iPAQ battery dwindeling...

Yo all. I've recently had troubles with mu iPAQ battery. I have used it only 2 hours (Sound off, backlight off) And it gives me the very low warning. I used my mobile charger at that point.. But today I've only used it for 30 mins and Im already down to %70. I'm starting to get worried. Any idea's? I have an iPAQ 3765 with no expansions.