The battery area shall be ventilated, either by a natural or mechanical ventilation system, to prevent
accumulation of hydrogen. The ventilation system shall limit hydrogen accumulation to less than 2% of the
total volume of the battery area. The location should be free of areas that might collect pockets of hydrogen.
Maximum hydrogen evolution rate is 0.127 mL/s per charging ampere per cell at 25 °C and standard
pressure (760 mm Hg).8 The worst-case condition exists when forcing maximum current into a fully charged
battery.
A battery area that meets the above ventilation requirements should not be considered a classified
(hazardous) location;
It depends on the building codes. Some codes allow you to exhaust at 1 cubic foot per minute per square foot of floor area as an alternative to calculating the gassing volume.
Allowable concentration of Hydrogen varies from Code to Code. NFPA gives a value of 1%. I am attaching a spreadsheet which will help you in calculating no. of hours required to reach the dangerous concentration. Max Hydrogen evolution is readily available with Battery vendor.