motor is brushless DC motor, cos the speed controllers do not deliver true AC sinusoidal (sniff) current, but rather pulsed on/off current.

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

Johnnie/Timbo - I reckon the following website is going to be very helpful fr both of you - an internet database for logging motor performance;

http://www.peakeff.com/

I'm going to bed


AlistairT

[Tim Mackey]

Yes Johnnie your right, a lower electrical resistance (ohms )would indeed cause higher current, and I think I got my terminolgy slightly askew !!
I suppose I meant resistance in the context of drag, which does cause the motor to show up as a lower resistance / impedance, IE it does not impede as much.
It is akin to stalling or trying to stall the motor, - if you imagine holding the motor shaft as it is turning, the tighter you grip, and therefore the slower the motor is allowed to turn, the greater the current drawn, as it fights to maintain the revs it is designed to do on a given voltage (Kv). This is why I would question your statement that a smaller prop would allow the motor to turn faster, incresing any back EMF - yes it would...but not much. It is one of the oddities of electric flight set-ups that causes some of the most confusion to died in the wool IC flyers! If you bolt a bigger prop on the front of an IC engine, the revs drop off subtantially, however, bolt a bigger prop onto an electric motor, and it just sees it a challenge, and continues to try and turn it at the same revs, but in order to do so, it requires more juice IE Amps.
Of course there will be some fall in revs, but nowhere near as much as on the IC engine.The converse is of course also true. Apologies, and I hope that clears up any confusion my previuous post may have caused amongst the newbies.
PS yes..it was EMF who had the modest hit with the rather good song "your un-believable" :-)
PPS - good links there Alistair ( thumbs up icon inserted here )

[Mike Wyatt]

Morning to you Timbo - looks like I've opened the can of worms even more by introducing the concept of variable voltage into the equation ..... !!!

I have access to three excellent fields here; one is a municipal sports ground not 5 miles from the city centre, but weekends you can only fly until 8:30 am when the soccer/cricket players pitch up. The other two are further out of town at the polo field of an equestrian club and on a farm. All have grass runways. The problem here is not that of finding a suitable space (there's plenty of it), but of finding an area which is safe - a couple of my mates have been flying quite happily, only to find a gun pointing at their head, followed by being relieved of their wallets, mobiles phones, wrist watches and anything else of value. We've had to stop using one site with a lovely concrete runway for that very reason.

Winter (which we are just starting) is the best time to fly, as the weather is more settled, as it's the dry season. Typical day time temperatures are mid 20s (C) with a cloudless sky and light breeze.

I've got several electric models ranging from a little foamie through a couple of hot liners to sports aerobatic jobs. I still have several ic models and persuade myself to take out from time to time, but by the time I've lugged a) Fuel b) Battery c) Starter d) Power Panel up to the car, the enthusiasm has already waned !!

How about you ??

[Tim Mackey]

Good morning Mike....yeh well done mate - more fodder to feed the arguments :-) :-)
LOL
Our site is pretty good really, certainly not been mugged at gunpoint yet -unless you count the huge rise in rent we were "asked for" by the owner a couple years back! I have about 12 miles drive each way to the field and back, but it is in a nice location with moutain views from 3 sides, a nicely tended strip - not hugely long, but we all manage ( well most do :-) ). 'tis club meet night tonight ( every Thursday during "summer" months) with usually a barbecue and banter and flying until around 10PM just at the moment.
We have no restrictions at all on flying days /times...other than electric only before 10AM any day. Pretty good, especially as I am now mainly electric, although still retain 3 IC models ( radio is powered by LiPO though)...yippee..I feel another debate comin on hee hee.
One of those in particular I would NOT envisage changiing to leccy is my 62" Spitfire, retracts, landing lights, and flashing cannons. I know about the sound chips and amplifiers and speakers to simulate the real sound of the merlin, but I have already got carried away with the toys on it, and it ( like me ) is now slightly overweight!!
I think my fav electric at the moment is my ALFA Sabre EDF, sits really nice in the air, has a good turn of speed, easily hand launches, and sounds pretty nice too with its fan spinning at circa 45000RPM :-)
I think I posted some pictures of some of my stuff in the gallery a while back. I have also now gone totally 2.4Ghz with the DX7 Spekky stuff, and so far am mightily impressed.
peg system? ...how quaint,
glitches? what are they then :-)
Keep writing, your forum needs you !!

