only 1.2 volts. lets say that you draw 1 amp at 6 volts. thats about normal for a weaker servo. amp x ohm = volt. so your ohm rating is 6 ohms. at 7.2 volts with 6 ohms your amp draw is 1.2 amps, a 20% increase. most 6 cell packs charge up to 8 volts at least, some are closer to 9. a fresh pack at 8.5 volts would force the servo to bite 1.41 amps, a 41% increase over normal voltage. since volts x amps = watts, 6v is 6 watts, 7.2 volts is 8.64 watts, and 8.5 volts will make the servo dissapate 9.71 watts. thats a 62% increase in load from 6v to 8.5v, and a 44% increase of load from 6v to 7.2 volts. even though its only 1.2 volts, its a big step for a servo to cover. even going 25% past a manufacturing spec is pushing it, going 62% past takes luck or careful planning.
these numbers were pulled outta my ***, and servos' resistance changes according to load. but, most servos do draw about one amp at 6v under load.
these numbers were pulled outta my ***, and servos' resistance changes according to load. but, most servos do draw about one amp at 6v under load.