The Verizon ONT has a Battery Backup Unit (BBU) that is installed provide battery power to the device during a power outage. The backup battery is able to provide 8 hours of continuous power to the ONT, so that emergency dialing services and certain voice services will continue to function. However, internet and television services will not be supported by battery power. This battery housed inside the BBU is covered by a 1-year limited warranty.
When and How to replace the backup battery...
A low-battery indicator light will appear on the BBU when it's time for a backup battery to be replaced. Using a 12V, 7AH SLA backup battery, like the UB1270, is recommended. Once you have it, simply open the BBU, remove the old battery, and replace it with the new SLA battery.