Nexxtech 1000VA UPS

Question

I have a NexxTech 1000VA UPS that power down after a few hours. The front panel indicates the unit can charge to full capacity. Would like to know the likely cause, and possible way to fix it. Thank you. 

Note: it is possible the it is a rebrand CyberPower unit, as some of NexxTech UPS are made by CyberPower.