In a recent electrical storm, my FIOS Internet backup battery was damaged. I discovered then that the battery is the primary power source, as well as backup. Rather than a parallel setup, apparently all power is routed via the battery, in a serial fashion.
As a result, with a damaged battery, I have no Internet access, even though I have no power outage. Until I get a new battery from Verizon.
A backup battery should be backup only. All other devices I own with battery backups do not require the battery to operate (from alarm clocks to UPS to home alarm systems). The FIOS design introduces a single point of failure with the battery, which is not necessary. Future upgrades, and new equipment, should incorporate a design in which either mains power or battery power is sufficient to have Internet access.