I hope it is firmware error and not battery error.
I have Eaton 5px 1000i rt2u g2 with installed gigabit card Eaton Network-M2. The error "battery fault" appeared almost after one year of operation without any problem. The UPS was still during warranty period so in my service they replaced batteries. But after 3 days the error "battery fault" reappeared.
I have made firmware upgrade of Eaton gigabit M2-card and and than UPS.
1, firmware upgrade of Eaton Network M2-card to the last firmware 3.1.12
2. firmware upgrade of UPS Eaton 5px 1000i rt2u g2
Why didn't you send it back?
It can be as simple as a bad connection, but 3 days is pretty far from acceptable repair.
It is one month from firmware upgrade and UPS works correctly.
My UPS protects Synology NAS, PC, home network, Wifi router.
If I send UPS for repair once more I couldn't operate NAS and home network.
And the UPS is heavy for manipulation moreover.