Keeps my HP 3715 running for days. I generally use it in place of books. Even without turning the PDA off, kust letting the backlight dim out, I still get several days of usage. If I used the PDA for much more, I would probably loose some up time though. The only issue (and it's not really an issue for me) is that it changes the size of the PDA. It still fits the cradle just fine (good going HP) but hard case and such no longer fit. The seperate charger that comes with it is a bonus. No extra power supply to plug in (it taps into your existing cords) and it charges either the regular battery or the extended battery.