Watts up (and down) with Amps Cs and Volts?

[Henning Strand]

I confess! I didn't pay enough attention at school. What I should have learned about the magic of electrics passed me by. Lately I have been trying to understand by reading articles both on the web and in RCM&E, but alas I still fall short of grasping all the elements.

Could someone please explain the basics? I for example need a setup of 150 watts+. I have bought a 3S 1000 mAh 20C LIPO with a motor that should draw around 12amps an ESC that is designed for 18.

What effect does the 10 or 20C marking of the battery have, what effect does propellor choice have, what effect does choosing a larger or smaller mAh battery have, when will the motor or ESC draw the limit, would it be sensible to have a larger motor?????

An electric flight for dummies manual is asked for!

By the way my plane flies great, but I want to understand the does and don'ts.

Henning

[Tim Mackey]

3s means 3 cells in series
3p would be 3 in parallel
series connection increases the voltage but not amp rating. LiPo has a nominal voltage of 3.7V per cell so connect 2 in series = 7.4 three in series = 11.1 and so on.....BUT the current capability stays the same as the individual cell ( assuming they are all the same ) so again
IF the cell(s) is 1000m/ah that means it can supply 1000m/a (1 Amp)for 1 hour
The 3 in series, although now 11.1V, can still only supply the same 1A per hour.

In parallel, the reverse sort of happens...
1 cell = 3.7V, 1A for an hour
2 cells = 3.7V 2A for an hour
3 cells = 3.7V 3A for an hour

Or of course...1 cell can supply 2 A for half an hour and so on...UNTIL you reach the claimed C rate - which was one of your Qs.
"C rate" refers to the CLAIMED MAXIMUM multiplier of the m/ahr rate...
So again....
1 cell 3.7V 1000m/a 10C means you can "drain" the cell at a maximum of 10 times the capacity (1000m/a ) so therefore this cell could discharge at 10 X 1000m/a....or 10A.
3 cells in series (3S) gives 11.1V but still at 10C X 1000m/a -so it is still only capable of 10A.
Bigger props ( pitch or diameter) will cause the motor to work harder, drawing more currrent, so thats why if you increase the number of cells ( increasing the voltage )the current will rise, and possibly burn out the motor or ESC....so Prop DOWN !
lastly....Wattage.
This is simply a sum of volts times amps.
3s pack 11.1V - setup draws say....12A
Then theoretically the input wattage will
be 11.1 X 12 = 133.2 Watts. In practice however, the voltage of the 3s pack will actually drop when "under load" probably down to around 10 V or so ( depends a lot on pack quality )so now we get...
10 X 12 = 120 Watts.
Your pack of 3s 1000 m/ahr 20C could THEORETICALLY supply 20A MAXIMUM, but 2 things to note
1)At 20A the pack will supply voltage for only 3 minutes ( 60 minutes at 1 amp, is the equivalent of 3 mins at 20A )
2) It is NOT advisable to discharge the packs at their maximum C rate as this considerably shortens their life.
I have many LiPo packs in my collection( ditched all my nicads and brushed stuff years ago ) and I NEVER discharge them at above half of their claimed C rate
So I recommend you set up your powertrain to "consume" no more than about 10A ( half the 20C rate )
12A should be fine if that is a maximum, and we dont all fly around on full throttle all the time DO WE :)
One last thing, you should really get a "whattmeter"or similar if you are doing much with electric flight. This allows you to see real time voltage, amps, and watts of any chosen setup.
PS current draw will drop slightly in the air as the prop "unloads"
HTH

[the unmagnificent man]

Hi Timbo, Ive just been reading your reply to Henning and although I have had to read it through a few times, slowly but surely the electric confusion is starting to clear. I wondered what the C rating was for. Judging by what you have written I will need to sell both my 2s 7.4 and buy something a little more substantial for that all important first flight if I want it to last more than 14 seconds. thanks

[JohnnyB]

Wow Timbo!! Just printed off your great reply to Henning above so that I can read and re read and gradually take it all on board. This ELECTRICERY is really something else!! My brain cells are in a real spin.
Thanks for a great effort.
John.

