Battery rooms or stationary storage battery systems (SSBS) have code requirements such as fire-rated enclosure, operation and maintenance safety requirements, and ventilation to prevent hydrogen gas concentrations from reaching 4% of the lower explosive level (LEL). Code and regulations require that LEL concentration of hydrogen (H2) be limited to 25% of LEL or 1% of room volume. The room ventilation method can be either forced or natural and either air-conditioned or unconditioned. Battery manufacturers require that batteries be maintained at 77ºF for optimum performance and warranty. This article will look into the battery room ventilation requirements, enclosure configurations, and the different ways to accomplish them.



Typical applications of SSBS are as backup power in uninterruptible power supply (UPS) systems for telecommunication rooms, electrical substations controls, and data centers. SSBS provide temporary power during momentary power failure while the main backup power, typically diesel or propane generators, synchronize with the power demand.

Typical battery SSBS are composed of batteries of the flooded lead-acid batteries, Valve Regulated Lead-Acid (VRLA), or nickel-Cadmium (Ni-Cd) batteries, a battery charger, rectifiers, inverters, converters, and associated electrical equipment. The SSBS are normally housed in a room as a part of a main building or as a standalone enclosure. During the charging and discharging of the batteries, H2 gas is released, which can be explosive and catastrophic at levels between 4% of LEL and 74% of the HEL.

The applicable codes and regulations for designs, safety operation, and maintenance of battery rooms are the Building Code, Mechanical Code, Fire Code, National Electrical Code (NEC), Occupational Safety and Health Administration (OSHA), and the Institute of Electrical and Electronics Engineers (IEEE) Standards.


Codes and Regulations

The following summarizes codes and requirements for ventilation at the national level. State and local jurisdictions may have more stringent ventilation requirements. For example, the California Code of Regulations or CAL-OSHA, requires H2 gas concentration to not exceed 20% of LEL or 0.8% of the room volume, compared to 25% of LEL or 1% of the room volume in the national level. Check with your local jurisdiction for specific ventilation requirements.


Mechanical Code

The International Mechanical Code (IMC) specifies similar requirements as NFPA 1 and International Fire Code. The Uniform Mechanical Code (UMC) does not directly address battery room ventilation but specifies that if a proposed occupancy is not listed in Table 4-4, the requirements for the listed occupancy that is most similar to the proposed space in term of occupant density and occupancy type shall be used. The occupancy that resembles that of a battery room on Table 4-4 is a science lab classroom where flammable or explosive gases may be present. The ventilation rate required is 1.0 cfm/sq-ft.

These codes have additional requirements that specify the design, construction, and installation of the ventilation system.


International Fire Code

The International Fire Code (IFC) requirements are such that when the battery storage system contains more than 50 gallons of electrolyte for flooded lead-acid, nickel cadmium (Ni-Cd), and valve regulated lead-acid (VRLA) or more than 1,000 pounds for lithium-ion batteries, the ventilation requirements are as follows:

  1. For flooded lead acid, flooded Ni-Cd, and VRLA batteries, the ventilation system shall be design to limit the maximum concentration of hydrogen to 1% of the total volume of the room; or
  2. Continuous ventilation shall be provided at a rate of not less than 1 cubic foot per second of the floor area of the room.
  3. Mechanical ventilation system shall be supervised by an approved central, proprietary, or remote station service, or shall initiate an audible and visual signal at a constant attended on-site location.

NFPA 1 — Fire Code, has similar ventilation requirements as IFC, with the exception of electrolyte quantity. NFPA 1 ventilation requirements apply to stationary storage battery systems having more than 100 gallons of electrolyte in sprinklered buildings and more than 50 gal of electrolyte in unsprinklered.

National Electrical Code (NEC)

Article 480.9(A) of the NEC states that provisions appropriate to the battery technology shall be made for sufficient diffusion and ventilation of the gases from the battery, if present, to prevent accumulation of an explosive mixture.

Adequate ventilation is required to prevent classification of a battery location as a hazardous (classified) location in accordance with Article 500.

Occupational Safety and Health Administration (OSHA)

Title 29 Code of Federal Regulations — Ventilation shall be provided to ensure diffusion of the gases from battery and to prevent accumulation of an explosive mixture.

The Institute of Electrical and Electronics Engineers (IEEE)

Standards 1188, 450, 484, and 485 provide guides that focus on the battery system design, maintenance, and operation. All these codes and regulations require ventilation or some type of provision to prevent H2 gas accumulation to LEL level.


