Can AC800F low battery can alarm be created when the battery goes low
If I understood correctly you are looking for how to get bit or variable to create alarm indication in graphic for low battery, which appear by default as system alarm in message list. If so you can find BTL system variable in I/O editor of controller. so in HW tree right click on controller and click on IO editor and based on single or redundant controller you can find following variable to use.
Hope that answers your query, else please replyback to help you ...