I have not have any batteryproblems with my E66
Power
BL-4U 1000 mAh
Talk time (maximum): GSM 7 h 30 min; WCDMA 3 h 30 min
Standby time (maximum): GSM 264 h; WCDMA 336 h
Are u use lot of GPS ? I always have my device in carcharger when
use the GPS in the car
There are some solve to safe battertime if u use much wlan ,
I always have my devices in charge evry night when I sleep
and have not have any problem with the batterytime .
There are little tips about the E66 here
Nokia Europe - Nokia E66 support