Since Comcast employee's answer is unsatisfactory, it is almost like a bot had answered it, I feel the need to add a few points:
1. I was also looking at what the battery does (that is how I found your post), and it is only to keep your phone line up for an extra 6-12 hours, not the wifi. If you have a cell phone and wireless signal, then having that extra battery feature in case of emergency is not usefull, and your cell phone battery will die in the same amount of time anyway. So you are better off with a battery pack to charge your cell phone.
2. Comcast doesn't supply a battery in their modem anymore. You can have them install one ut it would come at tripple the price of doing it yourself. What Comcast should do (and maybe they are doing it) is during the installation look at whether the client doesn't have a cell phone or wireless signal and determine that this person needs the battery.