BackUPS CS 500 (BK500) - Flickering "On Battery" with/without battery installed, won't power up

[Anonymous user]

S/N: 4B0843P50799

I have the above mentioned unit.  Regardless of whether there is a battery installed or not, the "On Battery" light flickers very rapidly and only the surge side of the unit provides power.

Pressing the power button does nothing, except dim the rapidly flickering indicator a little.

I pulled a battery from another, working unit, the exact same model/make, etc. so I know that is not a problem.

Any suggestions?

Thanks -- lee