[Tim Mackey]

Glad to help :)
I have some electronics background( but WAY WAY back when "tellys had tubes") that's valves to you kids...valves are sort of - oh never mind :)LOL
Although I have been aeromodeeling around 20 years or so, I had a lay off for around 5, and only came back to it around 2 years ago. For me, LiPos and brushless have been a phenominal leap forward. At the end of my last stint ( around 12 /13 years ago ) I was flying a home built electric mid wing jobby called "battery hen". Powered by a buggy brushed motor, and 8 heavy Nickel cells - performance was...marginal.
Over the last 3 years since returning to the game, I have "embraced" the new electric stuff, although still enjoy the sounds and performance of 4 stroke IC models - my favourite currently being my 62" Spitfire, which i have just finished fitting out with flashing machine guns, and landing lights.
Another brief rule worth remembering is based around "ohms law" the electrickery wizards ten commandments as it were :)
I=V/R in other words....
I ( current )is equal to V (volts ) divided by R ( resistance )
Now to us modellers this means that the current consumed is affected by either ( or indeed both ) voltage and resistance.
In simple terms the volts is the battery supply potential, and the resistance is the motor ( there are other factors but lets KISS) Unlike IC engines, electric motors tend to just keep on going with more effort when you try and stop them ( IE by putting a big prop on them ) and in doing so, the effective resistance decreases, and therefore the current goes UP. This of course can lead to the magic smoke being released, from the motor, or wiring, or ESC etc.
Likewise, keeping the prop the same, but increasing the volts ( by say....using a 3s pack rather than a 2 s pack )leads to the same thing more current
( remember Current equals volts divided by resistance )
generally speaking it is CURRENT that kills motors and the like, NOT voltage...thats why it is quite common to see a cheapy 400 size motor stated as 6V or similar being run on twice that amount....you just need to prop it right in order to keep the current down :)

[Danny Fenton]

Timbo that's brilliant, you have cleared up so much. and I guess the 100 watt per lb of model now fits in to the equation as well. Well done!

[Henning Strand]

Timbo, I was hoping for my thread to create some interest, and thereby some answers to my questions. Thanks to you it has.

I agree that the last couple of years have done a lot for the electric scene, so much so that even I have started in that direction. My last project is the great Tony Nijhuis' 31,5" corsair from RCM&E Sept 06, which also is the model my questions arise from. Ready to fly weight: spot on half a kilo. It's powered by a Welgard A22-12-13 with the Welgard 18amp ESC and using a 9*4,7 prop. It has "enough" power and flies fine at 60-70% throttle. My 1000mAh 3s 20C battery gives 10+ minutes of flying time.

http://www.modelflying.co.uk/gallery/image.asp?sp=&v=5&uabn=126&uin=288

What would be the immediate effect of going one motor size up to also enable the possibility of vertical performance? Would I also have to change some of the other factors?

Henning

[Tim Mackey]

I do NOT hear great things about the Welgard ESCs - they are apparantly rated optimistically regarding current capacity, whereas something like the Jeti ESCs are usually good for their stated rating....and then a bit more :)
SO....I would expect that increasing your motor size may likely result in greater current , and therefore your ESC may not cope. Of course...without the specs of your likely replacement, it is difficult to say. Of course, a bigger motor also means more weight, and then you need more power, and then you have more ......you get my drift Im sure :)
Without wishing to endorse or decry any particular brands, I would only say that in my experience, cheap electric powertrain components do NOT give long term value for money.
You could do no better than call someone like John Emms at puffin models for sound advice regarding a suitable upgrade of motor. I only really contributed to your thread with the intent of answering your specific and basic questions....I do NOT profess to be an expert in the multitude of various motors that exist today...I myself usually rely on John Emms to advise on the most suitable motor for many of my projects. HTH

[Tim Mackey]

I have had a look at the Welgard website, but it is lacking in detail, and generally pretty poor :(

