Yeah silly me forgot motors require power, not just amps
The TRUE ANSWER about efficiency is “it depends”……
The quest for efficiency is much more complex than just considering resistive losses.
If the operating point (speed and load) requres the ESC to greatly reduce the operating voltage sent to the motors, then the efficiency will be poor. The ESC is working like a buck converter and “chopping” the battery supply a lot which is inefficient (much more significant than I2R losses).
If the required operating point can be achieved adequately with a lower 1S battery pack voltage, then in theory this can be more efficient.
Propeller and motor matching is also a very significant part of increasing efficiency.
- Deciding to reduce the maximum “target thrust” for a drone can open up possible changes for efficiency.
- A larger diameter, shallower pitch propeller is much more efficient than a small dia. steep pitch prop. (but you need to match a different motor to the larger prop)
- Lower propeller RPM generates more thrust per watt than high RPM.
- Unfortunately, motor efficiency goes the opposite way. A motors used for drones generally are more efficient at higher rpm.
Search the web and you can find charts and graphs providing snippets of information on all the above.
If you are pirmarily chasing “efficiency” rather than “performance” like for a long range drone, then the trade off between the two becomes easier. Expecting both is unrealistic.
This post will probably trigger discussion. There are many opinions on all this, so I welcome any fact based comments……HaHa 
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Please see my earlier posts on this. Here is the Toyo robotics link again:
This is correct.
Simplifying greatly, the current through the motor winding can’t change rapidly, unlike current through a resistor. This is the property known as inductance.
Applying a voltage across the inductor causes the current through it to start to change “slowly” proportional to the applied voltage.
At switch on the current through the power leads ( ignoring their small inductance) will instantaneously become the same as the current in the inductor, independent of applied voltage,
however at (say) twice the applied voltage, the power only needs to be switched on for half the time to produce the required change in current through the inductance of the motor winding, so the current only flows in the power leads for half as long per cycle.
Therefore the average power lost through the resistance of the motor leads is actually inversely proportional to battery voltage and so power lost in the leads at twice the supply voltage is halved,
hence why mains power is often carried around at very high voltage to dramatically reduce resistive losses in the wires.
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impressive!i wanna give a try
Please see the youtube videos by SiieeFVP regarding Efficiency V Battery Voltage.
https://www.youtube.com/watch?v=ijKXsclb9eE look at 2:30 and 5:30 graphs.
The red points are 2S, blue 3S and green 4S. Left axis is Efficiency, bottom axis is RPM for same motor and prop. (the same RPM is the same thrust for all three battery voltages). These are real measured data points, not theoretical.
KEY POINT: Efficiency of a dronecomes from the complete system, not just the copper losses in the wires and motor. The efficiency of the ESC is rarely measured or discussed, but the above chart clearly shows it.
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The observations by skyscraper are correct regarding copper losses due to wire resistance.
The ESC however has a larger, opposite effect on efficiency.
A higher input voltage requires more “chopping” to reduce the output voltage to the motor. This chopping of the current is not efficient.
Please see the chart and youtube video
Yes. It is an interesting video. Its always good to do the experiment. At the end of the video he does ask the question why this is happening, and that would be a good follow up video if he can get some answers.
You could start by finding out where the extra power is going. Do the motors get hotter at higher voltage or does the ESC. Looking at the switching waveforms on the motor and fets under the different voltages would also be interesting and might help to understand what is going on.
I believe blheli32 ESCs have an option for FOC mode rather than using square waves.
Those data points are real, so that’s enough information to make a decision about trying a lower battery voltage (so long as the 100% throttle rpm provides sufficient maximum thrust).
Here is an answer from Google:
Using a higher voltage for the same RPM and power output in a brushless motor setup
actually decreases current, but increases switching losses and core losses (hysteresis/eddy currents) in the ESC and motor, making it slightly less efficient because the ESC’s MOSFETs switch faster (narrower pulses) and the motor’s iron core saturates more, generating more heat, though the overall efficiency difference for small voltage changes might be minor compared to the significant current reduction.
Here’s a breakdown of why:
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Current Reduction (The Good):
- Lower current reduces resistive (I²R) losses in windings.
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Increased ESC Switching Losses (The Bad):
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Pulse Width Modulation (PWM): The ESC rapidly switches the motor phases on and off. A higher voltage requires the controller to use narrower pulses (shorter “on” times) to achieve the same average voltage and power.
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MOSFET Switching: Each time the MOSFETs in the ESC switch, they briefly draw significant current, causing energy loss as heat. Faster switching (narrower pulses) means more frequent switching events, increasing these losses.
