Totally agree. 5 years of 4s and still happy. Did try 6s but not worth keeping both or converting everything. Love the physics stuff. Wish Bruce did some more whiteboard videos, I miss those.
Pick a cell count and stay with it. Good advice I received when I started flying. I like the whiteboard presentations too. For some reason, it makes more sense to me.
You are only considering the wires. Which is only a part of the story. Connector resistance is much higher, especially after 100 cycles. MOSFETs at temperature suffer from 3x the Rds,on at 25C and with the small ones used on 20x20 ESC's that is significant. Battery impedance is significant. And losses are I^2.R, which adds up given the several tens to a hunderdsomething amperes we push through. Then we have iron losses in the motor; the number of windings on the stator is proportional to kv-value, but inductance rises with the square of the number of windings. Given a fixed PWM frequency that increases the current ripple and therefore the eddy current losses. Going from 2400kv to 1600kv reduces those losses. And the list goes on and on.
I totally agree. I’ve been flying 4s packs since I started flying FPV back in 2016 I’ve tried multiple different 6S Lipo‘s, but didn’t see enough difference to make the switch. When actually comparing apples to apples as you say. Most of the time when people change to 6S they will end up putting a higher KV motor or making some other change that they did not realize and they feel that the quad flies better when in reality they’re not really comparing it correctly. For me, it is not worth it to pay a lot more money for something that is not giving you that much more benefit
Thanks for covering this Pawel, I've been preaching exactly this for years and no one seemed to believe in the maths... one person telling me that I was wrong because although his outright punch was reduced, his flight time was nearly a minute longer and so instead of not finishing the race with his 4s lipos hitting LVC, now he did finish the race. I reckon it must have been that his particular smaller lipo 6S setup was more current limited which had really just removed his natural propensity to not fly very efficiently... a natural governor I wonder if all the 6S hype was just a clever marketing stunt to make everyone feel that they had to go 6S or they were disadvantaged and that generated a fresh new load of component sales for the manufacturers and retailers.
Depending what you fly I found lower voltage on the same motor and kV is more efficient, opposite to my expectations. This is because the motor draws 100% throttle current in broken up "packets" so there is more loss due to resistance than expected, lower kV has higher resistance but lower 100% throttle current. Also higher voltage batteries are adding the internal resistance of the battery in series increasing the total effective internal resistance, though each cell does then output less current
I always thought 6s was introduced to overcome limitations of Lipo batteries, mainly in racing. If you can’t draw more current from them to increase power, add voltage instead 🤷♂️. It was just couple years ago 100C batteries were unusual and very expensive. Nowadays, flying 4s or 6s is just matter of preference, for average Joe, since they would not be able to utilize that additional power 6s potentially provides anyways.
4s has another advantage over 6s. If you also use a 5 inch ultralight long range drone a 4 and 3,5 inch freestyle drone on 4S you can use your 5 inch freestyle drone lipo packs for them as well... So you safe money and storage space...
You overlook the Joule heating inside the LiPo (whose ESR - at just 1.5 Ah - is likely to exceed 8 mΩ per cell), which is significant since it supplies all of the current for the entire drone.
Good discussion. But one thing you missed is that higher amperage causes efficiency losses in the coils and esc mosfets. 6s operates the motor and ESC cooler with lower amperage means less losses. High voltage is always more efficient and also in theory make your electrical system last longer because it's not getting as hot
I think that the battery sizes do not have a perfect match between 6s and 4s. The batteries that are more popular for 6s do have a little more power due to their size. I don’t feel like 6s adds more top speed but I do feel like there is less battery sag. This is the largest noticeable difference. I have to be more careful with 6s to prevent over discharging the battery.
Lipo sag! This is dominant factor in practice and should be the primary reason to move to a higher voltage "equivalent" setup or alternatively keep the same voltage but move to higher discharge rated cells. This can only be individually evaluated based on flying style and the quality of cells a pilot has access to.
Yes yes yes keep talking about 4s vs 6s :D makes my 2s lipos very cheap!! Erm 1 600mah 2s lipo gave me 9:59 of gentle pottering about on 5 inch props with a hexacopter lol
@@FPVUniversity Yea, but it is not because of battery, its "bigger battery", different motors, etc ... They want to say, "I like 1900kv better than 1600kv", and all that comes out is "6s is better than 4s".