[Mike Wyatt]

Hi Timbo

Your field sounds lekker, as we say in this part of the world (Afrikaans for nice). Wherabouts in UK ?? - I came from Surrey more years ago than I care to remember (over 40, actually).

When you say you use LiPo for radio do you mean BEC - I have never yet had to use a separate pack for the radio.

I have a .60 powered Fokker DR1 which I've thought about converting to electric, but like your Spit, it's quite heavy, so Ill probably leave as is.

Am also considering going over to 2.4GHz, but have the problem that I have about 10 35 MHz receivers. The strip we fly off at the farm is close to a 4X4 track, and these guys are licensed to use their walkie talkies in the 35.250 range, so we do tend to get some interference, particularly over weekends.

What's the power train set up on your EDF - have always found that fascinating, but never built one yet.

Back to LiPo's briefly, saw a guy at the field last Friday charging a 800 mAh pack at 2.5A - he said it was a bit sluggish and needed a boost !!!!! He seemed quite unconcerned when I pointed out the dangers of this folly - before putting a very comfortable distance between the two of us !!

Off down the coast next week for a few days leave - will take the foamie and fly off the beach, if not too windy.

[Tim Mackey]

Hi Mike. I fly from a patch at Rhuddlan in North Wales, and of course take the sloper to the great orme, and any number of mountain slopes within spitting distance of home :-)
My IC powered planes use LiPo packs for the radio is what I meant - sorry should have clarified that. I use either home made regulator circuits / commercial regs, to drop the 2 cell LiPo down to constant 5.0V. Apart from anything else, this allows me to charge my flight packs and leave them ready for a flight at any time.
Once my warantee is up ( November ) my DX7 set will have a LiPo tr pack too, although I have to say the duration of the supplied 1500 nmh pack is excellent. I had 8 flying ready planes at the time I changed to 2.4, so converting them was not too expensive, just did the last one a couple of weeks back, and sold my PCM9X for 120 quid immediately, funding almost 3 more spekky rcvrs :-)
My sabre uses a 3 blade carbon fibre composite Alfa fan unit, and an AON 4900 brushless inrunner. On the 1200m/a 3s F/Power pack, it sppins at around 45000 RPM, and flies very well for around 6 minutes. I have 5 of these packs, so never have any bother getting sufficient flights in any one session. I tried other packs, and other capacities, but the 1500 were a tad heavy, and performance no better...indeed worse, as this bird likes to be kept LIGHT ( 17oz AUW ). Other brands of pack could not hold voltage under load as well as the F/P, and they are one of my favourite brands. The Sabre F86 is a real beauty,probably my favourite plane of all at the moment, and always impresses the crowd whenever she flies - get one..... I promise it will give a huge grin factor.
Charging liPos at over 3C huh. This man is a fool, and just like overdischarging, charging at this rate will at the very least shorten their life, and at worst they could puff up and ignite. I should have serious words with your club committee and himself, as this is dangerous practice. As for boosting them, I should think it would probably have the opposite effect in the medium /long term, he may well upset the chemical composition due to internal heat, and that aint good :(

[Mike Wyatt]

Timbo -
F/P are excellent. I find Hyperion are also very good, although quite a bit pricier. I balance my packs quite frequently, although I've never yet had one seriously out of balance. Did recently have a Kokam which started giving out smoke signals whilst under charge - never found out the reason, though I suspect an internal short. Fortunately, it wasn't in a model at the time.

