Li-Po novice

[Royden Baker]

I have a glider with a worn out Nicad, so I naturally wish to buy the latest technology.  It has been suggested at the Club that my old fashioned brush motor is not the right animal to handle the Li-Po I intend to buy (7.2 volt, same as the old Nicad).  To my simple mind, current is current, and I presume the motor will take the amount of power it requires to rotate at its given RPM - how wrong am I?  Whilst I recognise the benefits  (and dangers) of these beautiful packages ot power, there's no witchcraft is there?

Having read one or two items about speed controllers, that is the next question, is my existing speed controller capable of handling a Li-Po, and I do realise I am rubbishing my own beliefs by asking this question.

 Roy Baker

[Tim Mackey]

Royden...I am working on an article for the forum to help people such as yourself, so keep a look out over the next week or so

Meanwhile, I am afraid you are a bit wrong on the "motor's requirements" .

A motor ( indeed any electrical resistance ) is part of Ohm's law, and this states that current (I) is proportional to the resistance (R) and the Voltage (V). Therefore if you increase the volts you will increase the current, and if you reduce the resistance the same applies. The motor does not "take the amount of current it needs to rotate" it will continue "taking" more and more current as the volts go up or the resistance goes down until it melts! However, a 6 cell nicad has a nominal voltage of 7.2V, and by happy coincidence a 2s LiPo has a nominal voltage of 7.4 - which is close enough. Now if you use a 3s LiPo of course, then the volts are considerably higher, ( 11.1 ) so ......see above !

As to your ESC - the thing to watch for here is the LVC ( low voltage cutoff ). Nickel cells are happy to get down to 1.0V per cell, and if your ESC cuts power to the motor at 6V, then again you are OK as this is the point at which a 2s LiPo will need to be cut too ( 3V per cell ). It is not actually good practice to allow the LVC to cut in anyway, and you should always aim to land as soon as you notice a deterioration in speed / noise of motor.

Of course if you really want to drag yourself into the 21st century, then when the brushed motor dies from exhaustion, you should replace it with a far more efficient brushless motor - you will never look back !

PS congrats on showing such brave initiative at your age - a credit to the older generation

[Royden Baker]

Hi Timbo

This is the second time I have answered your message, I have the feeling I got it in the wrong box the first time because it did not have the same heading as this one. and I can't find it anywhere!!

As far as the age is concerned, you have doubtless been reading another message (that's cheating!!).  I find challenges the best way to spend a lasting retirement.

Yes, I do still remember VIR, it's the triangle that does it!!

I must point out that I do realise the power source for any motor needs to be the correct voltage.  I say this because I got the feeling your message inferred that I was saying the motor would only take what it required whatever the voltage.  I apologise if that is not the case.  In my ignorance I said I would purchase a 7.2 Li-Po, but of course there is not one, although I would have thought 7.4 would not tax the windings too much.

I hear what you say about the LVC on the ESC.  Is there any writing or symbols to look for on the ESC that would put one's mind at rest about its suitability?

Finally, there is definitely a brushless motor on the horizon, governed by two criteria.  Firstly, I cannot handle my existing glider too well, I was brought up on ailerons which this one does not have so I may delay the moment until I find a nice Breeze or similar.  Secondly, especially taking the Breeze into consideration, I thought I would wait to see if the nice Mr Brown might raise the pension to the level of our European neighbours. 

[Tim Mackey]

Apology absolutely not required Royden yes , I did spot another posting from you which gave away your seniority There is not usually anything on the actual ESC - but if you tell me the model I will have a google around - besides, as you rightly assumed, the 2s LiPo is close enough to a regular 6 cell nickel.

I wouldnt hold out too much hope for the "nice Mr Brown" but best of luck in all you do...

whereabouts are you ? 

[Royden Baker]

I'm in the wilds of deepest West Norfolk.  Will you have the thesis completed before I buy my Breeze?

 Roy

[Piper Cub]

A lot of kits are now being marketed as IC or electric. Sometimes, the manufacturer will state the size of brushless motor required which is great.

But if they dont, what the easiest way to find out what brushless motor is most suitable and how do you determine which size lipo to use!

Any sign of the article for newbies yet? Havent been able to find it on the site.

[Tim Mackey]

Hi Piper C. The article has been submitted but it is up to the editor to decide release dates and methods....it may well appear in magazine print first, then the forum. Dont forget that Tony Jones has been running a very good 3 part series in RCME over the last three issues dealing with electric flight

I have answered this / this type of question several times, so I will be brief here, and also suggest you browse the forum for similar threads as I honestly dont remember which ones to point you to

Anyhows... VERY SIMPLISTICALLY - if you are wanting to power a large slowish model, with a suitably large and slower revving scalish prop, choose a largish outrunner motor with a low KV ( revs per volt ) say 500-600 and aim to get about 80 -100watt per pound. Most motors will state the recommended prop size, KV, and maximum current or Watts input allowed.

