Classification of the room also depends on the size of your room, charge/discharge rates of the batteries, ventilation, size of your batteries.
We have battery banks in substations, in the same room as the switchgear.
Typically, if the total volume of the batteries is less tahn 1% of the total free air volume in the room, H2 evolution is not a problem. That's what I can remember from a class I had a while back on Area Classification (API RP 500).