Many thanks for all of your advice ! I have now bought 2 Ego C Twists which certainly seem to be an improvement from the standard 650 Mah's. I am still using the CE4's as like their simplicity and that the mouthpiece screws in properly rather than just pushing in as I have found with other tanks. I have now also tried an Evod and it seems to work better on 4 volts or above on the Ego Twist. I guess that the battery will put out over 4 volts even at less than 50% charge! but just run down more quickly. I have been told an innokin Iclear 16 is like a CE4 but some find them preferable. The Ego Twist seems to have given me the extra performance needed !