Hi guys,
This isn't a "which is best for my bixler" style question- more a general question.
I, like most people, started RC on 3 cells. However, after making the jump to 4 cells I don't think I will ever go back. To me, a 4 cell just turns everything up to 11 :D
So- my question/ramble is as follows
Ignoring very small planes
Ignoring the benefits of fpv gear running on 12V and no need for step down etc
Are there situations where 3 cells are more efficient or appropriate?
Both my planes will run on 3 cells- but they obviously lack the punch and power of the 4 cells.
Or does the 4 vs 3 cell argument vary so much from setup to setup that it's a case of measuring amps drawn and wattage with each battery and prop setup?
Thanks
Quote from: Billy_boy_2010 on September 25, 2014, 08:11:59 PM
Ignoring very small planes
Ignoring the benefits of fpv gear running on 12V and no need for step down etc
Are there situations where 3 cells are more efficient or appropriate?
Both my planes will run on 3 cells- but they obviously lack the punch and power of the 4 cells.
Or does the 4 vs 3 cell argument vary so much from setup to setup that it's a case of measuring amps drawn and wattage with each battery and prop setup?
Thanks
4s is always going to be better than 3s.
More power and more efficient. Its really simple as that
Quote from: dogzilla on September 25, 2014, 10:28:30 PM
4s is always going to be better than 3s.
More power and more efficient. Its really simple as that
Could you explain why?
And do the benefits of an additional cell apply again and again?
If so- why don't easy stars and beginner planes all come with 1000mah 6 cell? Why isn't it the industry standard instead of the 2200 3 cell? Or at least a 4 cell 1800?
basically what you are asking is like comparing fuel tanks. A battery is a fuel tank, the Mah is the amount of fuel it holds and the cell count is how much power it can deliver to the engine.
Now ask yourself why mopeds dont come with a 30 litre fuel tank and a 2.0 litre engine or a Ferrari with a 1 litre fuel tank and a 30cc engine?
Any clearer?
One reason 4S is more efficient is due to Internal resistance of batteries and current through them.
For a given power output W , then W = V * I . IOW lower voltage means more current is required for the same power output. The current runs through the battery and its like there is a resistor sitting inside the battery connected inside to positive terminal. You can only get at the terminal after the resistor. That is called the internal resistance. So Lets say the internal resistance is 2 ohm . The current is 2 A, the nominal battery voltage is 10 Volts. Power in therefore = 10 volts * 2 amps == 20 W
The voltage lost in the internal resistance of the battery is found from the equation V_lost= I*R_internal_resistance;
so V_lost = 2 amps * 2 ohms == 4 volts.
Therefore the voltage now seen by the motor is only 6 volts. Power out is 6V * 2A == 12 Watts
Now lets say the internal resistance is doubled to 4 ohms ( higher voltage battery) voltage is doubled to 20 volts and current is halved to 1A. Input power is same == 20 W
V_lost = 1 A * 4 ohms == 4 Volts
so we now have voltage seen by motor is 16 volts. Power out is 16 V * 1 A = 16 Watts.
In both cases total power used is the same but the motor gets much more of it in the second case, as its running at lower current.
That siad many motors are designed to run at low voltage. Putting a higher voltage causes too much current to flow and saturets the magnets causing motor to overheat.
If you have a low powerd plane its prob fine on 3S. Only starts to be important when you draw a lot of current! But then you need some way to get 12V so you have to start using voltage regulators etc..
regards
Andy
Quote from: Billy_boy_2010 on September 26, 2014, 12:09:34 AM
Quote from: dogzilla on September 25, 2014, 10:28:30 PM
4s is always going to be better than 3s.
More power and more efficient. Its really simple as that
Could you explain why?
And do the benefits of an additional cell apply again and again?
If so- why don't easy stars and beginner planes all come with 1000mah 6 cell? Why isn't it the industry standard instead of the 2200 3 cell? Or at least a 4 cell 1800?