Design Considerations

Applying Most Stringent Codes and Regulations

Considering the most stringent requirements from the different codes, the following codes apply: OSHA, NFPA 1, IFC, and UMC or IMC. Also, it is worth noting that by providing adequate ventilation at 1 cfm/sq-ft, the NEC Article 480.9(A) requirement is met and the battery room need not be classified as a Hazardous location, Class I, Division 1, Group B, per NEC Article 500.

The codes allow for natural or mechanical ventilation. However, only mechanical ventilation systems will be discussed due to the unreliability of natural ventilation. The two code-required approaches to satisfy the ventilation requirements are to continuously ventilate the space at 1 cfm/sq-ft or intermittently ventilate the enclosure as needed by monitoring and limiting H2 concentration to not exceed 25% of LEL.


Continuous Ventilation

Of the two code requirements, continuous ventilation at 1 cfm/sq-ft is the easier approach since no calculation is needed for the H2 evolution rate. However, this option may be impractical when the space is air conditioned, because exhausting conditioned air is a waste of energy and the 77ºF room temperature normally required by the battery manufacturer will be hard to maintain in high ambient temperature applications —  a decision the owner must consider in order maintain to batteries warranty.

The continuous ventilation system must be monitored for airflow and can be done by installing a flow switch upstream or downstream of the exhaust fan at locations with minimum air turbulence and stable static pressure readings. Flow switches are available with Form C relays and can be used to send local or remote signals to notify maintenance personnel if the exhaust fan is not operating.

The downside of continuous ventilation is that it increases maintenance and energy costs, and reduces the service life of the fan. An estimated cost of approximately $200/yr of energy would be wasted at rate of $2/cfm, assuming 100-cfm exhaust fan. Therefore, continuous ventilation is recommended only when the space is not air conditioned.

An alternative variation of continuous ventilation in air conditioned battery room spaces is to utilize, as makeup air, the conditioned air from other occupied spaces that would require ventilation as part of the indoor air quality requirements.


Intermittent Ventilation, Monitoring, and Limiting H2 Concentration

The second of the two code-required ventilating systems is the intermittent ventilation coupled with H2 monitoring and limiting. This approach requires the design of a gas detection and ventilation system that will operate only when it is needed. The ventilation system requires the calculation of H2 evolution rate generated by the battery systems. This H2 evolution rate is crucial to properly sizing the exhaust fan. When calculating the H2 evolution rate, the following factors need to be considered: types of batteries used (VRLA, flooded lead-acid, or Ni-Cd), charging mode (float or boost mode), battery system charging current and voltage, and the quantity of batteries. The H2 evolution rate can be calculated using IEEE 1635/ASHRAE Guideline 21 — Guide for the Ventilation and Thermal Management of Batteries for Stationary Applications formulas. However, it is best to obtain the H2 evolution rate directly from the battery manufacturer since the battery chemistry varies between battery manufacturers.

The gas detection system monitors and limits the H2 concentration from exceeding the 25% of LEL level in the space, and it interlocks with the exhaust fan to operate when H2 is detected. The gas detection system is composed of a H2 sensor head, communication wiring, and a H2 controller. The sensor monitors the hydrogen levels and when the predetermined concentration is reached, sends a signal to the H2 controller to activate the proper alarms. The H2 controller is recommended to be installed on the outside of the enclosure if possible so H2 concentration levels can be verified before entering the space.

Most H2 controllers come configured with two alarms. Alarm one normally energizes the exhaust fan and motorized make-up air dampers to open. Alarm two reports normally to a remote occupied location to notify personnel and can be designed to activate local horns and strobes. Alarm two satisfies the monitoring requirement since it will notify personnel when the exhaust fan does not operate.

The battery system layout and ceiling construction will determine the best location for the gas sensors. Follow manufacturer’s installation guidelines for H2 sensor location and sensor coverage area. H2 is lighter than air and rises to ceiling areas; therefore, the ceiling must be smooth to avoid pockets that can trap H2 gas. The detection system requires regular maintenance and cost can be minimal if scheduled at the same time the battery system is checked.


Battery Room Configurations

The suitability of the ventilation approach can be influenced by the configuration of the battery room. The battery system may be installed in a dedicated enclosure or within a space that is shared with other office spaces, or in areas much larger than the battery room like in a warehouse.


Dedicated Enclosure

For battery rooms with a dedicated enclosure that are not air conditioned and are relatively small, continuous ventilation at 1 cfm/sq-ft is a simple and practical design. The exhaust fan can be ceiling or wall-mounted. Wall-mounted exhaust fans must be installed as close as possible to the ceiling, providing enough clearance for service and maintenance between fan and ceiling. The exhaust fan must discharge to the outdoors per UMC and IMC as described above. A flow switch can be installed upstream or downstream of the fan to monitor air flow.

