I try to find out the relation between rpm vs. thrust for a battery+motor+propeller combination. the image shows my setup and also the measurement result. Can anyone explain how l should use this datas (I know Kv.v gives the rpm but my voltage values decreasing because of P=V.I relation etc.)
Okay I thought I'd move this to an answer because I think this will help clear up some confusion you have.
First, if you are trying to plot RPM vs Thrust, then your battery, ESC, power, etc. have nothing to do with RPM vs Thrust, assuming the battery and ESC are sized correctly to provide adequate current to the motor.
It's like saying you want to measure a car's gas tank to see how much traction a car has at a given speed. As long as you're getting enough fuel (current) to keep the motor running, the size, shape, capacity, or chemistry of the fuel really doesn't matter.
Secondly, your voltage is not lowering because of P=IV. The only way that would work is if you are consuming constant power throughout the range of motor RPM, and this is absolutely not the case. In actuality, your battery voltage drop is more likely due to the fact that a real battery is well modeled by an ideal battery and a battery resistor in series, where that resistor represents internal resistance in the battery (generally a few milliOhms). As output current increases, V=IR, so the voltage drop across the internal resistor increases. Because it is in series with the battery, output (terminal) voltage goes down.
As a side note, P=IV, and V=IR, so you can combine those two to write $P=I^2 R $, where P now represents the power being "burned off" by internal battery resistance - this is why batteries under heavy load get very hot.
You didn't publish your test procedure, so depending on how long you took to do the test, coupled with battery C rating and capacity, your voltage drop may also be influenced by regular voltage-state of charge curves. Try performing your test in reverse (high speed first) and see if you get the same results.
Ultimately, my advice to you is, if you want to know rpm vs thrust, measure those two values and ignore everything else. They shouldn't have any impact.
Try to couple the batteries with voltage regulator. A simple IC DC regulator would do. It would help maintain the input voltage of the motor and helps getting the relation between RPM vs Thrust independent to the voltage irregularities.