Because it's a balancing act between motor, prop and airframe.
Your airframe determines the amount of thrust and speed you need and also the maximum weight of the motor. So now you need to find a motor of a certain weight to provide you enough thrust and speed according to your airframe.
A 6s easy star would require a small but very low kV motor and they just don't exist. Typically the lower the kV the bigger and heavier the motor, as they are designed to turn big props.
And going with the car analogy above it would be like putting a Ferrari engine in a fiat punto. It just doesn't need it and won't handle the power.
However going to 3 or 4s is a little easier, lots of motors fit into both categories and you should be able to find a 4s combination for almost any type of fpv aircraft.
Higher cell counts are better because higher voltage is more efficient electrically. Also lower amp draw which means you can use smaller wires and even save weight if you want to. Lower amps are easier on your batteries so you don't need such high C rating.
Have started to see more and more mini quads in 4s config. Also seen more Skywalker's and bigger wings run 6s, mostly for guys wanting very long range efficiency turning big props.
the best example of high cell counts being used is 3d and speed helicopters.
even little 450 size helis are going to 6s power because it runs so much cooler and more efficiently.
speed helis are running 14 cell setups designed to run 200 amps for 15-20 seconds.
the only time it makes sense to use 3s over 4s is if there is a restriction on using 4s. (pylon racing) or small bodied planes that physically cant fit it in.
I use 3s for my fleet purely to keep the wiring simple but I am increasingly looking at 4s.
/Steve
I´m thinking of upping to 5s. I use a 700Kv with 4s so my rpm´s per volt are :
700 x 14.8 = 10360
700 x 18.5 = 12950
Thats an extra 2590 rpm´s or 43.1 turns more of the prop per second.
So this means I can throttle back using less amps which it turn will give me a longer flying time.
Important is though can your plane take the weight? IMO no wingspan under 2 meters or 78 inches warrents a 5s.
it depends on what you fly and how you fly it. everything i've got (not much, yet) is best on 3 cell for it's size and type, and for my flying. this may change in the future, but i don't expect that to be "near" future.
Your airframe determines the amount of thrust and speed you need and also the maximum weight of the motor. So now you need to find a motor of a certain weight to provide you enough thrust and speed according to your airframe.
[/quote]
I read this Thread this morning but didn't have time to post, I would add my 2 pennies worth tonight but the above nails my thoughts exactly...
What I do is build the airframe and weigh it to see what the probably AUW will be and decide how I want it to fly,
A Biplane will never fly like a jet regardless of how much POWER you put in to it,
Lots of people assume more POWER is the solution to speed... you can have a very fast jet will poor vertical performance and you can also have a slow craft with endless vertical and everything in between,
So to answer the question 3cell vs 4cell ??? Whats wrong with 2 cell ?
Flying on 2 Cells Vid...
Cam Flight HD (http://www.youtube.com/watch?v=kIwPr3tt6RQ#ws)
All you need to do is find the sweet combo between battery motor and prop... it doesn't matter how many cells in my mind, its all about the best blend to make the airframe fly as it was intended or designed for :)
Ray
Well said Ray, a friends f450 clone framed naza lite copter will fly for about 6mins on a 2200mah 3s lipo using 920kv motors swinging 9x4 props when the lights turn red he lands. I put the battery on my home made frame weighing in at 1lb 5oz and get another 2or 3 mins flying using 1200kv motors and 8x4 props. It don't matter what he sets cutoff on the naza to either so its got to be the weight. Same thing applied when we piggybacked two 2200s to a DJI phantom only managed another 3 mins flight time so weight takes its toll again. P.s also 8x4 props handle wind loads better than 9s or 10s which seem to drain lipos quicker (hear the motors working hard in gps mode on a naza when its windy)
cheers Tony
Quote from: Gundummy on September 26, 2014, 07:31:12 AM
basically what you are asking is like comparing fuel tanks. A battery is a fuel tank, the Mah is the amount of fuel it holds and the cell count is how much power it can deliver to the engine.
