The standby battery should be located in the metal enclosure that houses the control panel.
Obtain a replacement battery that meets the manufacturer’s specifications as listed in the Installation Manual or on the wiring diagram.
Before replacing the battery, if the system is monitored, place the account "on test" with your central station to avoid a false dispatch.
Unplug the system's transformer, being careful because it is connected to a high voltage supply source, or if you are unable to locate the transformer, examine the panel's schematic for a pair of terminals labeled AC. Remove a single wire from one of the terminals, being careful not to touch it to the circuit board or enclosure. A piece of electrical tape may temporarily help.
Disconnect the battery from the circuit board. Usually the units are color coded red and black for positive and negative connections. If your panel had been neglected in the past, you may need to replace a terminal or two on the battery leads due to corrosion. A local electrical or electronics supplier should have a suitable replacement. Usually the leads are 18AWG wire.
Install your new battery, paying attention to polarity and then plug the transformer back in or reconnect the wire disconnected.
Depending upon the panel, you may need to reset your panel's clock. If applicable, instructions for resetting the clock should be in your user's manual.
Don't forget to call and clear your account from test after you are done with your servicing.
Image courtesy of Elk Products