Two factors impact the backup time, the load power and battery real capacity. If the load is different from the original design, double check if its total power consumption is much higher now. The battery real capacity could vary when the battery is getting old. Usually it’s risky when the battery is used for more than two years. Besides the hardware changes the capacity, the software might impact it. For examples, check if the UPS is set to shut down after a certain time frame no matter the battery status, check if the quantity of battery is correct in settings, check if the battery voltage level is correct in settings.