the SE needs a charged and functioning cell phone to send any messages other than the pre programmed messages like the spot, as it must be paired to send custom messages to any number other than the pre selected messages. if you're cell is dead, you are not able to send custom messages. I leave my cell phone either shut off or in airplane mode to not deplete the cell battery when not in use.
one of the best mods I have done is get the 12 V cig adapter for my doo. Can charge my camera, phone, sat phone etc.