If you have a multi-meter, check battery voltages. A 12V pack should be over 13V fully charged, at least 26V for a 24V pack. If your UPS cannot get there, the battery has shorted cells. Cells can also become sulfated, in which case they may still have voltage but won't be able to pass enough current under load.
My Cyberpower UPS did the spontaneous shut-down on brownout to me a while ago and when I investigated it, I found out that its 24V battery couldn't charge past 17V (very much dead) with the UPS still pushing 2A into it (the battery was very warm) yet the UPS still reported 24V with 60% charge, no battery warning. Either something in the battery management failed or the firmware is messed up, maybe both. The UPS was only three years old at the time. I still have it, planning to eventually have a look for a possible repair (try to find out why the UPS was misreporting battery voltage) and see if there is anything worth salvaging otherwise.
Edit: weird, I couldn't see my first post the first time I got notified of a reply in this thread, re-posted and now my first post has re-appeared. That's why I have two nearly identical posts in here.