Have you tried unplugging it with things powered on to make sure the battery is working? The batteries are supposed to be replaced every couple years, I found out when I started having power outages last winter and my (seasonic) PSU just turned off when the power flickered and PC was running. I bought a replacement battery off Amazon for cheap and it was easy to swap out.
PSU is different than UPS. PSU is the Power Supply Unit and doesn't have a battery. UPS are Uninterruptable Power Supplies and do have batteries. Seasonic only makes PSU's.