RC India

I have built my first quad the discovery clone with pixhawk2. 4.7 and using Bonka 4S 4200mAh 35C lipo to power the same. The quad is having low battery failsafes much earlier than what I have kept as low battery failsafe value. I kept the low battery failsafe at 14.8V but after landing the quad the battery checker shows around 15.3V, maybe the lipo is having "VOLTAGE SAG". Don't know what is the exact problem??
 I did bench testing and found something. First I did calibration of power module for voltage then I attached the props on the motor in a different way such as CW prop on CCW motor and vice versa, so that it will keep the quad at ground and will generate thrust towards sky hence it will push the quad towards the ground ( Just for testing purpose such that it will simulate the inflight characteristics especially the voltage measurements by the pixhawk power module) . I took the readings from battery checker at balance plug and the power module of pixhawk (Volt readings in mission planner) to compare the voltage difference. When quad is armed the voltage reading is quite OK! but as I increased the throttle the voltage difference is increasing rapidly. The approximate values are....at ~0% throttle voltage diff is ~0.06V, at ~17% throttle voltage diff is ~0.11V and at ~34% throttle voltage diff is ~0.19V and at ~50% throttle the volt diff is ~0.29V..........The difference between voltage at balance plug of the lipo and the voltage measured by the power module of pixhawk in the mission planner, is increasing with increase in throttle, Is this normal??
Because of these voltage difference the quad is having early low battery failsafes, does anyone knows what could possibly be wrong............ There's nothing in between lipo and power module, just a xt60 extension cable about 8 to 10cm. Both the battery checker at balance port and the power module voltage sensor should show equal voltage considering the load and resistance ahead of power module, isn't it?