The following steps and examples are provided for guideline and reference purposes only.  Provincial, State or Federal regulations and codes may vary.  We strongly recommend working with a reputable certified installer and/or inspector for guidance.

1. Calculating Hydrogen Concentration

A typical lead acid battery will develop approximately .01474 cubic feet of hydrogen per cell at standard temperature and pressure.

H = (C x O x G x A) ÷ R


(H) = Volume of hydrogen produced during recharge.

(C) = Number of cells in battery.

(O) = Percentage of overcharge assumed during a recharge, use 20%.

(G) = Volume of hydrogen produced by one ampere hour of charge. Use .01474 to get cubic feet.

(A) = 6-hour rated capacity of the battery in ampere hours.

(R) = Assume gas is released during the last (4) hours of an 8-hour charge.

Example: Number cells per battery = 24

Ampere size of battery = 450 A.H.

(H) = (24 x 20 x .01474 x 450) ÷ 4


H = 7.9596 cubic feet per battery per hour

2. Calculating Room Volume

For a room with a flat roof volume is calculated W x L x H less the volume of chargers and other fixed objects in the battery room.

W= Width

L = Length

H = Height

Example: Room size 80 feet long, 60 feet wide and 30 feet tall.

V = 60 x 80 x 30

V = 144,000 cu.ft.

3. Determining Ventilation Requirement

Assume 75 batteries stored.

7.9596 x 75 = 596.97 cubic feet per hour (7.9596 calculated in Step 1)

Battery room 144,000 cu. ft. from example in Step 2

V = R x P ÷ H x 60 minutes

(V) = Ventilation required

(R) = Room cu. ft.

(P) = Maximum percentage of hydrogen gas allowed

(H) = Total hydrogen produced per hour

V = 144,000 x .01% ÷ 596.97 x 60

V = 144.73 or the air should be exchanged every 144.73 minutes (2 hours 24 minutes)

4. Determining Fan Requirement

Fan Size = R x 60 minutes ÷ V

(R) = Room cu. ft.

(V) = Ventilation required

144,000 x 60 ÷ 144.73 = 59’ 697.36 cu. ft. per hour or 995 CFM.

The ventilation system should be capable of extracting 59,697.36 cu.ft. per hour or 995 CFM.

5.: Do You Need Forced Ventilation

In theory the 596.97 cu. ft./hr. only represents .004% which is < 1%. Therefore forced ventilation would not be required for this example.

However, the following should be considered before ruling out forced ventilation:

Is the battery room closed in or open? If closed in no natural ventilation may be possible.

Since hydrogen gas rises are there areas in the ceiling where gas may collect in greater concentrations.

The above calculation represents worse case scenario assuming all batteries are gassing at the same time. This is highly improbable.

If natural ventilation is sufficient in an open area forced ventilation should not be required.

If your calculations determine a percentage <1% hydrogen concentration, we recommend a Hydrogen Gas Detector for safe measure, part number HGD-1.

Hydrogen Gas Detector

Hydrogen is colorless, odorless and is the lightest of all gases. The detector, therefore, should be installed at the highest, draft-free location in the battery compartment or room where hydrogen gas would accumulate.

The size of the area one detector will protect depends upon battery compartment room. The detector measures the hydrogen gas in the air immediately surrounding the sensor. If hydrogen gas might accumulate in several, unconnected areas in the compartment or room, individual detectors should be placed at each location.

Should the concentration of hydrogen gas in the air surrounding the sensor reach 1% by volume, the "1% caution" yellow LED will light and the 1% internal relay will close. Should the hydrogen gas concentration reach 2% by volume, the "2% warning" red LED will flash and an 80 db alarm will sound; the 1% relay will remain closed and, if a Dual-Relay model, the 2% internal relay will close. Either relay can activate a remote exhaust fan and/or alarm.

In addition to protecting your employees and your property, the detector also may reduce the following costs:Electricity – Heating – Air Conditioning. Instead of continuously running an exhaust fan to prevent hydrogen gas accumulation, use the detector to activate the fan only if the concentration reaches 1%. Insurance. Installation of a detector in areas where batteries are charged may result in a premium reduction.

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