In most areas, the Comcast equipment should have it's own backup. It also depends on how widespread an outage is. Also, if you are talking about a battery for the modem itself, and it also handles your phone, then that battery typically only powers enough to run the phone. You are better off getting a regular UPS to power the modem/router or gateway. Even if you don't have regular power issues, it's still a good thing to have to prevent problems during brief issues.
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