A method and apparatus for adjusting transmission power in a portable two-way radio device allows the device to determine an expected battery voltage drop for a subsequent transmit event, based on present transmit power settings and battery impedance, and change the power setting for the subsequent transmit event if the battery voltage is likely to drop below a shutdown threshold at the present power setting or if the expected battery voltage drop will cause the battery voltage to be substantially above the shutdown voltage level and the present power setting is below an optimal level.