We have the same issue with "justme7"
APC Back-UPS ES500 Model BE500R-PH with serial # 7B1118L08818. At first it will operate as normal but after in a few seconds i can hear twice tick sounds and then a loud nonstop beep sound. We have no other option but to turn it off.
Actually, we are running chain of fastfood stores and pastries and all branches using you APC Back-ups. Our problem started since our Power Electric Provider supplying us insufficient and fluctuating power. Our concern now: how to fix this problem is it replace the battery (tried already but no luck) or look for the specific circuit component on the board.