An apparatus for charging a battery of a user device includes a charge pump that converts an input voltage, received from a power adapter, to a system voltage that is less than the input voltage based on a ratio of the charge pump, a regulator coupled between the system voltage output by the charge pump and a battery of the user device, the regulator configured to control a battery charging voltage applied to the battery of the user device and to provide isolation between the system voltage that is applied to one or more components of the user device and the battery charging voltage applied to charge the battery of the user device, and a controller configured to determine a difference between the system voltage applied to an input of the regulator and the charging voltage output by the regulator.