A smartphone battery interface should provide ener-gy-related information efficiently with high accuracy be-cause it is the basis for user actions on battery consumption. Previous studies have mainly focused on battery information per se, but the effectiveness of user interaction with the interface has barely been studied. In this paper, we first discuss the results of a survey on different types of ener-gy-related information and features to investigate what users want in terms of battery consumption for smartphones. We then introduce Powerlet, which is a new battery interface that attempts to actively interact with users to provide battery usage information. We validated the efficiency of Powerlet with real users' experiences derived over a seven-week study period. We found that the users actively managed and changed their phone usage based on the energy-related in-formation provided by Powerlet. With the Powerlet active battery interface, participants reduced daily energy con-sumption by 8.2% on average.