Mick,you could say which model of receiver you have.
One of the latest 3-ch synth receivers needs 6v bec or 5 cells
Futaba and Hitec go into detail on their websites.
5-cell (6.0V) receiver battery packs – All Futaba airborne equipment (except that which is specifically labeled otherwise) is
designed to work with 4.8V (Ni-Cd 4 cells) or 6.0V (Ni-Cd 5 cells or alkaline 4 cells). Using a 6.0V pack increases the current
flow to the servos, which accelerates their rate of response and their torque. However, because of this faster current draw, a 5-
cell battery pack of the same mAh rating will last approximately æ the time of a 4-cell pack.