Easy to find on eBay, just search for Alarm Battery, and you’ll need a 12v sealed lead acid battery. Before you buy, open up alarm control box, locate battery and determine what specification of battery you’ve already got.. it will probably be 1.2ah or 2.0ah. Use that info to buy correct battery.
When new battery arrives, open up control panel, do not touch circuit board or mains wiring, unclip old battery, pull off black and red connectors on battery, and reconnect those to new battery making sure polarity is correct. Clip in new battery and close up alarm control. At any stage be prepared to enter alarm code if it goes off whilst you are doing the work.. alarm system will see it as tampering, but will reset when cover back on and battery installed. Just don’t touch any live wires.
Rupert, March 2019