Batteries have internal resistance - as the current flowing through a battery goes up, the internal resistance will cause the battery voltage to drop. If you're dropping below something like 4.2v on a 4-cell pack, I think that's where you start worrying about servos stalling. For instance, a pack that's already low (4.7 v, say) in a plane in a dive... pull the elevator to full travel and the air drag pushes hard against the elevator - current draw by that servo goes way up, as does the current through the battery - battery voltage drops, and as it goes below 4.2 (or some low value - the techies will correct me I'm sure), the servo starts to lose torque, the elevator tends to flatten out due to air pressures, and plane never pulls out of the dive....