Now ask yourself why mopeds dont come with a 30 litre fuel tank and a 2.0 litre engine or a Ferrari with a 1 litre fuel tank and a 30cc engine?
Any clearer?
Thanks. But I can't agree with your fuel tank analogy.
Whilst the amount of mah is equivalent to the size of the fuel tank- the analogy falls apart when involving engine size.
For a given task- say a cruise at 70mph- A bigger engine will use more fuel. Whereas, with a battery- the higher cell count will use less fuel (fewer mah).
Skyscraper- the internal resistance factor makes a lot of sense. Why wouldn't you want the lower apm draw for the same amount of thrust. But again, I find it hard to understand why you wouldn't always choose the larger cell battery....
Dogzilla- thanks. I think I'm starting to grasp it. Couldn't you run a highish kv motor on a really small prop- and run it on a 6 cell? (If you were lucky enough to find a high kv motor which is compatible with 6 cell- which I'm sure would be tricky as you say?)
Re the C rating I'm so glad I switched to 4s in parallel- it essentially doubles the C rating of my batteries.
Steve C- interesting. I suppose heat is more of an issue for copters. I don't think a normal motor would run hotter on a 4s than a 3s would it? Assuming the correct sized prop was used?
Lola- assuming your 4s setup is maxing the motors wattage in your example, wouldn't you need to reduce the prop size to make those additional 43 turns/second safe for the motor? I assume so? Although the benefit, as someone else said, could be the same thrust for less current draw?
Quadboy- can you explain why, what you fly, is best on 3s? Based on what everyone has said, I would guess you have high kv motors with fairly low watt ratings and a switch to 4s would require smaller props- but wouldn't this reduce your current draw? Could allow your quad to stay in the air for longer.......
RaySky- I (think) understand that the airframe determines the thrust required and motor weight. But my issue is there seems to be several different options to achieve the same task.
What started this whole thread off was when I saw the TBS Caprinhara- a premium product by a fairly switched on company- uses a 2s battery. Why did they choose it over a 3 or 4s? The cynic in me thinks it could be because they got an amazing bulk price on a 2s motor from a supplier.......but they do have a good understanding of long range and, to maintain their reputation, they must have known it was a good choice.
Would you agree that the Caprinhara would fly on a 2,3 and 4s setup equally well? Even if you sacrificed the size of the battery a little to keep the weight down and balance against the (inevitably) larger motor. A slightly heavier small wing isn't the worst thing in the world anyway is it?
The below is pure fiction- but would the below look roughly right?
2s 3000mah battery. 35g motor. 8 x 4 Draws 20a max, 10a cruise
3s 2600mah battery. 45g motor. 7 x 5 Draws 15a max. 7a cruise
4s 2200mah battery. 45g motor 6 x 4 Draws 12a max. 5a cruise
Wouldn't the 4s out perform the others? The more efficient setup more than compensating for the lower capacity battery? Why would tbs have chosen the 2s setup?
Perhaps I'm over thinking this. Maybe it's as much of an art as a science? That's the impression you gave me RaySky anyway.
Please take the above apart guys and educate me :)
The easy one first. The caipirinha is only the same thickness as a 3s so unless you want a hump on the top or to weaken the wing too much by hollowing it out a 4 s is out. I suppose you could run 2 2s packs in parallel.
At the end of the day you have to spec the power system based on the profile of your flying and the wattage requirements of it.
A motor wound for 4s shouldn't get any hotter than a 3s motor, things only get hit when you are being silly, unless you use scorpion motors... They run hot.
Running a small prop is just a frig for a badly designed power system. There wouldn't be much benefit from running a tiny prop at five times the speed, the efficiency gains come from the extra torque of a higher cell count and mean you can spin a bigger prop slower, look at the aerial mob arms combo for the discovery they go to 6s with huge props running really slowly.
A proper power system should be designed from the ground up starting with watts per pound, once you know how much the thing will weigh then you can look at motor prop combos and then depending on the current draw you can spec out the battery you need. Specialist uses like 2 hour flight times are different to the average use scenarios.
An example of a plane I have that would not really benefit from 4s power is my easystar. I can get a 20 minute flight from it using a climb glide flight profile, but to be honest after ten minutes I am ready to land.
I don't have any long range aspirations at the moment so there would be absolutely no benefit in going 4s, in fact the power regulator required for the apm and video would just over complicate it.
My discovery has 3s 6100 packs in it, running the larger props for a discovery and I get about 20 minutes or so, but again I bet bored before I hit my reserve. And as I fly close in 1-2km max, ther is no harm in landing and swapping out the packs.
Just my 2p worth
/Steve
Sent from my iPad using Tapatalk HD
Quote from: stevec on September 27, 2014, 11:05:03 AM
The easy one first. The caipirinha is only the same thickness as a 3s so unless you want a hump on the top or to weaken the wing too much by hollowing it out a 4 s is out. I suppose you could run 2 2s packs in parallel.
At the end of the day you have to spec the power system based on the profile of your flying and the wattage requirements of it.
A motor wound for 4s shouldn't get any hotter than a 3s motor, things only get hit when you are being silly, unless you use scorpion motors... They run hot.
Running a small prop is just a frig for a badly designed power system. There wouldn't be much benefit from running a tiny prop at five times the speed, the efficiency gains come from the extra torque of a higher cell count and mean you can spin a bigger prop slower, look at the aerial mob arms combo for the discovery they go to 6s with huge props running really slowly.
A proper power system should be designed from the ground up starting with watts per pound, once you know how much the thing will weigh then you can look at motor prop combos and then depending on the current draw you can spec out the battery you need. Specialist uses like 2 hour flight times are different to the average use scenarios.
An example of a plane I have that would not really benefit from 4s power is my easystar. I can get a 20 minute flight from it using a climb glide flight profile, but to be honest after ten minutes I am ready to land.
I don't have any long range aspirations at the moment so there would be absolutely no benefit in going 4s, in fact the power regulator required for the apm and video would just over complicate it.
My discovery has 3s 6100 packs in it, running the larger props for a discovery and I get about 20 minutes or so, but again I bet bored before I hit my reserve. And as I fly close in 1-2km max, ther is no harm in landing and swapping out the packs.
Just my 2p worth
/Steve
Sent from my iPad using Tapatalk HD
I'm with you for the first half Steve (except the caprinhara uses a 2s but I expect yours was just a typo).
And I appreciate that the thrust required etc based on the spec and requirements of the plane should be the first calculation and the motor and battery selected accordingly.......
but is the point that there is often more than one way to achieve the same result? Ed the desired amount of thrust- with a couple of different battery and prop options?
If 4s batteries can offer the same thrust as a 3s battery- whilst consuming fewer amps- why would you choose the 3s option?
And I can't see your Easystar as being a good example. The plane may well benefit significantly from a 4s setup. Longer battery life and better climbing & acceleration being my guess.
However- I accept tthat if you're tired after 10 mins there is no benefit to you. But that doesn't mean there's no benefit to the plane ;)
You nailed it, it is a case by case thing. I am happy with the climb rate on 3s. Happy with the duration on 3s, if I wanted to extend it I could throw in a 3300 3s and fly for 30 minutes.
Physically fitting a 4s in with all the other crap I have on it just wouldn't happen either.
Not sure what I types about the capi but I meant to type 2s...
Sorry read it back, the wing is the same thickness as a 3s pack so if you want to run 3s it cannot be flush. 2s hides neatly below the airfoil section.
A 4s would stick out like nuts on a bulldog.
/Steve
Quote from: skyscraper on September 26, 2014, 09:08:52 AM
One reason 4S is more efficient is due to Internal resistance of batteries and current through them.
