Back-UPS CS 500 shuts off for some reason

[Doomster_apc]

I have a Back-UPS CS 500 that I bought in 2014.

It worked fine until recently.

I have a power strip connected to the Back-UPS (by the battery backup sockets, not the surge protection sockets).  My PC and some peripherals are connected to the power strip.

I leave the PC on 24/7.  A week ago, I noticed that the PC was shut off in the morning.  I first thought it might be the PC so I checked it but I didn't find anything wrong.  The next morning, the PC was shut off again.

I tried to troubleshoot this so I connected another PC to the same power strip and left both PCs on overnight.  The first night, both PCs stayed up. I left them on for 2 more days but then a few days ago, I noticed in the morning that both PCs were shut off.  The UPS was on though, and the Replace Battery light was not on.

So I isolated the problem down to the UPC and the power outlet on the wall.  The UPC only uses one socket.  The other socket was free so I connected an electric clock to it, the one that resets and blinks 12:00 if the power ever goes off.

The UPC worked fine until Sunday morning, when I noticed that, again, both PCs were turned off, and this time, the UPC was turned off.  I turned the UPC back on but the Replace battery light was still not blinking.  The electric clock was still working - it had not been shut off.  So I could eliminate the wall socket as the problem and narrow the problem down to the UPC.

I took the battery out and checked the volts using a Fluke multimeter and it showed about ~11.5 volts (I don't know if this is sufficient or too low).

Is there a way to check the status of the UPC?  These models do not have a "Test" button like some older models do.

Has anyone else encountered this problem?