Do you have Verizon FIOS fiber internet? Have you been hearing a weird intermittent beeping from your FIOS equipment? And not the modem or router by your television, but rather that strange box hidden away in a closet by your power box. Then you probably need a new ONT battery. Verizon calls this a Battery Backup Unit (BBU).
Fiber internet comes into your home into a Optical Network Terminal (ONT). This is not something you ever touch or think about, it's a box that is setup when you get FIOS and then it's left alone for years on end. But then, one day, maybe after 5 or 8 years of wonderful fiber internet... it beeps. And then, 15 minutes later, it beeps again. It usually takes a long time to identify, and then it's painful to listen to.
The point of the ONT battery is for backup purposes. If your home were to loose power and you have your emergency number (911 in most cases) through your Verizon interent telephony then you need the battery to place your emergency call. That's why the ONT unit has a rather larger 12V 7AH battery in it. Even if you never use that battery it will wear out. When it's done for, the ONT unit will recognize this and start to beep every 15 minutes.
This website will help you decide want to do next.
Do you hear a brief (about 0.5 seconds), sharp electronic "beep" every 15 minutes? Then there is no doubt about it, your battery backup unit (BBU) needs to be replaced. With some newer ONTs the BBU can simply be removed, but for the most part you are going to want to replace it if you want to preserve your emergency calling through your Verizon fiber telephone connection.
If your battery is dead, the easiest thing to do is replace it. In some circumstances you don't need to replace the batttery but, let's face it, it's the path of least resistance. Verizon requires the use of a 12-volt, 7.2Ah sealed lead acid battery.
There are many ways to stop your ONT battery beeping. The first is the easiest: just press and hold the "silence battery" button for 3 seconds! That's it, problem solved... for 24 hours. The beeping will resume and the next day, exactly on schedule. This isn't a bad solution considering it takes three seconds, it's free, and it'll buy you a full day of peace and quiet.
You can also disconnect the battery and reconnect it. This will make the unit think, under some circumstances, that you've gone ahead and replaced the battery. If you actually have a faulty/old/etc. battery than this will only buy you a few hours or, at most, a few weeks of time, but it should work in a pinch. This is best used if you are fairly confident that your battery is good and you just want the ONT to take a second look.
Some Verizon FIOS units don't need a battery connected at all. After all, in the case of emergency and power loss, most people would use their mobile phone anyway. This tends to be the case with newer ONT units, so your's may or may not need a battery connected. To test, disconnect the battery and see if the ONT unit beeps within the next few minutes. If it doesn't you don't need a battery at all. You can confirm this with a call to Verizon tech support.
The typical ONT has a battery backup unit tucked away in a comparment. This is the only user-serviceable part of the ONT and, as such, it can be opened without tools. Inside you will see the 12-volt, 7.2Ah sealed lead acid battery, a battery connector (the wire with a plug at the end), four LED indicators, and two buttons. The most useful part, aside from the battery, will be the silence button, which will mute your Verizon ONT beep for 24-hours. You just need to press it and hold for a few seconds.
If you lose power you can expect to loser internet connectivity even if you have an ONT battery backup. This is the case because the ONT battery exists to ensure phone service during an internet outage, but not internet!
What about if you want to maintain phone and internet service during a power outage? Then you need to backup the entire ONT using something like an uninterruptible power supply (UPS). This will ensure the internet is working proper so long as the other required pieces are working as well! That means your computer, cable modem, routers, switches (etc.) need power too! If you have a simple setup then you just need to be concerned with the ONT unit, cable modem, and your computer or phone.