Well, usually if you past 15 minutes the output FET's will overheat and be stressed which will blow when power is restored.
Maybe your UPS has a battery timer cut-off, also.
So, doing this for more battery time may fail.
I use automotive batteries solely for a better replacement/availability and not for increased battery run-time and don't recommend latter.
Maybe you can try placing a 12v 'puter case fan inside on FET's. if running past 15 minutes, if allowed.