Battery Room Hydrogen Ventilation (IEEE 1635)

ScopeCharging vented lead-acid/flooded cells liberate hydrogen, and each ampere through each cell makes 0.000269 cfm of hydrogen (IEEE 1635 Annex A), so Q = 0.0269 x I x N cfm holds the room average at the 1% IFC / NFPA 1 limit (a 75% margin under the 4% explosive limit); I is the current per cell (charger current / parallel strings). The catch that undersizes real rooms is the formula counts individual 2 V CELLS, not jars -- a 12 V jar is six 2 V cells, so twenty-four 12 V jars is 144 cells, not 24. A 24-cell string at 20 A needs 12.9 cfm (1.0 ACH in an 800 ft^3 room); counted correctly as cells (twenty-four 12 V jars = 144 cells) it is really 77.5 cfm, six times the airflow. Sealed VRLA in float gasses far less. A design aid; the applicable code and room design govern.

Formula or tableQ_cfm = 0.000269 x I x N / (limit/100), 0.0269 x I x N at 1% (I = current per cell, N = individual 2 V cells); ACH = Q x 60 / room_volume.

Source and editionIEEE 1635-2018 / ASHRAE Guideline 21-2018 Annex A battery-room hydrogen ventilation rate, holding the room-average hydrogen at a chosen limit (1% by default, the IFC / NFPA 1 limit; 4% LEL); the applicable code and the room design govern.

Free accessThe 1.27e-7 m^3/s per ampere per cell hydrogen rate is the published IEEE 1635 Annex A value (and follows from Faraday's law); the cell count, charge current, and room volume come from the battery installation.

What governsEstimate. AHJ and licensed professional govern.

API field namescell_count, charge_current_a, room_volume_ft3, q_cfm, ach

• Cells not jars N counts individual 2 V cells; a 12 V jar is six cells, so counting jars undersizes six-foldIEEE 1635
• Hydrogen rate 1.27e-7 m^3/s (0.000269 cfm) per ampere per cell; Q = 0.0269 x I x N at 1%, 0.01345 x I x N at IEEE's 2% designIEEE 1635-2018 / ASHRAE Guideline 21 Annex A.3.5-A.3.6
• Bounding case sealed VRLA in normal float gasses far less than this vented-cell bounding ratebattery practice