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I am in the starting phase of building a fairly large octo-copter drone. I am currently thinking about the possibility of powering my octo-copter design using 4 separate batteries where each battery powers 2 motors each, in order to get around the problem of the large current that would pass through the single central line (causing heat and voltage drop etc.) if I where to connect all batteries in parallel.

For sure there would always be a slight voltage difference between each battery and if you keep flying for too long you are gonna get one battery that runs out before the others, but I am theorizing that it shouldn't really matter as long as I make sure never to completely exhaust any of the batteries. The flight controller should compensate for the lower voltage provided to each engine. That is my theory at least. I saw a video of these guys doing the Hobbyking Beer-lift challenge with a really large drone, and they seemed to be using something that seems to be 4 separate circuits, one for each motor. Has anybody done something similar to this, or have any experiences about if it's a good idea or not?

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I agree with lenik first statement, you idea is totally sound.

Also your grounding to not have all the current drained passing through a single 'hub' is sound as otherwise you will need this central hub to be able to handle the total current drained which would result in extra weight.

The two drawbacks of your solutions that I see are as follow:

  1. changing battery pack is going to be more tedious than in the case you could have snapped several battery together

  2. as your copter is not going to be perfectly balanced and symmetrical, you will have one pair of motors draining its battery faster than the other, this will reduce your flight time

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  • $\begingroup$ But if you notice that one battery is often drained first you could try shifting some weight around to compensate for that. This would require you to do so, such shifting the mounting position of each battery (so not a fixed socket where they fit snugly). $\endgroup$
    – fibonatic
    Jun 29, 2018 at 2:11
  • $\begingroup$ I guess you could compensate some non-symmetry by shifting weight, but in the end you will still have some difference of parameters in the motors and propellers (the cheaper the more). Also external disturbances like wind blow or motion of the payload while be hard to repeat from one flight to the next $\endgroup$
    – N. Staub
    Jun 29, 2018 at 6:48
  • $\begingroup$ Good point Staub. Thanks. Yeah I was just feeling hesitative to have a single point/wire in the system having to take all the current loads of the whole vehicle, as would happen if you connect them all in parallel. I guess for the uneven discharge problem I'll just have to tell the weather gods to stop blowing wind while I'm flying. :) Actually, about the start-up, do you see any problems with the power-up of each individual ESC not being at the same time, and not at the same time as the flight controller, seeing as they usually all have to sync with each other? $\endgroup$
    – RalleGAB
    Jun 29, 2018 at 8:42
  • $\begingroup$ I don't really see an issue there, but I never worked with commercially available products so i can not guaranty anything in their variety. If you see issues may be you can make a kind of switching system based on relays and a small AAA0 battery. $\endgroup$
    – N. Staub
    Jun 29, 2018 at 9:17
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There's nothing wrong with your idea. You may build your copter with multiple batteries and it would work just fine, provided you do everything right.

The only thing I would like to raise is, that your stated goal (getting rid of the heat and voltage drop) has absolutely nothing to do with this design choice. Using multiple batteries you'll get exactly same heat, and exactly same voltage drop, provided you use similar wires in both cases.

From my point of view, building an octocopter with separate batteries is reasonable if you want to solve the problem of one or two motors/controllers dying mid-flight and still being able to pilot the thing and land it safely.

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I'm a little late to this one but i think there is still valid points not made:

The idea is ok, but i would not consider it sound, primarily because of safety reasons. If one battery fails or gets too low you still lose the whole platform. As far as i know there is no autopilot which compensates for more than one random motor/esc failure.

The safe way for big drones is using high capacity Schottky diode to make redundant battery harnesses, which prevent over-draining or failing batteries to be loaded by the good batteries.

Like this you have the advantage of monitoring your batterie performance over telemetry because you can see the combined voltage level as one. In your suggested separate case you would need to monitor each voltage which is quite a burden.

Personally I don't think you save a lot of weight by not using a central PDB which you can get for very high voltage ratings, the wires may be chosen a little thinner but if you go octocopter you go large anyway, so i think safety is more important than 200g of extra weight.

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