UPS battery systems are not normally grounded. This gives them a degree of immmunity to ground faults: the first ground fault will not have a detrimental effect on the battery. Monitoring is essential and ground faults must be hunted down and removed promptly. A second ground fault at a different position on the battery string is usually messy.
Check with the individual UPS manufacturer for the best advice for their product.
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Many DC battery systems in the U.S. are negative grounded systems including those installed for UPS systems and Telecom systems. The design and size of the system determines if the system is grounded or not. I would suggest you check with the UPS Manufacturer or the Electrical Engineer of record for the system you are workin on.
