Agreed with David....the 4150 is definitely a compact beauty.
Listening to music with earphones(stored in SD ram) for hours did not seems to drain out the "normal" battery...at least for 5-8 hours. David...u should consider getting an SD storage and u will really love this baby...
However, doing with WIFI for 1 hour will definitely see a dramatic drop in power. It will normally give a warning when it reaches 30%.
If necessary, a backup battery should do the trick. I was wondering how much more power can the "larger" battery provide..another 1 hour or less????
Anyway, 4150 is meant for quick download of emails or checking the net...and certainly not for long surfing. As such, it has served its purpose. If there is a need to do it for a long period of time, then I would advocate plugging to the charger simultaneously.
Note: In case, you need to reply your downloaded emails (TONs of them). then disengage your WIFI or Bluethooth (David note this as well). Reply all your mails and then reengage your WIFI and send them all out. This way, yr battery will last enough for its decent function..