APC smart-ups 1500 “replace battery” led always on, and beep for a minutes somtimes

You should only be receiving a notification of a replace battery for one minute every five hours. I would recommend disconnecting all equipment from the UPS at the end of the day, and allow the UPS to remain on overnight, to receive a full charge. At that point, depress the test button for 10 seconds, and that will initiate a self-test. That should clear the replace battery LED and audible tone. If not, the battery would need replacing.