I started making my first ever quadcopter last week, I didn't really have any idea about the components to use beforehand and I ordered somewhat randomly after reading up a little bit about the ratings and incompatibilities of components online. The components I have are;
- 4x A2212 930KV BLDC motors rated at a max current draw of 12 Amps
- 4x Hobbypower brand 30 Amp ESCs (Electronic Speed controllers) w/ SimonK firmware
- 4200 mAH, 3S, 30C Lithium Polymer Battery (DJi Brand)
Other components are unrelated to this question, as such I'm skipping them.
The problem I'm getting right now is that the quad doesn't seem to be operating at lower battery voltages when taken to full throttle. Once the battery falls even a little bit, the quad motors start to "Jerk", quickly losing speed and gaining it back again only to lose it again.
My first instinct was to measure the current flow out of the battery. I don't have anything to measure high DC currents, my DMMs only go up to 10 amps so I used a resistive shunt (made of some very thick wire) and put it inbetween the battery and the Quadcopter and measured the voltage drop across it. These are my observations:
The first readings are with a fully charged battery, the idle battery voltage (with just the quad's electronics powered and no BLDCs running) is 12.5 volts which goes down to 11.7 volts with a 48 Amp current draw at full throttle. The motors are rated at 12 Amps each full current draw and I'm assuming there are 15-20% losses in the ESCs. The spec lines up almost perfectly with the results here.
The second set of readings are with a battery discharged to about half capacity, the battery sits at 12.05 volts while idle and goes down to 11.2 volts at max current draw. The problem happens when I go above 60-70% throttle at this voltage level. The motors speed up as I take the throttle up until I get to 11.2-11.3 volts on the battery, then the motor "Resets" and "Bounces" back to a lower speed until the voltage pops back up to the 11.5 volts level, it then tries to speed up again until it gets to the same 11.2 volt level again and the cycle repeats.
From this experiment I concluded that the ESCs I'm using might have some sort of minimum voltage cutoff built into them, so I set up another experiment to test that out. I took the propeller off of the motor and ran it from my bench power supply, I hooked the ESC with an Ammeter in series and measured the current flowing into the ESC at different voltages (randomly selected). The results were;
With no load attached to the motors, the Motors / ESCs operated right down to 10.2 volts which is actually below what's considered safe for a LiPo, below this voltage, the motors powered up once in a jerk and then slowly shut down. I experienced no such cutoff / jerking in this test that I experienced in the previous test and the ESC's battery cutoff seems to be well below the 11.2-11.3 volts that I get down to in the previous test.
Now I'm wondering if the problem is with the battery not being able to supply the current it needs to. The battery is rated 30C, at 4200 mAH capacity this should be good for atleast a 100 Amps (taking into account battery degradation). The battery supplies 48 amps in the fully charged test without problem and the limiting factor there was the motor's max current draw rating (12 amps x4 = 48 amps + losses)
I don't actually have an electrical load large enough to draw 100 amps from the battery to test it but if there's no other way around it, I'll have to make/buy one (glow wire? heating element?).
I'm unsure of what else to test / measure and how to do it, from what I can gather, the bottleneck in the system is either the Battery, the ESCs or the Motors. I don't think the motors are at fault so its either the ESCs or the Battery, what is the easiest way to test this out and deduce the bottleneck? (Short of getting another battery and different ESCs to test with).
I'm really really sorry for the wall of text, but this problem has been troublesome to troubleshoot and I wanted to give out as much information as possible so that someone might be able to think of a solution.