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Multirotors can usually operate with a larger battery than recommended for longer flight times but slightly less performance. However, there is a point of diminishing returns at which increasing the battery size will just make a drone so heavy that it uses more power to stay in the air than the added battery size provides.

How do I know when a battery will be too big and won't provide significantly more flight time? Is there some way to calculate the diminishing returns of increasing the battery size for a freestyle quadcopter?

My goal is to get the longest flight time that I can while only losing the upper-end performance which I don't use anyways.

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    $\begingroup$ I don't really think that questions like this are as useful to the community as they could be. A detailed answer would be too long and overarching because it would have to account for each of the different use cases for a drone aircraft. e.g. airplanes vs. freestyle multirotors vs. racing multirotors vs. camera platforms. Maybe most others think otherwise, but I think that questions asking about these specific types of aircraft would be more unitary and useful to the community. $\endgroup$
    – ifconfig ♦
    Commented May 7, 2020 at 21:29
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    $\begingroup$ @ifconfig In my question, I asked specifically about quadcopters, and the type of quadcopter (racing, camera platform, freestyle, etc) is irrelevant because any way to calculate diminishing returns of flight time in relation to battery size would involve the power usage/thrust of motors and the total weight of the drone so one answer would apply to any type of quadcopter. $\endgroup$
    – Jacob B ♦
    Commented May 7, 2020 at 22:12
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    $\begingroup$ Sure, but what exactly is a good battery mass fraction on a drone would differ dramatically depending on what it's trying to accomplish. Maybe this is something to address in an answer, not the question? Something here doesn't feel right to me. $\endgroup$
    – ifconfig ♦
    Commented May 7, 2020 at 22:37
  • $\begingroup$ @ifconfig I have added in some specifics about what I want the drone to accomplish which should help narrow down the answers and make the question more useful/relevant. $\endgroup$
    – Jacob B ♦
    Commented May 7, 2020 at 22:53
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    $\begingroup$ @JacobB, in your original question you asked about drones, and not multirotors. The world of drones is large and diverse, as ifconfig rightly points out. I have adjusted your question to reflect your clarification. $\endgroup$
    – Kenn Sebesta
    Commented May 8, 2020 at 12:55

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Short of very complicated maths, here is what I would do, and may very well help you:

You need to know the specifications of your motor - how much power do they consume for a given amount of thrust? You can find this out either by looking at the spec sheet, or I would just add masses in 25g increments and take a look at the black box recording.

Once you know this, you can know how much power it will take to lift the extra mass of the battery, you can use P=IV to figure out the extra energy stored in the battery itself and see if it is enough to counteract the extra mass.

This is obviously very rough - you would need a large sample size and to operate in lots of different conditions, but it would give you a good idea of if it is worth it before you even buy the larger battery.

Disclaimer: don’t overload your quad and risk burning out your gear - do this at your own risk.

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  • $\begingroup$ Also if you're using at least semi-known brand motors you can check the miniquadtestbench.com/motors for performance tests. Usually there are multiple results with different propellers and info about how much thrust does it provide and how many amps it draws. $\endgroup$
    – Nikita Pedorich
    Commented May 7, 2020 at 22:40
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I've heard that you want to keep your battery weight no heavier than 1/3 your drone weight.

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  • $\begingroup$ I heard this from Joshua Bardwell during one of his live streams recently. $\endgroup$
    – Schome1
    Commented May 8, 2020 at 17:44
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I don’t think there are diminishing returns, if the propeller+motor+controller efficiency stays the same regardless of overall weight.

Imagine a drone where all the weight is in the battery and the motors have just enough power to hover the drone. Double the battery and you’ll double the capacity but you’ll also double the weight (and required power output of the motors). So your flight time will always stay the same.

However, a real drone’s weight isn’t in batteries alone, though ideally it would be. Everything except the batteries is just dead weight when it comes to energy storage. So the more batteries you can add the less effect the other components should have and your overall flight time should increase.

All of this of course assumes that your drone has the same efficiency when running at higher power/rpm.

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  • $\begingroup$ I don’t think this is the case. Let’s take it to a ridiculous extreme to prove a point: if I try to use a 3000mAh LiPo on a Toothpick. The extra energy it takes to spin up the propellers just to get the quad to hover (if it is even able to hover) will be so much greater than if I had used a regular sized battery that at best the flight time would be unaffected (though it is unlikely) and at worst, the quad would be severely lacking in power and may even fry a motor trying to lift the LiPo. It is also worth noting that this is also o do with the other hardware: larger props are more efficient. $\endgroup$
    – Drones and Whatnot
    Commented May 8, 2020 at 10:31
  • $\begingroup$ This is axiomatically true, but unfortunately even from a theoretical standpoint a system's propeller/motor/controller efficiency cannot stay the same as thrust increases. The basic premise of this response cannot work. $\endgroup$
    – Kenn Sebesta
    Commented May 8, 2020 at 13:00
  • $\begingroup$ @Kenn - I think this response is the correct answer, but the question is flawed because it assumes that the limit for increasing battery size will be flight time, when it's actually performance - and the various rules-of-thumb (1/3 or 1/2 total weight) indicate that different people have different minimum levels of performance. $\endgroup$
    – Robin Bennett
    Commented May 10, 2020 at 10:26
  • $\begingroup$ You're making some assumption here that you're not stating. If you have just enough power to hover and then you add more battery weight, your flight time will be zero because you won't get off the ground. $\endgroup$
    – hobbs
    Commented May 14, 2020 at 6:09
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If you're like me and don't want to calculate: stay under 50% of your drones whole weight.

Probieren geht über studieren ;) - Trying helps more than calculating

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This question did not tell us if you are investigating only one change, the battery capacity, to an existing commercial quad, or whether you are designing a new custom quad that is not yet built. Trial and error methods such as adding weight to the current quad may be easy as suggested by @Drones and Whatnot. And I would refine this test method by looking at the available battery packs that you might consider and simply test the flight characteristics with the additional weight that this alternative pack would represent. There are only a limited number of batteries that would be feasible, so there would only be a limited number of tests that you would need to try. If the drone is still in the design phase, I would enter the parameters into eCalc. That would quickly tell you the calculated (estimated) flight duration with an alternative battery pack.

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