Mobile devices are only as useful as their battery lasts. Unfortunately, the operation and life of a mobile device's battery degrade over time and usage. The state-of-health (SoH) of batteries quantifies their degradation, but mobile devices' support for its estimation is very poor due mainly to the limited hardware and dynamic usage patterns, causing various problems such as shutting off the devices unexpectedly. To remedy this lack of support, a low-cost user-level SoH estimation service is developed for mobile devices based only on their battery voltage, which is already available on all commodity mobile devices. The design of the estimation service is inspired by an empirical observation that the relaxing voltages of a device battery fingerprint its SoH, and is steered by extensive measurements with 13 batteries used for various devices, such as Nexus 6P, Nexus 5X, Xperia Z5, Galaxy S3, iPhone 6 Plus, etc.