Abstract: A mobile telephone handset (1) has its circuitry (11) to be driven by a rechargeable battery (17). The handset can to perform a data processing operation such as a download or a memory defragmentation, which requires battery power over a considerable period of time and a battery sensor (20) is coupled to the circuitry for indicating whether the battery has sufficient capability to complete the data operation. The circuitry (11) is configured to cease the data processing operation in the event that the battery is indicated to have insufficient capability to complete the operation. A battery charging sensor (19) indicates when an external battery charger is connected and the download or defragmention process is preferably carried out using the power of the charger rather than the battery.