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. If you have had a power outage recently and your panel is telling you it has a low battery the panel will need to be power cycled.
Follow the steps below to power cycle your alarm system:
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 your system is new and its still telling you there is a low battery, let it charge for a full 24 hours. If the low battery still persists after that then you may need to contact technical support.