EG: Model weighs 6 lb - you want approx 500Watts of electric power. Motor should be capable of taking around 30-40 Amps, and for 500Watts you will therefore need about 16V or 12 V respectively. 4 Lipo cells in series will be about 14V ( under load ) and for your 40Amps, you will need a capacity of about 3000m/ah with a C rate of about 15 to 20. This pack will be working at about 13C to supply your 40 Amps.

Faster smaller models will need small high revving props, and for this choose either a high KV inrunner, or indeed some high KV outrunners are available these days too.

Finally, remember some suppliers such as E flite are now naming their motors to match their equivalent IC brothers.

[Piper Cub]

I see lots of kits stating electric flight should use 4S2P Lipo packs or even 2 4S2P packs so have a couple of questions;

Can Lipo packs be wired in series? One Piper Cub kit I've seen (quarter scale) recommends two 5SX cells or I think one 10S2P. Would one  combination give a better flight duration than the other?

Im assuming the 4S2P means 4 cells in Series 2 in parallel but what is the like for like difference in capacity between a 4S2P and a 4S1P or isnt there any? Would there be a difference in performance between these two pack configurations ie flight time, or has one pack just got more capacity?

When buying Lipo packs, should one generally buy say a 4S2P as opposed to a pack that only has 1 parallel cell ie 4S1P?

Thanks for all the info!

[Tim Mackey]

Q1 part 1 Yes, providing they are the same mah and C rate ( capacity ). part 2.... see below.

Q2 Not in itself no...flight duration is down to capacity of cells irregardless of whether that is 1 x cell at 2000 mah or 2 x cells of 1000mah each ......paralleled to give 2000mah.

Assumption1 Yes and the rest is explained in Q2 answer !

Q3 see Q2

The only thing I would say is, providing you are happy to use a "Y" type lead to connect your 2 x packs of 4s1P batteries together to double thier capacity, then this is a bit more flexible in as much as they can then also be used seperately if required in other models which do not require so much capacity and its associated weight. If doing this, it is important to ensure that both packs are at the exact same state of charge before connecting them together or charging.

[Piper Cub]

Ouch! My brain hurts!

Looking at the Lipoly you suggested Timbo. What does the 15 - 25C signify and what's the difference between this and a similar Lipo pack that's stated as 20 - 30C?

More importantly as an example, how do I know when 15 - 25C, 20 - 30C or something else is suitable for whatever set up \ plane its intended for?

Sorry to ask dumb questions!

[Tim Mackey]

Piper...wait til you read the post I just did to your other thread then !! LOL

Which LiPo did I suggest ?

Anyway.. 15 - 25C means it can THEORETICALLY be discharged ( used in other words ) at up to 15 -25 times its stated capacity, so if its a 1200 m/a battery ( 1.2 Amp ) that's 18 to 30 Amps. Take the average as being around 20C which is.....24Amps ( 20 X 1.2 )

The letter "C" simply means its Capacity, so a 1C pack would be only useable at 1 X its capacity, or in this case 1.2A.

What is suitable for your model depends on how much current it is going to pull, which is a factor of motor choice, and more especially prop size. The higher "C ratings" claimed by some packs are to be used with caution, and anyway if you used that example pack at 25C ( 30 Amps ) two bad things will happen

1) The pack will have a pretty short and hard life

2) Your flight time will be just 2.4 minutes ( assuming the throttle stick doesnt move much from "GO"

This is calculated thus.....

The pack is rated to supply 1.2 A ( 1C ) for 1 hour ( 60 minutes / 1 = 60 )

So to supply 25 times that amount you again divide the minutes (60 ) by the C number (25) and you get..... 2.4 minutes.

PS there are no dumb questions..... but many dumb answers

[Royden Baker]

Hi Tim

 I have read with awe all the answers you have given to questions I had not dared ask!!

I find I don't have time to read too much, although that is probably an excuse because I prefer a hands on approach, so all in all (yes, my brain is beginning to protest!) I have to say, at the moment at least, I will be sticking to IC engines with numbers that I do understand.

I think your information is invaluable to a beginner like myself, but I am sure you would agree, you never really fully understand until you have taken the plunge and done or used it or broken it!!

In conclusion, I do like your final statement, how true.

Best wishes to you and yours for the New Year.

[Tim Mackey]

Hi Royden, and H N yr to you too !

The numbers do look a little daunting at first, but honestly - I am NOT good at maths, in fact this was the main reason I dropped out of tech college back in my youth when studying electronics ( that, and the fact that my mates were all earning huge money ( £10.50 a week !! ) in "proper jobs". As you rightly say, hands on experience is invaluable, and if you start off with something cheap and cheerful, you will soon get the hang of things. I do NOT profess to know anything like as much as I probably should, but did / do find the learning experience a fun one, if a little steep