Hell mhlb
This issue can be caused by old firmware, loose connection, faulty battery, or a faulty UPS. I would suggest that you clear the error, update the firmware if a newer version is available, and swap the battery with the one from your other UPS. If the error returns then parts will need to be replaced. If it returns on the other UPS that you moved the battery to then it would appear to be the battery. If it returns on the original UPS the error was on with a different battery then I would suspect a faulty UPS.
Thanks