-
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Increased Motor Core Losses (The Bad):
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Magnetic Saturation: Higher input voltage can drive the motor’s iron core (stator) closer to or into magnetic saturation.
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Hysteresis & Eddy Currents: Saturation increases the magnetic field strength, leading to greater hysteresis losses (energy lost in re-magnetizing the core) and eddy current losses (circulating currents in the core).
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In Summary:
While you reduce I2R losses by lowering current, the trade-off at higher voltages (for the same output) comes from the ESC’s MOSFETs working harder (more switching) and the motor’s magnetic core experiencing more losses due to saturation, making the system slightly less efficient overall, even as peak currents drop.
That’s interesting. It may require a hall sensor motor. I wonder if anyone has tried this for a drone.
I ran it by ChatGPT(YMMV 
) For the 1 hour 250g quadcopter application, it seems to agree that 2S is about right, but for larger quadcopters higher voltage is better. ( so you select a proportionally lower kV motor to match) Quoting from a somewhat larger discussion:
For a given RPM and torque:
The sweet spot is the lowest voltage that allows
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Reasonable current (<~30–40 A per motor for 5″ class)
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Wire diameters below skin-effect dominance
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Phase inductance high enough to suppress ripple
Translated into quadcopter practice:
Prop size Typical sweet spot
≤ 2″ 1S
3″ 2S
4″ 2–3S
5″ 3–4S
6–7″ 4–6S
lol when you ask gemini about 1 hour 250g quads it starts quoting this thread.
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It’s funny what you get from an AI in a niche field. I work in one, a type of sensor, and they all produce mostly useless response. They do usually produce the company I work for though so for marketing purposes there is that
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BLDC motors are a “niche field”? AI is just a tool and can make stuff up, but ChatGPT seems to have it about right in this case. Larger quads tend to use higher voltage batteries and small ones lower voltage batteries. Looking around that appears to be the case in practise.
LoL, I love how it contradicts itself. 2.6h theoretical flight time of “all battery” craft means that you could achieve 1h with 40% battery fraction. MRs here have ~60% battery fraction.
20-30W is a bit optimistic, my heli takes ~28-29W in hover @~230g (2-2.5W for avionics, 4-6W will be added for digital FPV). @geofrancis do you have power draw for your 250g quad?
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Unfortunately none of the small boards have current monitoring or even a sd card for logging. I looked into an i2c power sensor like the INA219, but they are either heavy or have a current limit that’s too low. I would need to up rate one with lower resistance shunt resistors.
I built an APM hobbyking 450 heli years ago that was really efficient, it flew for 35 minutes on a 5000mah 3s battery and I seriously thought about doing another helicopter to try and get the hour before I looked at the mavic minis and ran the numbers.
Simply the mavic minis can fly for 30 minutes on a 2500mah battery so if you can remove all the extra parts like the gimbal and weight optimise all the other components then you can double the battery without increasing its weight over 250g, doubling the 30-minute flight time to give you the hour. This was the logic that I used to come up with this design and why it uses mavic motors and propellers.
by optimising it so that it’s now:
20% under 250g, (210g for the latest version)
6250mah battery thats 2.5x the capacity of the mavic,
it now flies for much longer than an hour.
Just out of curiosity, I connected the v2 quad with its AIO flight controller, AIO analogue FPV camera and mini GPS/compass to a bench power supply, and it pulls 2.15W at idle with RC and telemetry connected and camera on.
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Fair, AFAIK Ardupilot doesn’t work with INA219, at least I didn’t get it working with it. You need INA226 or 228, they are better than 219 anyways. Replacing the shunt is what I did.
Wow, that is impressive, hard to beat with heli complexity at this scale, maybe if it was monocoque airframe out of a high-performance resin with decased servos with more tightly sized motor and integrated avionics on 1mm boards, I need larger R&D allowance 
Wow, I get 2.5W at idle without the camera though I stuffed in the optical flow sensor now. H7 and Digital FPV suck a lot more power .
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You have to realise that ChatGPT is the ultimate “Yes man”. If that is the answer you want , that is what you will get.
I got the opposite
Is a 1-hour hover at ~250 g theoretically possible?
Yes — if you achieve:
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Extremely low hover power (e.g., ~30–45 W or less)
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Very high-specific-energy battery (~220 + Wh/kg)
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Very lightweight and efficient airframe and motors
That combination could allow ~60 min hover in benign conditions. The ArduPilot build reports such a result, which is not impossible in principle. ArduPilot Discourse
Does this violate physics?
No — it simply operates at the lower power regime that small quads can sometimes hit if everything is perfectly optimized. But the margins are tight and achieving >60 min under real world conditions (wind, payload, GPS nav) is far harder.
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