I could not actually find an A22-12-13 listed -just a 22-12-12 0r 22-12-10. With such skimpy detail on the site, I do not know for sure the meaning of the numbers, but hazard a guess that the last figure (12 /13 ) is the number of winds. GENERALLY speaking higher winds give more torque, but less revs.
It may well be that the motor you have is ok up to higher current than you are currenty drawing (12A?)in which case going up an inch or so in diameter of your prop will give you the climb performance you need ( smaller diameter with greater pitch will increase top speed but NOT torque /climb.) This of course will increase the current, and as well as reducing the duration - MAY push the ESC beyond its 18A rating. The only way to really tell is by using a meter as mentioned earlier.
I would GUESS that an inch or so on the prop may show an increase of around 2-3A which assuming the battery holds its voltage at around 10V gives a wattage of 10 X 15A = 150watts....an increase of 30 Watts (25% up on before )
At .5 kilo in AUW this gives you around 135 Watts per pound as opposed to the 109 or so previously.
PS nice looking job on the Corsair :)

[Danny Fenton]

Sorry must be having a stupid moment :)I have just noticed a battery pack quoted as 3s1p have I missed something? I get the 3s = 3 cells in series, but the 1p bit??

[Tim Mackey]

simply means 3 cells in series - 3S1P
If there were need to increase the capacity without actually using physically bigger CELLS you ( or a manufacturer) would put two (or more ) lots in parallel giving for example 3S2P
This would be 2 of the 3S1P packs connected in parallel - 3S2P.
This means the voltage stays the same as the 3S1P pack ( 11.1 V ) but the capacity has now doubled, so if the 3s1P pack was 2000 m/a capacity, the 3s2P pack will be 11.1V but now 4000m/a capacity. Incicdentally the actual C rating as a multiplier will NOT double, so if the 3S1P pack was rated at 20C putting 2 of these together in parallel as above will still give 20C rating NOT 40.
The advantage is of course that you now have 20C times 4000m/a instead of 2000 m/a, so at its maximum the pack COULD be discharged at 20 X 4000 m/a which equals 80 AMPS. Get it ??

[Danny Fenton]

I must go and lie down lolol
so a 3s2p actually contains 3 cells in series, in paralel with another set of 3 cells in series, ie 6 cells total. So three times one cell voltage, with twice the capacity. But the C rating doesn't alter....... where's my OS 40 and some nitro LOLOL. Thanks Timbo

[Tim Mackey]

yep...you have got it spot on - see.... it is easy really :)
The reason the C rate does not change is because it refers to the maximum discharge current of the individual cell, so it does not matter how many cells you have in what configuration (parallel /series / combo) the C rate of the cell does not change.
The only way a cell ( and therefore a pack made up of those cells ) can claim a higher C rate than any other is by the manufacturer inproving the performance of the cell, usually by improved density, lower internal resistance, better general quality of materials etc etc, and GENERALLY speaking the end result is a slightly heavier cell.
Now just before you start flicking and priming, and choking, and more flicking of that 40 engine, will you use straight fuel, or maybe 5% nitro, perhaps 10% ? or even 20 or 30 %. How about the oil content, do you use castor, synthetic, or a combo, and if so what ratio ?? Is that needle set just right, or slightly lean.....whats that - a bit smokey ?? - oh well it is rich then, so screw in a little more....and are you sure the idle needle is set right, how is the pick up from tickover _ what ?? it wont tick over reliably...maybe the plug is knackered, or maybe it is the wrong type- perhaps a hotter plug is called for etc etc....oh never mind, just charge the leccy job up and go fly :) LOL

[Danny Fenton]

Thanks Timbo I am not getting into that debate LOLOL besides the 40 is an OS, you just flick it and it goes, whats all this adjusting stuff???

Just to keep this thread going one more question, well actually a few in parallel, or maybe series?? Can you feed the Rx 4.8v from the lipo packs using all the types of ESC's or only certain types? And what's this BEC thats thrown about, and opto decoupling whats that when its at home?