One thing to consider is also pwn frequencies. High frequencies on esc is less efficient on ESC but mor efficient on motors. And frequencies can be higher at low voltage. So i dont know what is more important here the efficiency of the ESCs or the efficiency of the motors...
Agreed on all points for a constant wattage output. However 6s works better when you require large bursts of current. Since for a given wattage you pull less amps the batteries are able to not sag as much as 4s. Many years ago with 3D rc helis they also went from 3s to 6s to lower the amp requirements for the motors. Battery chemistry has always been the limitation so these are just work arounds.
Yes but you also need to also consider that a 40C 1000mAh 6s lipo will theoretically supply 40A constant discharge while the 40C (weight equalised) 1450mAh 4S lipo will theoretically be capable of 58A constant discharge. No?
@@paulhope3401 Good point however in this hobby when it comes to Lipos there is no standardized testing for C ratings. A lot of packs when subjected to the max C rating sag and output less current.
@@IanF-FPV Yes, I totally agree. I'm just trying to point out that it is just a big equation with known values that are inextricably linked, change one value and another has to also changed. Ie; a 1000mAh 6S lipo cannot be considered to be the same as a 1000mAh 4S, we must equalise via weight to be able to compare. I thought you missed that, thats all. Peace.
Really great video & profesional explanation ! In fact just switching from 4s to 6s in the same motor&prop might actualy lead to higher loses and overall reduced eficiency, as especially propellers are designed for maximum eficiency a certain speed to minimise drag & maximise thrust. And considering drag increases with V^2 , you might lose much more power due to aerodinamical efects in propeller.
There are optimum voltages for every sizes of quads. It is a compromize of 1) size and weight of the electrics because of voltage arc and leak insulation and between 2) the weight because bigger wiring and windings and cooling because of high currents because of low voltage...
As motors are switched with x kHz PWM, does higher voltage make any difference there and to what direction? Googled for a while but it seems to be a non-issue as not much was found.
Most people already know that watts is what flies your quad, and that current is what causes wires heating up which causes power loss. Being able to use higher voltage with lower voltage for the same amount of power allows you to lose less power to wire/connector resistance. But what it may have more to do with the use of XT60 connectors (rated to 60 amps continuous) for quads that pull up to or even over 100 amps. Some people have managed to *melt* these connectors in flight. Which means even when it doesn't melt, they're losing a certain amount of power to heat. So even if they do adjust the wire thickness for the voltage change, if they're not using eg XT60 for 6S and XT90 for 4S then the comparison is moot. Then there's the battery sag difference as mentioned by others...
Good discussion, but it's incomplete. What about voltage sag and it's effect on power? On 6s you quoted 14A and 4s 21A. The higher current on 4s will cause more voltage sag, unless using a higher C-rated cells. Less voltage sag on 6s will give more power if both 4s and 6s are same C-rating. Of course higher C-rated cells cost more (needed to reduce voltage sag on 4s so equivalent to 6s at max power). 6s electronics are likely to last longer as lower current, so not working as hard. There will be some price differences between 4s and 6s electronics, but usually higher current ratings drive pricing more than higher voltage ratings (in normal years). Would love to see this as a mini-series; building two similar quads; but 4s and 6s. Compare component costs, and weights, including batteries. Do some bench testing of quads at 50% and 100% power when battery are full and 50% to compare voltage sag, currents (thus verify power levels).
To be fair, the video is focused on efficiency, not flight performance. In more efficient throttle settings, there will be less voltage sag than if doing flipping flops with bursts to near max throttle (not efficient operation). In such cases 6s will have less voltage sag, thus output more maximum power. ie: Use case does matter ... free style, vs. cruising.
@@AerialWaviator I think this was clarified in a different comment. Effective capacity of a 4s is more as compared to 6s to arrive at the same power. It's more to do with the capacity rather than the C rating.
This video was great, and the comments as well. There must be more variables present in the end products, than in the principles driving the math behind them. I would love to hear more about these limitations and why the industry always leans towards higher voltage for larger multirotors. Seems like everything in the m600 or Alta x range run on 12s for a reason.
I'm not an expert by any means but I just thought of writing my thought on the subject as I'm also an avid RC enthusiast. Yes I do get the point you are making here which basically is a fundamental principle of physics - ie. the law of conservation of energy. But for our practical purpose in our particular application, don't you think it is only natural that we tend to get inclined towards what we experience in real life when flying 6s? If we do feel the difference and we like it, I'd say we would tend to go for it whether or not 6s is more efficient than 4s. Yes "better" is a relative term and yes, all else being equal, there would perhaps not be a difference. But as 4 is not equal to 6, theory may appear to contradict what we may percieve in real life. I switched to 6s and I'm loving it!😁
6S is MUCH more efficient, he screwed his calculation up badly. Electrical losses are based on (current^2) x (resistance), its not based on voltage drop like he showed.
