Back in the 1980's I worked at Northern Californias largest hobby shop. We used to set up some of the off-road R/C cars with servo power coming straight from the motor battery pack. At the beginning of the run the S148 servos would be getting a tad over 8 volts. Never smoked one. They will be perfectly fine with 6.5 volts. I used to set up my Hirobo Shuttle helicopters with the same servos and 5 cell nicad packs, the additional voltage would spin the gyro faster for faster gyro response.