How to use it

Review the methodology below to make sure it aligns with your project's requirements, then:

Choose US (ft, cfm, °F) or SI (m, l/s, °C) units. Switching converts the values already entered.
Choose the battery type and enter the cells in each string, the number of strings and each string's rated capacity in amp-hours.
Enter the charger's maximum output current into each string. This is the conservative reading of the worst case the fire code describes. If the code official accepts it, you can enter the battery maker's gassing current instead, in amps per 100 Ah.
Enter the room's length, width and height. Add the room temperature and elevation if they differ from 77 °F and sea level.
The hydrogen generated and the exhaust needed by each code path appear in the results and update as you type.
To check a fan you already have, enter its exhaust airflow and say whether it runs continuously or on a timer.

Use Copy link to this result to save or share the calculation, and print the page for a one-page record of the inputs, results and method.

Methodology, equations and example

Lead-acid and nickel-cadmium batteries make hydrogen and oxygen when charging current splits water in the electrolyte, mostly near the end of a charge and during equalizing. Hydrogen burns in air from 4 % by volume, its lower flammable limit (LFL). Above the code's size threshold, the fire code requires battery rooms to be ventilated so that hydrogen stays well below that level.

What the code requires

The International Mechanical Code (§502.4) sends stationary battery systems to the International Fire Code, §1207.6.1 in both the 2021 and 2024 editions. That section applies to lead-acid and nickel-cadmium systems exceeding 70 kWh (IFC Table 1207.1.1 in 2021, Table 1207.1.3 in 2024, with kWh = rated voltage × Ah ÷ 1000; 50 gallons of lead-acid electrolyte also counts as 70 kWh). It allows either of two paths:

Path 1, §1207.6.1.1, ventilation based on LFL. The exhaust must hold hydrogen to 25 % of the LFL, which is 1 % of the room's volume, "during the worst-case event of simultaneous charging of batteries at the maximum charge rate, in accordance with nationally recognized standards".
Path 2, §1207.6.1.2, ventilation based on exhaust rate. The exhaust must be at least 1 cfm per ft² of floor (5.1 l/s per m²). It runs continuously, or it is started by gas detection at 25 % of the LFL. It needs 2 hours of standby power and supervision.

Either path satisfies the section, so the lower of the two airflows is the least exhaust it allows. Where the local fire code is NFPA 1 rather than the IFC, check its battery and energy storage provisions instead.

Hydrogen generated

Faraday's law gives one molecule of hydrogen for every two electrons that pass through a cell as overcharge. As a volume at the room's temperature and pressure (an ideal gas):

G = I × n × s × R × T ÷ (2 × F × P)

Where:

G: hydrogen generated, cfm (l/s)
I: charging current into each string, A
n: cells in series in each string
s: strings
F: Faraday constant, 96,485.33 C/mol
R: gas constant, 8.3145 J/(mol·K)
T and P: the room's absolute temperature and the atmospheric pressure at its elevation (ASHRAE Fundamentals, chapter 1)

At 77 °F and sea level this is 0.000269 cfm per amp per cell, the figure commonly quoted in battery room ventilation guidance. All of the current is taken as making gas. Once a battery is fully charged, any current still forced through it goes to gas. A healthy battery on a working charger draws far less than the charger's maximum at that point, so the full current at full charge is a fault condition, such as a failed charger or a forced overcharge. This calculator takes that fault as the code's worst case. No credit is taken for recombination in VRLA cells, since a failed or dried-out VRLA cell gasses like a flooded one.

Exhaust for path 1

With exhaust Q running continuously and the room well mixed, hydrogen settles at G ÷ Q. Holding that to 1 %:

Q = G ÷ (K × 0.01)

K is a mixing factor, 1 for a fully mixed room. A value below 1 allows for exhaust that does not sweep the whole room. The calculator uses 1 unless you enter another value.

With no exhaust, and with a fan on a timer

With no exhaust, hydrogen in a volume V rises at G ÷ V, reaching 1 % after t = 0.01 × V ÷ G minutes (G in cfm, V in ft³). You can take V as only the upper part of the room, since hydrogen rises.

A fan on a timer lets the gas build while it is off, then clears it while it runs. Once the cycles have settled, the peak comes at the end of each off period:

C_max = G ÷ (K × Q) + (G × t_off ÷ V) ÷ (1 − e^{−K × Q × t_on ÷ V})

with G and Q in cfm, V in ft³, and t_on and t_off in minutes.

If the off time alone fills the room past 1 % (G × t_off ÷ V ≥ 0.01), no fan size can make the timer work. A timer does not meet path 2 either, which needs the exhaust running continuously or started by detection.

This timed-fan model is Adicot's own derivation, not a method the code names. §1207.6.1.1 does not require the exhaust to run continuously, but before relying on a timer, confirm with the code official that it is accepted.

Example: A room 23.3 ft × 24 ft × 11.5 ft (6,431 ft³, 559.2 ft² of floor) holds one string of 120 flooded lead-acid cells rated 620 Ah, on a charger with a 50 A maximum output, at 77 °F and sea level.

Energy capacity: 120 × 2.0 V × 620 Ah ÷ 1000 = 148.8 kWh, over the 70 kWh threshold, so §1207.6.1 applies.
G = 50 A × 120 cells × 0.0002686 = 1.612 cfm of hydrogen.
Path 1: Q = 1.612 ÷ 0.01 = 161.2, so 162 cfm continuous. Required airflows are rounded up, since they are minimums.
Path 2: 1 cfm/ft² × 559.2 ft² = 559.2, so 560 cfm, continuous or on gas detection.
The least exhaust that meets §1207.6.1 is 162 cfm, by path 1.
With no exhaust, the room reaches 1 % in 0.01 × 6,431 ÷ 1.61 = 39.9 minutes.
A 350 cfm fan running continuously holds hydrogen at 0.46 %, which meets path 1. The same fan on a timer, 5 minutes in every 60, lets hydrogen peak at 6.25 %, which is over the LFL. The 55-minute off period is longer than the 39.9 minutes it takes to reach 1 %, so no fan size would fix that timer.

Limits of this calculator

It covers lead-acid (flooded and VRLA) and nickel-cadmium batteries. Other types for which IFC Table 1207.6 calls for exhaust ventilation, such as nickel metal hydride and flow batteries, are not covered. Lithium-ion is not covered either: Table 1207.6 does not call for §1207.6.1 exhaust ventilation for it, but it does call for explosion control.
Exhaust ventilation is one of several IFC §1207.6 requirements for these batteries, as Table 1207.6 applies them. Explosion control, spill control, safety caps and thermal runaway protection are separate.
The airflows are at room conditions. Locate the exhaust high in the room, where hydrogen collects, and make sure makeup air can reach the batteries.

Battery Room Hydrogen Ventilation Calculator

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