A respiratory device, such as a ventilator, for use in treating respiratory disorders and for preventing respiratory disorders. The respiratory device is configured to be powered from a range of different power sources including an internal battery, an external battery, AC power source or a DC power source. The device may be electrically connectable to a plurality of external batteries in a series and the power from each external battery is used sequentially along the series. A controller of the respiratory device is configured to detect the connection of the different power sources and control use of the different power sources using a power priority scheme. The controller may determine an estimate of the total available battery capacity from all the electrically connected batteries and display the total battery capacity on a user interface display of the device.