Battery O.C Voltage ( UPS Batteries )

Don't have a clue.

Maybe these guys will know, Yuasa Batteries, Inc.

Maybe the UPS maker will know.

Maybe the Shadow will know.

................NOT.................

What sort of UPS Domestic/Small Office or Commercial?

What size Battery Bank? 12V, 24V, 48V, 500V, 600V

How old are the UPS and battery(s)?
A small UPS battery set has an approximate life of 3-5 years and a large UPS battery set should last 8-10 years in realistic world.

Before I worried about what voltage would result in a Open Circuit battery, I would be looking at the condition and age of the batteries as this looks from the very limited information in the question like you have other problems.

The voltage regulation in UPS rectifiers\chargers (depending on the type of UPS) is usually pretty good and unlikely to be your problem, I would lean more towards age and condition.

Regards,
Stuart.

the dc bus voltage was 340 v and there was a battery reached 8 volt after 30 minutes of loading. could this battery open the string???

This Battery String is Not Open Circuit yet, as you can still get a voltage reading.

What appears to be happening is that the Battery String(s) have individual batteries with a High resistance and therefore the UPS is going offline due to the voltage being too low, this is a normal design function.

If this battery string is over five (5) years old, it might be time to replace them all.

Get the manufacturer or service agent to check out the full system, they will soon let you know if it is just one or two batteries, or if the whole string has reached End Of Life.

Regards,
Sapper

The batteries are in series which means that the voltage into your UPS is the sum of all the individual cell voltages and that the current out of the string has to flow through each cell unimpeded. What this means to you is that the condition of each cell is important, a "bad" cell anywhere in the string affects the overall performance of your battery. The bad cell is not necessarily "open" but has a higher internal resistance than normal which brings down its voltage, if it were truly open no current would pass through it. Assuming that you have made sure that the electrolyte is at the right level for each cell then it is time to replace the bad battery, it will not get better.