It depends on what kind of battery backup unit (BBU) you have. Look at this Verizon link for more details. Assuming you have the white one (the one on the right from that last link), the battery is a standard 12V 7.2Ah sealed lead-acid battery. If you have a battery store near you, you can probably pick one up today. I don't have a phone number for Verizon, but the online link to order one from Verizon is this one. The cost from Verizon is $40 + $9 for shipping, which is pretty high. There are compatible ones that cost much less like this one or this one. My experience is that these last about three years though the battery resellers claim more.