Disclosed are a battery power control device, a portable device and a method of controlling the portable device. The disclosed portable device includes a workload calculator which calculates a workload necessary to provide the specific service to the user according to at least one of time request information related to a desired time during which the specific service is provided to the user and QoS (Quality of Service) request information related to a quality of the specific service, the necessary workload being smaller than a default workload that is preset for providing the specific service; a task performer which performs a task for providing the specific service according to the necessary workload during a time that is reduced to be shorter than a task performance time according to the default workload and stop the task performance during the reduced time; and a battery unit which supplies a power for performing the task to the task performer.