Hi, a little update, I switched to NUT as suggested, the problem was partially solved, now doing some tests, I stopped the array, turned off the power and the UPS kept the system on for over 30 minutes, once reached 25% battery, it turned off as scheduled.
Sometrimes the "battery have failed" error is triggered but it disappears immediately afterwards. All self tests continue to pass without problems.
However, I discovered why NUMXFERS is so high, the voltage in my house should be 220 volts, but often it is 240-245 volts, this unfortunately causes the UPS to regulate the voltage which I think is the reason why it doesn't reach never 100%. I'll try keeping the UPS on charge with nothing connected, to see if it gets to 100%.