Battery Room Max Charge Current from Available Airflow
The highest maximum charge current a room's exhaust can safely support.
Example
You enter
- Available exhaust airflow (cfm) 100
- Cell count (individual 2 V cells, NOT jars) 24
You get
- Max charge current per cell (x parallel strings = charger total) 154.9 A
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeI_max = available CFM / (0.0269 x N) at the 1% IFC / NFPA 1 limit (IEEE 1635), the current per cell, where N is the number of individual 2 V CELLS, not jars. An exhaust of 100 cfm over a 24-cell string supports 154.9 A; counted as 144 cells (twenty-four 12 V jars) it is only 25.8 A, so mistaking jars for cells overstates the safe current six-fold. Answers 'how much charge current can my fan handle' instead of sizing the fan. A design aid; the applicable code and room design govern.
Formula or tableI_max = available_cfm x (limit/100) / (0.000269 x N), available_cfm / (0.0269 x N) at 1% (N = individual 2 V cells; I per cell); the inverse of Q = 0.000269 x I x N / (limit/100).
Source and editionThe IEEE 1635 / IEEE-ASHRAE Guide 21 battery-room hydrogen ventilation relation solved for the maximum charge current a given exhaust rate can support, holding the room-average hydrogen at the chosen limit (1% by default, IFC / NFPA 1; 4% LEL); the applicable code and the room design govern.
Free accessThe 0.000269 cfm per ampere per cell hydrogen rate is the published IEEE 1635 Annex A value; the available airflow and cell count come from the room and battery installation.
What governsEstimate. AHJ and licensed professional govern.
API field namesavailable_cfm, cell_count, max_charge_current_a
- Cells not jars N counts individual 2 V cells; a 12 V jar is six cells, so counting jars overstates the safe current six-foldIEEE 1635
- 1% default limit the current is bounded so the room-average hydrogen stays at the entered limit, 1% by default (IFC / NFPA 1), a 75% margin under the 4% LELIEEE 1635-2018 Annex A
- Bounding case sealed VRLA in normal float gasses far less, so this is a conservative current limitbattery practice