This is explained by our Eris guru Caddyman here along with many other tips for optimizing battery. The most important of the tips was to disable mobile networks in settings-->wireless settings--->Mobile network. Uncheck this when you don't need your data. I made a shortcut to wireless settings to quickly turn on/off this feature Anycut. I've noticed a HUGE increase in battery life. My phone can go 2 days without a charge until the end of the 2nd day. Seriously, that's big! If you just want to check your emails and stuff here and there, turn off your data until you are ready to do that. Some don't care about this, so not a big problem to them, but for some of you who need your phones to make calls throughout the day this is your biggest TIP/Trick. Do this for a day or two and you will see this is true. No joke.