How to replace battery

["m35603" <m35603@xxxxxxxxx>]

We have a ADT PC5508z alarm system installed in our house. It is not
active (i.e., we don't have a contract with ADT) right now. Two days
ago, the thing started beeping every few seconds, and by looking
through the manual, I figured out that it was a low battery warning.
Where in the world is the battery on this thing though? We took the
keypad off and there doesn't appear to be a battery in it anywhere. The
manual talks about replacing the battery every three years or so, but
nowhere can I find anything that tells me where the battery is located.
I would appreciate any help I can get. I dont' want to call ADT because
we haven't had one in the last two years, so they probably wouldn't
want to help us.

Thank you very much!
Angie