Just an FYI, the battery will only keep your phone service running, not the internet. If you want both running in event of a power outage, you'll need an Uninterruptible Power Supply. (UPS).
I have one of these on my XB6 gateway and my cordless phone base.
I just got this modem myself, as a replacement, and want to get a battery. Thanks for the link to that xfinity support doc, @EG . It gives the price for the battery as $165! Ridiculous even for Comcast; might as well spring for a UPS. Especially since Arris lists the price as $60. If they had any in stock, which they don't. Wonder who bought 'em all.
Your welcome ! Dead tread from last year now being closed.