APC Smart UPS SC620 Suddenly Shuts Down

[dp-50]

I have an elderly SC620 Smart UPS.  The battery has been replaced several times over the years.  The present battery is less than 1 year old and scheduled self tests are passed OK.

Recently the UPS has shut down at few times without warning.  There have been no mains failures and nothing unusual is reported in the log files.  The voltage and frequency are normal and the load is about 25%.  All self tests have passed.

When I power the unit on after such an event, the battery charge is showing as 95%.  I have checked the shutdown parameters (600 secs on battery plus 180 secs for operating system) and I think they're OK.  In any case, they've not been changed recently.

The UPS is set to 'reduced sensitivity' but it's as if there is a transient mains disturbance and the UPS is immediately shutting down rather than going into battery mode.

Has anyone experienced similar or know of a solution?