Security system batteries are used to provide back-up power when the AC mains power fails. Generally speaking, an alarm (security system) battery will provide anywhere from 30 minutes to several hours of backup power during a power failure. But alarm batteries should be replaced every 2-3 years on average so they are ready when they're needed. Otherwise you may find that your alarm system will fail during a power outtage.
When looking for a replacement battery, the main consideration is the size, voltage, capacity (AH rating) and terminal type. Most standard alarms use 12V batteries, ranging from 4AH to 8AH batteries. Larger batteries allow your alarm system to run longer when power fails. An alarm battery typically has an F1 187 fast on tab which is 0.187" or 4.75mm wide.
Alarm Batteries - Lead Acid Alarm Battery | Canada