Lower KV with the same thickness of the windings would mean more windings, right? Would not the same power through more windings give a more powerful magnetic field, and then higher efficiency / stronger magnetic field for the same electrical power?
Lower KV on the same motor size means you have to have more windings and you have to use smaller wire. So resistance and inductance goes up. Ergo, max current is more limited. And it adds nothing to efficiency. As yes, lower KV motor will require less amps for the same torque, but more voltage for the same RPM. Everything balances out and energy is conserved
@@FPVUniversity and keeping with the theme of magnetic fields, one way the strength of the field can be expressed is with "amp turns." If you have a nail and wind 100 turns of wire on it and run 1 A through it, it is the same as if you had a single massive turn with 100A. And if you have a limited "winding window area" then ideally you fill it 💯% and the current density and power loss will be the same. With motors, it gets more complicated because of the effects of back EMF as well. More turns means more back EMF and higher Kt and lower Kv. But even still it balances out. Impedances get transformed by a square relationship. Double voltage means half the current which means 4 times the impedance.
If i have a TATTU R-LINE 4.0 1550MAH 6S 130C Lipo, TATTU R-LINE 4.0 1300MAH 4S 130C LIPO Lipo Battery Packs with XT60 Connector, could i use DJI FPV AC Power adapter to charge it? DJI FPV power adapter is the one i use to charge my DJI FPV drone battery (Lithium ion polymer). Please advise. Thanks.
what might one study regarding battery internal resistance with respect to cell geometry (like prismatic, cylindrical, spherical?) and number of cells such that 'c rating'?
Generally individual cell resistance is very low for healthy cells. if same chemistry and cell capacities, a 6s pack will have lower resistance vs a 4s pack. There difference is small in comparison to the resistance of wire between the battery and motors (including ESC) What is more significant is the voltage sag on 4s vs 6s if pack have same C-rating. Thus 4s can not sustain same level of continuous power at high throttle as 6s. This is related to 6s needing to deliver less current. Using higher c-rated packs on 4s can even out the difference. This can be somewhat the case naturally as a 4s will have higher capacity (thus c-rating) vs 6s pack for same weight. This where debate gets a bit complicated as will vary based on battery weight and budget for what c-rated packs one purchases.
Have used 6s for 4years now Biggest difference i found is lipo lifetime due to less sag involved my 6s lipos have outlived my 4s lipos almost twice over
Oh well. I've been saying the exactly same since the beginning, Pawel. People won't hear. 😕 Anyway, thanks for the video! Stay safe there with your family! 🖖😊
Irrelevant in this aspect. 6S lipo with the same energy (1500 vs 1000mAh) will have smaller cells with higher internal resistance. But lower current, so it will be basically the same.
Question. I have a 4s hd whoop running 1303 5000kv xing's motors. Iflight Alpha A 85 HD vista version weight of 92 grams without battery. I have burnt out two of the motors. I have 6 nin 1203 3400kv motors on my bench. If I replace the 1303's for the 1203's will this damage the esc/fc? the 1203's are 2.5 grams lighter each saving 10 grams overall. Will the weight difference and kv difference make a big difference? Flywoo NIN V2 Motors 3400kv,
Less sag is because of less amp draw. If you are flying a 6s 1300, then it should be compared with 4s 2150mah. Simple physics. Then check the voltage sag. Ultimately 6s, thrust is not increased.
Like @ForbiddenFPV wrote. Less sag because current is lower. But is you scale C rating and capacity, sag will be very similar. Smaller on 6S maybe, but only a little smaller
6s should not be about more punch, and that leads to the only one thing you missed about the flighttime since batteryproducer do not offer proper 6s batterys. When you lower your amperage by increasing the voltage, you do not need 100c batterys. You could use lower c rates, what means the batterys would be ligther. I got a quad that flies about 10mins like regular freestyling or fast cruising with a lot of elevation changes and accelerations. Its a 5s that i run with a 1800mah 75c battery and i pull on full throttle about 90a. I could extend that if i find good batterys 6s 1500-1800mah and 50-60c. But the mainthing is the KV like you said. There is no point for having 6s with 1700kv for a 5", motor manufacturer should offer less kv motors like the flywoo nin 2306.5 1350kv for freestyle (2100kv on 4s)
All I know is my 6s packs lasted over 150 cycles before I had to retire them because I simply wouldn't get more than 2 minutes flight time out of them, where as all my 4s packs retired themselves before 70 cycles. 6s for me and I ain't going back.
