Why does my myTouch 4G lose battery so fast? 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
You can install BetterBatteryStats (from Play Store or from XDA, the latter is free) and see which is causing this.

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:  


Switch the phone to airplane mode (switches wifi/bluetooth/gsm off)
Then re-enable wifi (plus maybe bluetooth) again. Just leave it for a while and see if it's better now.