For a given power output W , then W = V * I . IOW lower voltage means more current is required for the same power output. The current runs through the battery and its like there is a resistor sitting inside the battery connected inside to positive terminal. You can only get at the terminal after the resistor. That is called the internal resistance. So Lets say the internal resistance is 2 ohm . The current is 2 A, the nominal battery voltage is 10 Volts. Power in therefore = 10 volts * 2 amps == 20 W
The voltage lost in the internal resistance of the battery is found from the equation V_lost= I*R_internal_resistance;
so V_lost = 2 amps * 2 ohms == 4 volts.
Therefore the voltage now seen by the motor is only 6 volts. Power out is 6V * 2A == 12 Watts
Now lets say the internal resistance is doubled to 4 ohms ( higher voltage battery) voltage is doubled to 20 volts and current is halved to 1A. Input power is same == 20 W
V_lost = 1 A * 4 ohms == 4 Volts
so we now have voltage seen by motor is 16 volts. Power out is 16 V * 1 A = 16 Watts.
In both cases total power used is the same but the motor gets much more of it in the second case, as its running at lower current.
That siad many motors are designed to run at low voltage. Putting a higher voltage causes too much current to flow and saturets the magnets causing motor to overheat.
If you have a low powerd plane its prob fine on 3S. Only starts to be important when you draw a lot of current! But then you need some way to get 12V so you have to start using voltage regulators etc..
regards
Andy
Andy
The internal resistance of a cell is so low as to be negligible... its almost considered as being next to zero.
if it was 2 ohms per cell then a 4 cell pack would be 8 ohms so if you shorted it out there would only be 1.85 amps flowing,
In my honest opinion more cells would increase the pack resistance minimally, I will agree that less volts means more amps so the circuit resistance comprising of pack, esc and motor will generate more heat and make it less efficient to degree.
Ray
Quote from: RaySky on September 27, 2014, 10:21:46 PM
l resistance of a cell is so low as to be negligible... its almost considered as being next to zero.
[...]
Lipo internal resistance is prob of the order of a few milliOhms. May seem negligible but will drop voltage proportional to current. E.g With 40A through bat an internal resistance of 0.025 ohms drops 1 Volts. etc. which that feels about the correct order of magnitude to me.
You can see the effect if you have Voltage on your OSD. Turn full power on and watch battery voltage drop. The only thing( apart from lead resistance which is pretty small) causing that is battery internal resistance. In fact if you have a current sensor you can calculate the intenal resistance.
The higher internal resistance in example was just to simplify calculations! :)
Lipo internal resistance also increases with age apparently, so replacing your old 3S with new 4S will give astounding results!
regards
Andy
Quote from: Billy_boy_2010 on September 27, 2014, 10:26:12 AM
RaySky- I (think) understand that the airframe determines the thrust required and motor weight. But my issue is there seems to be several different options to achieve the same task.
Perhaps I'm over thinking this. Maybe it's as much of an art as a science? That's the impression you gave me RaySky anyway.
Please take the above apart guys and educate me :)
Billy
It is so difficult to work with pure number crunching and science regarding best prop/esc/motor and pack... not to mention airframe aero dynamic/lift and coefficient drag blah blah blah lol
It would drive us all nuts so what I do is use an online Rc motor calc to get me somewhere near and try props changing pitch and diameter either side of the size and see if the craft performs better or worse.
The best piece of kit to help getting a sweet blend of motor and prop is a watts meter,
If you know the max amps for a motor and use that as the limit then you are 90% the way there,
Andy
Too True ;) We are both singing from the same sheet ;) but I have to say that some motors wont take a 4S voltage so swopping from a 3S to 4S needs a change of prop for an already installed motor and even then it may not fly any faster or better and may not climb as well as it did on 3S with a larger prop.
So to answer the original question of "Why would anyone use a 3S pack?" the answer is... Its the original designers concept or opinion as a starting point to suit general criteria of motor and prop :D
Ray