Apparatus comprises: a first radio modem having an associated first radio frequency power amplifier; a second radio modem having an associated second radio frequency power amplifier; a terminal for connection to a battery; a capacitor having first and second terminals; a current limiting switch having input and output terminals; and a direct current-direct current converter having an input and an output. The terminal is coupled to the first radio frequency power amplifier, the terminal is coupled to the input of the current limiting switch, the output of the current limiting switch is coupled to both the first terminal of the capacitor and the input of the direct current-direct current convener, and the output of the direct current-direct current converter is coupled to the second radio frequency power amplifier.