Off home now - I'm a pensioner working on contract for a chemical company, and don't normally work Fridays. Will be off to the polo field tomorrow, weather permitting - it's a bit unsettled right now with a cold front coming in from the Cape.

Cheers - be in touch in about 10 days.

[Tim Mackey]

Have fun....Hyperion are cheaper than F/P over in good 'ol Blighty...come to think of it, pretty much everything is cheaper than F/P !!

[Geoff Daunt]

A Question for you experts.
I have 3 Lipo's for my twister 3d helicopter. They are 3S 1500 each. After flying and running down to the minimum 2 of the batteries will take approx 1100 Mah charge with a dedicated Lipo charger and linked to a balencer. The 3rd battery will only take a charge of about 750 mah. This is on a regular flying basis. I sequence the use of each battery and they have been used about 20 times each.
My question is why is this third battery not also accepying a 1100 recharge. The cells all look ok and l have never crashed or damaged the cells to my knowledge. The obvious result is that this third battery only gives me about 70% of the flying time of the other 2. Any advice greatly appreaciated.
Geoff.

[Tim Mackey]

Hmmm, I wont discuss the oft used explanation of "expert" but do not consider myself one!
Have you tried actually ising the balancer to confirm the actual voltage of each individual cell in the poor pack, both at the end of a charge, and at the end of discharge ? perhaps one cell has gone naff?
Also, if i am right, the twister bell does not have any form of LVC protection on its combo unit, and relies on the flyer noticing a drop offin performance, at which point he should immediately land...but i bet he ( you) dont !
I know I didnt with my twister bell, and probably flew occasionaly to the point of allowing the 2 cell pack to drop too low.
Could this be it?

[Geoff Daunt]

Thanks for the reply.
I would not know how to chack the voltage of each cell with my balencer since it seems l can only plug it in and it does an auto balence either with no charge input or with a charging input. There is no lcd screen or other read out info on my balencer. The one l use is an e-station balencer.
You could be right about running down to low but l have treated all batteries the same and stop when l can acheive no lift.
I guess l could check the voltage with a simple volt meter but then l assume l would need to remove the protective wrapping. How do l tell where the positeve and negative ends are on these cells since l have never opened up a pack??

[jimbo]

SAPHION'S
in an artical in l of last years r c m &e.SAPHIONS WERE THE BEST THING SINCE SLICED BREAD,how do they copmpare with li po's,ie weight and power,are they charged as ni mh,they dont seem to get a mention any more.

[Tim Mackey]

You can use a simpledigital m/meter and carefully "prod" each of the connectors on the pack's balancer connector. They all tend to differ with there configuaration, but almost certainly one of the ned wires will be egative of cell 1, 2nd wire is positive, and also negative of cell 2, and so on.....
Each cell should show around 4.2V fresh off the charger.

Saphions are trying to make a second comeback, they did NOT prove as popular as the Lithium Polymers because of weight, and this in turn slowed sales, which kept prices high...etc etc...
A few retailers ( online ) are now trying again. Think VHS versus Betamax, or cassetes V 8 track.

[Geoff Daunt]

Thanks for the information Timbo.
I have done as you suggest and putting the Negative Prod on the black wre and going up the different colours l got the following results; 4.2: 8.4: 12.6.
So with my limited knowlege of electrics it would seem that each cell is currently balenced at 4.2V per cell.
Any other thoughts as to why it will not take the same charge in Mah as the other 2 batteries.? Many thanks.
Geoff.

[Tim Mackey]

Yep , seems that each cell is about spot-on. If you have the facility to discharge them to a set level ( 9V in the case of 3s ) try doing so, then see how much goes back in. I really dont understand why this pack should be different to the others...are they all the same make?
PS....You could always just leave the voltmeter connected across the pack, and connect a car bulb or similar to discharge them, stopping the process when the meter reads 9V, then repeat the voltage check for each cell, make a note, and then recharge.