What about the inductance of the motors? Resistance is not relevant when using high frequencies so inductance should be used as well. I believe a higher voltage will build the magnetic field quicker given the same inductance. It would be interesting to see how that balances out (If it does) when the motor inductance changes due to the change of kV. Technically speaking, does this also mean that the ESC frequency should change in order to make it a fair comparison and therefore there would be a difference in MOSFET switching losses? (Not RdsOn as that only matters when the MOSFET is saturated, I mean the resistance while it's transitioning which is the major source of heating and power loss in a MOSFET)
There is a lot of solid info here about electrical power, and you make a point about the small losses over small lengths of connection wires. But your model of motor efficiency seems far too simplistic and inaccurate. It seems like power dissipated in driving a motor is something like (a+b) I V + c I^2 R where the I V term combines (a) the mechanical power with perfect efficiency and (b) losses that scale linearly like friction, while the I^2 R term represents (c) losses due to electrical resistance or magnetic field eddies. It looks like good motors may hit 90% efficiency at peak, but that efficiency drops roughly linearly as rotation speed lowers and drops severely at speeds higher than peak efficiency. So it seems like the lower I = current required to achieve a target mechanical power on 6S instead of 4S could be significant if either (1) it's at low rpm where efficiency is poor, or even more so (2) it's a high punch out rpm where I^2 is huge. And particularly in regime (2) the 4S motors generate more heat, which increases R and further degrades performance. This comes from a few general searches, and I didn't put a full quantitative argument together. But it's enough that I worry something significant is missing from your presentation. What do you think?
@@FPVUniversity Yeah, but the induction will also be lower, respectively, the induction losses are also lower .. as a result, again .. the same spike, higher efficiency at the same power
I'm not trying to hate on you, I'm just trying to explain some things because it's pretty clear from watching this video that you don't entirely understand the physics involved here. Yes, power drives the motor, not voltage or current, but the conversion of power to heat is ONLY dependent on current, not voltage. So at the same power level, as you decrease voltage, and increase current, the heat production (wasted energy) increases, and therefore, efficiency decreases. More of the power is turned into heat, instead of turning it into thrust. This is why long distance transmission lines for electricity use high voltage, whereas the local lines use low voltage. Efficiency increases with voltage. Note that voltage drop has nothing to do with this. You are calculating efficiency incorrectly.
@Chris yeah but you need larger and heavier infrastructure on higher voltage for voltage arc isolation. Ok it is true that you also need heavier wiring and heavier windings for lower voltage. So it is complicated. Weight and size is important for aerodynamic aircrafts. That is why there is are optimal voltages for every size of quads... It is the optimum between weight and size because voltage isolation in cross to weight because wiring...
The alchemy is simple: its easier and cheaper to get crap technology 6s sell with discharge rate 90+ and it will have less sag than 4s that is similar technical level. If you get really high discharge rate 4s like 120+ you will have similar characteristics, but bats degrade, and after some time 4s will sag more than 6s..
I have 1 6s. It just worked out more efficient for 6s in testing. And I also can run 4s just less power. So went 6s. And still get use out of my 4s batteries on it when need be
@@FPVUniversity figured efficiency is making the same power using less wh. Doesn't always work out 6s. Depends on motors and weight and so much stuff.. cause I have seen a lot of motor thrust that 4s was more efficient. Just to be honest. The difference isn't much. And there can be error factor in human and weather. And sure motor stand test also have variances
@@divingfalconfpv4602 efficient is loosing less energy to things you don't need. In this case, heat. It's about energy conversion. How much energy will you loose somewhere between battery and motor magnetic field.
