The minimum transmit battery voltage (Vtx min) of a radio operating in a TDMA mode of operation or the stand-by battery voltage (Vstd-by) of the radio operating in a GSM mode of operation can be determined as a function of the combination of an in-slot transmit battery voltage (Vin-slot), an out-of-slot transmit battery voltage (Vout-slot), a stand-by compensation factor (&Dgr;sb), and a compensation factor (K). The stand-by compensation factor (&Dgr;sb) is related to a difference between the out-of-slot transmit battery voltage (Vout-slot) and a stand-by battery voltage (Vstd-by). The compensation factor (K) has a first value for the TDMA mode of operation and a second value for the GSM mode of operation. Each value of compensation factor (K) is related to the manner in which the radio utilizes battery power during transmission in the TDMA or GSM modes of operation.