What, specifically, triggers a DC Bus Over Volt fault?

[BillP]

I have a ~9 year old SMX2000RMLV2UNC that had been sitting unused for the past few years. I went to check it recently (expecting a dead battery) but what I got was a "DC Bus Over Volt" fault as soon as I turned on the output. Like I said, I was expecting the battery pack to be toast, but I was also expecting the unit to turn on even with bad batteries.

Now I'm faced with the choice of gambling $400 on a new battery pack. It might clear the fault, or it might be a battery pack incompatible with a new UPS I'll be forced to buy.

So my question is: what specific condition triggers this error? Which DC bus is being measured? What is the correct voltage for that bus and what voltage triggers this fault? And is there a specific component that has typically failed to cause this error?

Thank you!