Draining too fast

Three weeks ago, I excitedly rant here that my younger sister gave me her old iPod Video 30GB since she got her new iPod Touch 16GB. I’ve been using it almost every night before I got to bed and it works pretty well. But sadly after some weeks of using it I noticed it easily runs out of battery, unlike before when my sister still using it, it takes at least half day before it goes low bat. I think I need to replace the battery, sister told me that she haven’t replace it  since the day she purchased it. So this weekend I’m planning to get at least two replacement batteries so that I have back up battery in case the other one fails.