said by seaquake:How the heck can they tell the BBU is going bad? I didn't know there's some type of data transfer going on.
Either way, shop around for a new battery before choosing to purchase one from Verizon. You'll likely find one much cheaper. Replacing the battery is fairly easy.
Yes, there are signaling wires that go from the BBU to the ONT. It can tell if it's running on battery power only or if there is no battery installed or if it's running on AC only.