Current ADT system though centralized and is powered directly to the power source, there is a battery backup that support the system incase of power failure or outage. There are clear indicator in the Keypad along with the beeping sound that would be hard to miss. You would also get a call from ADT regarding the same. They would want to validate with you on this to ensure there were no security tampering.
For immediate fix (to stop the beeps) you could press the asterisk sign twice on the keypad. It would give you a 8 hrs reset time. It would start beeping if the battery is still out of charge by then.
By default our homes comes with a 12 V 4 Amps battery that has a charge life of 3-5 years. The first thing they would recommend you to do is ensure the trippers are up and let he system on proper power source (if tripped, after turning it on) for 24 hrs to see if the battery is able to charge back. If that does not work, you would be asked to replace the battery. They would recommend a 12V 7 Amps battery which has a better backup that the original 4 Amps.
Below is the Panel that has the battery housed. Typically found in our Utility room (Laundry) or the room that leads to garage.
Ensure to call ADT before you touch the wires inside of this panel as any accidental wire pulls could trigger ADT and could end up with Cops showing up your door step.
Inside of panel would look like below.
Simply unplug the old battery and replace it with the new one (black wire to black pin and red wire to red pin).
Let the Battery charge for few mins / hours and see the low battery warning sign disappear from the keypad.
Here is the instructions provided by ADT for battery replacement procedure.
https://clanslang.com/wp-content/uploads/2020/07/Replacing-the-12V-System-Battery.pdf
PCR – ADT customers do not have maintenance as part of the HOA package. And this service if to be done by ADT service personals would be charged if request. However the replacement battery could be sent to you for free of charge if you request them to do so. The replacement battery could also be brought from Amazon / hardware stores for well under 30$.