Methods of and apparatuses for varying and controlling the effective series resistance (ESR) of a power supply rail configured to transmit power from a power supply of a host device to a wireless network interface device (e.g. a wireless modem). Varying and controlling the ESR of the power supply rail, relative to an ESR of a super capacitor coupled to the power supply rail, allows control of the currents drawn from the host power supply and a super capacitor. During a time when the wireless network interface device is transmitting or is about to transmit, the ESR of the power supply rail is increased so that it exceeds the ESR of the super capacitor, thereby causing current for the power amplifier (PA) of the wireless network interface device to be drawn primarily from the super capacitor, rather than from the host power supply. During a time when the wireless network interface device has completed transmitting or is about to complete transmitting, the ESR of the host power supply is lowered so that it is less than the ESR of the super capacitor, thereby causing the super capacitor to enter a charging state.