Actually i guess is the charge cycle cause as i know Li-Ion can stand up 500 charges then it get started to defect. hmmm 500 charges which means should last 2 years unless u charge more than 1 time a day. Anyway besides that i believe is also part of firmware doing. For example now my W850i (R1KG001) consume more energy compare to the previous firmware. For later firmware i guess the most is 2 and a half days.