What is the power draw of an average 5 inch FPV-drone?

Question

I'm doing a project in school where I'm comparing different batteries for use with an FPV drone. To do that I need to know a how many watts the battery needs to be able to provide.

I found this motor test, but if I understand it correctly, the motor draws about 30 - 40 A on average depending on the propeller, and 150 - 190 A maximum. If this test is with only one motor (I assume it is, correct me if I'm wrong) that seems like a lot, considering that my drones draw about that much in total (4 motors + VTX, camera etc.) during flight. The voltage is about the same as my drones (4S), so that would result in a much higher power draw (About 9600 W vs. 2400 W).

Could someone help me figure out what is correct? Considering a typical 1550 4S battery can deliver about 200 A burst, it seems like I'm wrong about something.

Answer 1

I think those figures are looking at very short spikes as a motor accelerates from stationary. It's typical for permanent magnet motors to have a 'stall current' that's several times higher than the normal current at their maximum power speed.

Most 5" FPV quads use 30 or 40amp speed controllers on each motor. These would melt and catch fire at 150Amps unless it's only for a tiny fraction of a second. Motors, wires and battery plugs have a fair bit of mass to them, and can absorb the heat from being overloaded for a few seconds or even a few minutes. The solder connections and circuit board traces in an ESC do not, and overheat very quickly.

The 'average' column in those tables looks like the average current for a motor at full throttle on a test stand in stationary air, not the average current during flight. You don't use full power very often, or for long, and as the quad flies faster the motors don't have to work as hard to pull air into the props.

The proof of that is that a typical 1500mAh battery typically lasts 3-4 minutes (and we aim to leave 20-30% capacity in the battery for maximum lifetime). A 3 minute flight is a 20c discharge (30amp average). A 4 minute flight plus 25% safety margin is only 12c, which gives an average current of 1.5A * 12 = 18amps - and that's the total for all four motors.

FPV batteries need to be able to handle short spikes of power, up to the maximum of the speed controllers, but most of the time they're not working all that hard.

Answer 2

No, those figures are likely quite correct. High peak current draw is one of the reasons why drone battery leads are 12 AWG. Although, the MQTB motor test you're looking at is a fairly synthetic test that isn't 100% representative of what you'd see on an actual multirotor drone.

Here are some reasons why:

• The astronomical current figures at roughly >2/3 throttle aren't that common to see in a given flight. High throttle punches are usually a small percentage of total flight time.
• When the current draw gets too high, the internal resistance of the battery causes voltage sag that tempers/lowers power output. You can think of this as a kind of negative feedback loop.
• The peak figures he cites are almost certainly highly transient affairs, like for example when the motor initially starts from a stagnant state and requires a bunch of juice for a fraction of a second. I haven't tested this myself, but I'm fairly confident that peak current draw over a more reasonably long period of time would be a lot lower.

That said, peak current draw during a flight can still be quite high. Once well-calibrated, the current sensor on many flight controllers can paint a rather flattering picture of total current draw, sometimes showing hovering in the ~20-50A range and peaks in the ~100-150A range during punch-outs for reasonably powerful setups.