Smartphone power modeling is an important technique for users, application developers, and hardware manufacturers to build energy-aware systems. Compared to traditional offline techniques that use external power measurement devices, the online approach uses a built-in BMU (Battery Monitoring Unit), which has the advantage of generating a dynamic power model. However, the very low update rate of a BMU is an issue for online power modeling. In this paper, we introduce an autonomous power modeling tool for smartphones called DevScope, which overcomes the limitations of BMU-based online power modeling. DevScope controls components according to the BMU update rate, and derives the component power model automatically by analyzing the changes of power state. With DevScope, we construct an online and automatic power modeling that reflects both the diversity of users' environments and smartphone hardware complexity. By evaluating the scheme with various smartphones and configurations, we show that DevScope, indeed, generates a flexible and accurate power model.