Hi I have a HP2490 unit wm5 and have a problem with the battery life. if i use the unit constantly, the battery life is satisfactory. should i work the batt to between 50 & 60% and switch the unit off, the battery runs down at a tremendous rate (sometimes as quikly as 4 hours in standby mode!. this does not happen constantly and cannot be replicated at will. Any body any advice??