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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.

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closed as off-topic by Chris Stratton, PeterJ, Voltage Spike, Brian Carlton, Dave Tweed Sep 15 '17 at 11:14

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Chris Stratton, PeterJ, Voltage Spike, Brian Carlton, Dave Tweed
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Were this an engineering problem, you'd validate the components against their specs, or if they were original designs, you'd hook up debugging within, for example you'd monitor the command from your flight controller to your motor controllers, and monitor the motor controller internal algorithm. I guess you could still do something like add an independent data logging system reading the battery voltage and ESC commands and either record it or transmit it off device. \$\endgroup\$ – Chris Stratton Aug 27 '17 at 20:07
  • \$\begingroup\$ @ChrisStratton I'm sorry if this is the wrong place to post this, I'm aware this isn't Engineering per-se but I was unsure of where else to ask this question so I asked here. \$\endgroup\$ – Khurram Aug 27 '17 at 20:14
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From the entire explanation can be said that the voltage from battery is sagging on high load. Maybe have too small capacity, or the batt. technology doesn't allow a high flow of ions. At certain point the ions can't move so fast as the demand, so the voltage sags.

The ESC has inbuilt UVLO protection. The H-Bridge can burn at low voltage, since the on resistance can drastically increase, so this is a good feature of the ESC to shut down.

Try to focus on the chemistry of the battery, which battery can deliver such high current.

EDIT:

IMO, a 4200mAh battery is too small to supply 50A. At least it gots discharged almost immediately (some minute). Also, a wire with cross section of 14AWG is not to expect miracles.

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    \$\begingroup\$ The battery is spec'd to be able to deliver much higher currents than what I'm drawing from it (30C at 4200mAH = 126 amps) unless I'm not understanding the battery rating correctly. Either way, I suppose you're right, in that I do have to test out the actual capability of the battery somehow. \$\endgroup\$ – Khurram Aug 27 '17 at 20:22
  • \$\begingroup\$ Where do you measure the batt. voltage? It could be that you are using undersized cross section of connecting wires, or too long. \$\endgroup\$ – Marko Buršič Aug 27 '17 at 20:26
  • \$\begingroup\$ I'm measuring the voltage right on the battery terminals, I have a couple of 14 AWG wires going from the battery to the quad (through the shunt) and the voltmeter probes are connected right where the wires connect with the battery terminal \$\endgroup\$ – Khurram Aug 27 '17 at 20:36
  • \$\begingroup\$ Try doing the same measurements at the motor and the ESC, they may be considerably lower. \$\endgroup\$ – pjc50 Aug 27 '17 at 21:30
  • \$\begingroup\$ @Khurram My opinion still remains that the battery is the problem. You'll have to get a big dummy resistor to load it and then measure again. Some of the sellers are not to be trusted. I am waiting for some cheap battery tester from China, that measures cranking amps (lead acid bat). Maybe you will need some similar setup -to measure voltage within the rated load. \$\endgroup\$ – Marko Buršič Aug 27 '17 at 21:56

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