[Tim Mackey]

I was only joking around Danny...as I am sure you knew :)
This BEC thing is the device ( Built into pretty well all ESCs these days ) that allows your receiver to get its 4.8V supply from a larger battery such as 3S LiPo. It is really just a voltage regulator, "stepping down " the larger voltage to the required level. It stands for Battery Eliminator Circuit. It Eliminates the need to use a seperate battery for the receiver /servos. There are certain limitations, but I wont explain them here as it will get very looooooooong.
Opto Isolating or de-coupling is slightly more complicated, and is connected with problems of interference.
All electrical stufff on board your model creates some electrical interference or "noise" some things more than others. ESCs are quite noisy, and placing them close to the receiver for instance is usually a BAD idea. Long servo leads also potentially act as aerials, and can collect, then feed back to the receiver noise. Very large capacity ESCs have a LOT of work to do, and can be especially noisy, especially as they are likely driving a big motor, and possibly quite a few servos. These ESCs often have Opto Isolation circuitry built in. In simplistic terms, the potential noisy side of things are isolated from the rest by using optically coupled devices which allow a signal to be communicated within a device without wires - sort of like an infra red remote control unit communicating with your telly etc. This allows the ESC to be "electrically" isolated from the receiver, yet still communicate with it for throttle signal and so on, yet stop all the "noise" from being sent to the receiver which could potentially create interference.
It is also found in other areas of the hobby -not just ESCs.
Some of the better on-board glo drivers utilise opto isolating, remote switches for things like gear and bomb bay doors and so on, as well as complete servo drive units /powerboxes which effectively isolate ALL the servos and their long leads from direct connection to the receiver. Basically, the idea is to stop ANY potential electrical noise from sending spurious and unwanted "noise" from actually getting through to the receiver.
Hope that gives you the basic idea.
Clever stuff this opto isolation :)
Or of course you could just do what i have done, and change over to 2.4Ghz - which is totally immune from this noise, and has GREATLY improved my electric flight results.
Timbo now gets his coat and runs from the room before the 35 meg brigade capture and torture him for treason. TAXI !!

[Danny Fenton]

Thanks Timbo, your explanations are very clear I am impressed! So really there is no need for the opto isolation on the lower end of the game, the park flyers and smaller lipo packs. I do know you were only joking about IC v Lectric, I would miss the flipping of an ic engine, its kinda therapeutic in a masochistical sort of way, and appeals to us slightly older generation. However I am very keen on trying leccy as I used to be a very keen 1/12th leccy racer before going on to 1/8 (IC). So was in at the start of the Demon 2C speed controllers, and charging nicads at alarming rates. I also used to wind my own 540 motor or two. So I thought I would have a leg up on this stuff, but alas no. Why can't the motors be labelled with their max wattage or something so that we can see exactly what size we need for a particular job LOL ie .049 = X .09 =y .15 = Z .25 = a etc etc I do know that you can by lipo/motor/esc in IC ratings but those comapanies are few and far between, and are they charging for that ease of selection????
Thanks again your answers have been very informative :)

[Broken Prop]

Just got onto this thread. Many thanks for the explanations Timbo!
I just need to sit down quietly with a Scotch and get my head round it all.....

[Henning Strand]

Regarding my earlier question on stepping up on motor size. On the field yesterday there was a nice FW 190 weighing in just above my Corsair at 600 grams+. With a 1500 mAh 3s he had flying times of 20 minutes + and had plenty of power at just 1/4 throttle. He had an AXI motor that obviously was more powerful than my little Welgard.

I was thinking that being able to fly a more powerful motor at lower throttle gives longer battery life and longer flying times than flying a smaller motor that has to work itself warm to keep up?? Am I right? Of course propellor size also comes into the equation here, I guess...

Henning

[Tim Mackey]

Propeller size will probably have more to do with it. Have you tried my earlier suggestion of just sticking with the motor you have, and increasing the prop diameter a little...say .5 inch at a time? Email Welgard and ask them for the recommended prop sizes, maximum current rating, and so on.
I have a GWS 400 size Tiger Moth which weighs in at 1.2lb AUW with a 1200 3s LiPo - very close to yours - and changing from a 9 x 4.7 prop to a 10 X 4.7, gives an increase in power from 102 Watts up to 132 - a percentage increase of almost 30%.
The flyability increases a lot and duration has not been affected much, due to being able to throttle back a lot and still have the thrust required due to the larger prop. For info these are the full figures taken this morning with the "Astro whattmeter"
9 X 4.7 A=9.72 V=11.0 W= 102
10 X 4.7 A=13 V=10.5 W= 132

Notice how the voltage dropped slightly with the bigger prop, due to a greater drain on the battery capacity, this was more than compensated for by the increase in current and therefore wattage.