No battery in modem - DirecTV

My modem was replaced a few months ago. I noticed today that my new modem has no battery. My old one had a battery. Is the point of the battery to keep the modem charged in case of a power outage? If so, I want a battery and I would like to know how to get one.

This question, "No battery in modem," is about DirecTV channels, programming, equipment, and sports.