Battery life in dual-boot HTC HD2

I own a dual-booting HTC-HD2 with MIUI on SD card. I already knew, from tutorials, that in order to maximize battery life I should first-boot Android only with 100% of charge. This is currently unclear to me. First, I run Windows Mobile 90% of time, and I use Android occasionally only for some programs that are not available in WM. It happened that I needed to boot Android when on the street, with less than 100% battery. However, I recently noticed that my battery is descharging really fast, for example today from 7:30am to 10am it ran from 100% to 68%, and I really wonder if it could have lasted till 8pm (when I get back home). Is it because Android should **always** be booted with 100% of battery, or because I run 24/7 Skype and Windows Live Messenger in HSDPA from Windows Mobile? Thanks