For a prolonged outage, what is the recommended power management step for a smartphone?

Answer

Switch the phone to airplane mode immediately to conserve battery

Since the primary failure mode is power loss leading to tower shutdowns, keeping the phone charged and then switching it to airplane mode conserves the battery for necessary, short bursts of communication.

For a prolonged outage, what is the recommended power management step for a smartphone?
deviceweatherphoneimpactsolar storm