Abstract: A battery case for a mobile device, such as a mobile phone, makes a connection with the mobile device and also makes a connection with a peripherals module attached to the battery case. In one aspect, the battery case includes a built-in battery, and the battery provides power to the mobile device and/or to the peripherals module. Different power management options are available. In another aspect, the battery case includes a data path between the mobile device and the peripherals module. For example, the mobile phone may send control instructions to a camera module via the data path, and the camera module may return images via the data path. Various control and data transfer options are available.