So I formatted my phone today. I have not used it, and it was at 100% this morning. No apps installed, the screen was only on for about 10 minutes. Yet when I go to check it now, about 12 hours later, its battery is at 13%. I got a new battery a few days ago, so the battery isn't worn out. My phone is a myTouch 4G from t-mobile. My android version on it is 2.3.4
If you're running the phone without a SIM card this might also be the reason:
Some phones aren't tested well if there's no SIM inserted (my old Xperia X8 is such a phone for example).
Narrow it down this way: