I have a Phantom V1 that I operate with a Futaba 8J transmitter. Using Duracell alkaline batteries in the transmitter causes a voltage drop every five minutes...6.0 volts. 5.9 volts, 5.8 volts, 5.7 volts, etc. The Futaba manual says using 4 cell Ni-Cd rechargeable batteries (1.2 volts x 4= 4.8 total volts) is acceptable. (the manual also says that the Futaba 5 cell rechargeable battery pack (6 volts) will discharge quicker than the lower voltage 4 cell batteries. How low should the transmitter voltage be allowed to drop before risking loss of control?