Low battery?

I flied my copter, RTF X8, these days. It always fell into failsafe due to low battery even after I calibrated the power module. I tested the battery using voltage meter. It showed 15V and the voltage shown in the GCS is about 14.6V. However, the flashdata log showed that it return home due to low battery. It also generated sound to indicate it was low battery when it was landing. I have attached log file. Could you help me figure this problem out, please? I really appreciate it.

The battery voltage under load is different from the reading of the battery with a meter. When the motors are drawing current the voltage the voltage reading drops. If you clibrated the power module it should be correct.

Dear all,

Exactly, the the maximum volatge of 4S battery is 16.8 V. In the parameter list, the voltage to trigger the battery failsafe is 14V and the battery voltage to trigger the warn in the Mission Planner is 9.6V. When the motor is rotating, it will generate EMF which will decrease the voltage between the output pins of the battery. So the voltage between battery when motor is roatating is much lower than the one when copter is disarmed