A major concern for today’s smartphones is their much faster battery drain than traditional feature phones, despite their greater battery capacities. The difference is mainly contributed by those more powerful but also much more powerconsuming smartphone components, such as the multi-core application processor. While the application processor must be active when any smart apps are being used, it is also unnecessarily waken up, even during idle periods, to perform operations related to basic phone functions (i.e., incoming calls and text messages). In this paper, we investigate how to increase the battery life of smartphones by minimizing the use of the application processor during idle periods.We find that the application processor is often waken up by a process running on it, called the Radio Interface Layer Daemon (RILD), which interfaces the user and apps to the GSM/LTE cellular network. In particular, we demonstrate that a great amount of energy could be saved if RILD is stopped, such that the application processor can sleep more often. Based on this key finding, we design a Smart On Demand (SOD) configuration that reduces smartphone idle energy consumption by running RILD operations on a secondary low-power microcontroller. As a result, RILD operations can be handled at much lower energy costs and the application processor is waken up only when one needs to use any smart apps, in an on-demand manner. We have built a hardware prototype of SOD and evaluated it with real user traces. Our results show that SOD can increase its battery life by up to 2.5 more days.