6s are more efficient not so much at hoover thrust but more full trottle. Because the lower peak current less batterij sack. Where it is 10mOhm x 100A is 1V . So less need for thick cable is lower weight. The key to higher voltage is lower losses in the cables. The difference is bigger if you go 8S or 12S problem is there are no 2208 700 to 900kv motor it is even possible to use 16850 batts 12S pack for 7” 9” endurance freestyler
That's "pretty-close" to what happens, but definitely not the whole story. Current (Amps) is what generates HEAT, Heat is wasted Power. With 2 "identical" quads, 1- 4S, and 1- 6S, compare the TOTAL HEAT that is DISSIPATED by EVERY PART, NOT JUST THE WIRES. The Motors, Circuit-Board-Traces, and ESCs all generate HEAT, LOTS OF HEAT, the amount of Heat is directly related to the CURRENT flowing through these parts, NOT THE VOLTAGE, and NOT THE NUMBER OF WATTS, ( Volts X Amps ). The only reason these parts don't fail from overheating is because they have a constant blast of Air blowing on them. . . .
Using P = I^2* R would be a more straightforward way to go about the calculations, but algebraically it's all the same. He computed the total voltage drop across the constant resistance sources, wires and motor controller FETs and such. You can easily manipulate the system of equations for power and ohms law to show that power loss is the result of voltage drop across the cables just as easily as the current through them. P = I^2 * R = V^2/R. you do have to be diligent about which voltage and currents are used in those formulas. V is not the voltage of the battery or the voltage applied to the motors but only the voltage drop across the transmission line.
Just for simplicuty I decided to ignore them. Also had no real life values for the lengths on the junction and its resistance. But relationship between 4S and 6S voltage drop will be similiar
So.. bearing in mind the price difference between building a 4s and 6s quad... Which would you recommend in order to fly longer and to be able to freestyle?
Engineer here. 6S WILL fly longer, period. This guy screwed his efficiency calculation up horribly, voltage is much more important than he applied. Efficiency decreases with current^2. 50% more voltage equals 50% less current, thus much less losses to heat. This is why local power lines are 10,000 volts, and long distance power lines are 765,000 volts. The power companies don't increase power up to 765,000 V for fun, they do it because its orders of magnitude more efficient.
@@Chris-hn4lp yeah but you need heavier and bigger infrastructure. And weight and size is important for aerodynamic aircrafts. That is why there is an optimal voltage for every size of quad...
@@FPVUniversity I get bored and land long before my 7" quad uses it's 6S 3300 mah battery, I still don't know how long it lasts, somewhere past 30 minutes, maybe it's the way I fly.
Very aggressive freestyle and especially racing pilots don't care about science. They use what gives best results..and in both cases 6s wins. That's why all top racers switched to 6s 3 years ago.
I think there's another 6S big difference not mentioned here: long-term savings on electronics. When using 6S set up with 4S motors we need to do motor limit (not throttle cap), for example motor limit 70%, and then in crashes we have less current to burn the fc/esc.
At 11:30 Does not the motor only use 2 wires at the same time ? The third being used to raise the position / speed of the magnets ? Those who fly in 6s with 1950 kv engines but fly in cruising mode actually use a lower range of use of the engines but it is also a more efficient range ! And have more flight time and lipos have less current to supply so also heat less and there is less heat loss. Idem for ESC, etc... Another small joy, with a 6s battery, there is a better chance of ruining the battery for an HS cell !
Since 4s battery has less balancing wires, less walls between cells and allows to use thinner wires it is lighter than 6S, that means it is more energy dense. Also it allows to make all the wires on a drone thinner. Thus 4S drone will be significantly more efficient than 6S drone.
It's not more efficient in terms of watt hours used. 6s systems use more watt hours not by much though. That means it's not more efficient than 4s and if you compare 5s you'll find it's the best platform that is in between 4s efficiency and power of 6s. Ryan Harrell talks about this in his yt channel
@@FPVUniversity example motor 1000 Kv, same esc, same prop. running at 2a 4s: 16,000 + rpm 6s: 25,000 + rpm No? Ps: I never get a reply when I have an actual question! Lol
@@samsonfpv8902 no. I mean rpm is correct but current is not. Current = Voltage / Resistance. Since resistance of motor is constant, current will grow as well
Here you have a completely different problem. Li Ion have higher internal resiatance and will sag more with the same current. On top of that, you can not build the same weight and energy 6S and 4S Li-Ion as 18650 are 18650. But the efficiency on the motors and loss on wires stays the same.
@@romanwalchli8550 I think, you should try 7-incher on 6s lio, and on 4s lio (yes with different kv motors of course). 4s1p just cant give you the same amount of power as 6s1p - current limit is the same, say 15C, but no limit on voltage.
