Why are my motors doing this weird power loss when running at approx. 75% throttle ? Thrust and amps go up nicely till about 75%, then suddenly I loose a lot of thrust and the amps actually go down. Please see attached video for a better understanding.
(This is done while bypassing the APM, and with a 100 amp capable battery, the ESC is rated to 25 amp, and my wattmeter never shows more than 25 amp (approx. 15 amp when the issue starts))
First, don't believe the battery rating. Burst is not the same as continous. This is most likely the culprit.
What battery exactly are you using? I got the feeling the ratings are way out of whack.
The only other answer can be that wiring or a connector is getting hot increasing resistance and thus lowering the power. What does the voltage at the input to the ESC do? Betting you are seeing a drop.
Oh, I guess as a really distant answer is that the ESCs and motor are mismatched timing. The question here would be if one motor by itself runs fine, then it's battery or power distro for sure.
The battery in the video is a 5000mah, rated at 20C. I pull at the time no more than 20 amps, i.e. 4C.
Just checked, voltage direct on the ESC looks fine till the weird stuff begins.
And as soon as I drop the throttle, it's immediately is fine again (can't be resistance, can it ?)
Right, so it's what I said, you have a faulty (AKA rating is higher than what it puts out) pack. Maybe it's only one cell that is weak, but the pack is not capable of providing power (constant Voltage and increasing Amps) to support the load. Put it this way, the power has to go somewhere if the battery was providing it. If the voltage is dropping, that normally indicates power is going somewhere heating something up. You said that's not the case (something would be glowing red in this kind of instance to take the kind of power drop), so the only option is that the battery is not providing it.
Again, if the ESC/motor was drawing (conducting) enough power to see a significant voltage drop at the ESC leads, then the power (Amps X Volts=Watts) is going somewhere in the form of heat.You didn't say anything was hot or smoking, and trust me, it would be IF the battery was providing the power. Further, you put a power meter in between the ESC and battery, it shows the battery providing less power under heavy load (Both Voltage and Amperage drop). The only good answer is a battery not sustaining the load. In your case, yes, a 5000mAh should be fine with that load and clearly is not.
This actually happens a lot. I had a brand new 3cell 3300mAh pack die on the first run, and the cell never came back. In fact, that cell reads dead short, but never puffed or did anything. Thanks local hobby shop, who also won't take it back even though it never even made it through a single charge/discharge cycle and did not puff. It was just a defective battery.