While on 8.1.2 my battery life was hit or miss. It would go from 100% to 50% in 2 hours without even using it. Other days it would be fine and last all day. I never figured what what was causing it... probably some Cydia app. I just upgraded to 8.4 and jailbroke with TaiG... now my battery life is awesome. I've reinstalled most of my important tweaks, so one of the ones I haven't installed was probably causing the issue and if I figure that out I'll be sure to post it. The main one I'm waiting for is HomeScreenDesigner. Once it is updated I'll install it and see if the battery issues come back. I did found out that StatusBarWeather8 was causing apps to crash. ESPN was one of them. I emailed the dev but got no reply (big surprise). Anyways, if battery life is an issue for you on 8.1.2 maybe 8.4 will help you like it did for me.