I just successfully did this today, thanks to the generous instructions in other posts. I got the battery online from Amazon but actually from vendor Bedford Power.
12V 7 Ah Sealed Lead Acid Backup Battery Replacement for Verizon Fios Systems 12 Volt ...
$17.90
It was very difficult to open the panel, especially since mine is not like any of the pictured examples. Mine looks like this:
Start prying on the lower left hand side of the top panel. It will feel like it's going to crack. Eventuall the top snaps and it swings open on the hinges that are on the right-hand side.
The white strip on the right side of the battery snaps back easily and it slides out. The harness stays on the battery and unplugs itself from its one connection.
As another poster mentioned, you will need pliers to get the harness off the 2 connectors so you can put it on the new battery. As soon as I put the new battery with the harness attached to the connectors back in, and plugged the harness back in to the connector, the battery low light went off.
I don't have fios phone so this whole ordeal was a complete waste of time and money. Had to listen to the crazy beeping for about a month until I had time to order the battery. My smoke detector battery low sound is not half as annoying. Ridiculous that you can't just disable the alarm.
