If you have an "AC Power Failure" then you will need to "fix the power failure" before you troubleshoot the battery.
The first step is to check the panel status and verify that you have a "System Low Battery".
a) Press the "Gear Icon". Scroll down to "System Test"
b) Enter into "System Tests". Enter your "Master Code" default it is set to "1234" then hit "OK"
c) Press "Sensor Test"
d) Wait 60 seconds under the "Sensor Test" page and then "Close" until you reach the home screen.
If the panel still continues to show a "low battery", then you'll need to confirm that the wiring for the battery is connected properly. To do this, follow the instructions below...
The first step in opening the panel is pry the trim ring off with a screwdriver, using the slot in the bottom right corner of the panel (circled in red in this picture).
Once the trim ring is loose you can remove it from the panel.
With the trim ring removed you will lift up on the two tabs at the top of the panel to open it (circled in red in this picture).
The main face of the panel will swing open from the top.
Disconnect the "AC Power" and unplug the "Battery". Plug the "Battery" back in and re-connect the "AC Power".
This should have cleared the low battery condition, now lets check the panel status and see what it says.
Press the "Gear Wheel" at the bottom right of the screen.
Under Panel Status, Press "Clear" and then "Listen".
It should say "System Okay".
If the previous steps didn't clear the "Low Battery Message", or if it comes back again, check the battery voltage by...
a) Pressing the "Gear Wheel" on the home screen and then "Arrow Down" to the last page
b) Select "Enter" next to "Programming", then Insert your "Master Code" and press "Ok".
c) Press "Interactive Services", then "Module Status".
d) It will show you a voltage number for the battery on page. If the battery voltage shows 1.00 V, check the battery connections. Next, give the battery 24 hours to charge.
If it still shows the "Low Battery Message" after 24hrs, replace the battery.
If you need to replace the backup battery you can turn off the trouble beeps and tell the central station not to call you for a low battery while you wait for the replacement battery. You can get a new battery here or contact us if you are still under warranty. Here is a link that will show you how to replace the battery.
If your Simon XTi is telling you that it has a low battery it may be because you have just received it and it needs to charge. A new system is typically charged within 4 hours of being powered up but can take anywhere from 12-24hrs as well.