For battery rooms that are relatively large, the 1 cfm/sq-ft rate would result in a very large exhaust fan, which may be impractical and inefficient. In this case, the approach of monitoring and limiting H2 concentration from exceeding 25% of LEL is a better approach. The hydrogen detection system must be interlocked with an exhaust system that can provide the minimum air changes required to replace the air in the enclosure and limit the H2 concentration to no more than 25% of LEL. Under normal conditions, the exhaust fan is not energized but will get energized upon detection of H2 in the room.


Shared Space

For battery systems that are installed in a relatively large space like a warehouse which are not normally air conditioned, a localized ventilation system is ideal. The batteries can be installed under a hood with an exhaust system to remove H2 gas to the exterior of the building. The ventilation rate of 1 cfm/sq-ft rate is appropriate for this configuration since the area used for the ventilation rate calculation is the cabinet or rack area under the hood.

This type of design results in a relatively small ventilation system serving two purposes: to remove H2 gas and remove heat generated by the batteries. There are packaged hoods and exhaust systems available for this type of application. Hood design must meet UMC and IMC construction requirements. Similarly, as mentioned above, to comply with ventilation monitoring requirements, a flow switch shall be installed upstream or downstream of the exhaust fan.


Design of Ventilation System

For the battery room case study, a dedicated air conditioned enclosure will be used as an example. The enclosure is a sheet metal building with insulated walls and roof and dimensioned 12 ft by 32 ft by 10 ft high. The building has access doors on both ends and has no windows.

The battery system consists of the following:

  • Lead-calcium batteries, VRLA
  • Float voltage of 2.21 V per cell, three cells per unit
  • Equalize voltage of 2.33 per cell
  • Normal operating temperature of 77ºF
  • Two parallel strings (120 cells per string) at nominal 250 amp-hour
  • Total electrolyte – 408 gallons

For this case, monitoring and limiting H2 concentration is the best approach since the space is air conditioned. This approach requires the installation of an exhaust fan interlocked with a hydrogen detection system. The detection system monitors H2 concentration levels in the enclosure and activates the ventilation system when 1% of H2 LEL is reached.

The battery system specified has a H2 evolution rate of approximately 0.0031 cfm, while the formulas provided by IEEE/ASHRAE Guide 21 yielded 13 cfm. The IEEE/ASHRAE formula was based on a worst-case scenario when batteries are overcharged. The calculated required ventilation would take approximately three minutes to reach the limiting concentration, using the more conservative H2 evolution rate, the volume of the enclosure (3,840 cu ft), and 1% of H2 LEL as the limiting factor. The manufacturer’s H2 evolution rate would take approximately 200 hours or approximately eight days to reach the limiting concentration.

For this project, a 400-cfm fan was selected to provide approximately one air change in 10 minutes. The fan selected was a roof-mounted type with high enough static pressure to overcome the static pressure from make-up air dampers and fan losses. The fan was installed on the roof to meet UMC and IMC requirements for the exhaust discharge termination clearances.

The two approaches resulted in a fan of the same size. The advantage with the selected approach is that the fan will only operate intermittently when needed, resulting in energy savings and prolonged fan service life. The activation of the exhaust fan is controlled by the hydrogen detection system. The H2 controller is configured to run the exhaust fan for 10 minutes every time the 1% of H2 LEL concentration is reached, providing one air change every time it comes on.

Motorized make-up air dampers are installed approximately 12 in from the floor at both ends of the building to provide cross ventilation and to assure H2 is completely removed. The enclosure has a smooth ceiling to avoid H2 potential accumulating in pockets. The two battery strings are laid out along the 32-ft walls. The air conditioning unit is installed at the middle of the building and to avoid installing hydrogen sensors close to moving air devices, two sensors are located about 8 ft to 9 ft from both ends of the building. The H2 controller is mounted near one of the exit doors for easy access. Outdoors strobes are installed at both exit doors to warn personnel when there’s H2 concentration of greater than 1%. The strobes are color blue following the standard color code for environmental conditions. The hydrogen controller alarm one (1% of LEL) energizes the exhaust fan. Alarm two (2% of LEL) sends remote notification and activates local strobe.



The design of battery room ventilation involves compliance with multiple codes and regulations. Regardless of the size of the battery system, some type of ventilation is required. Even though codes allow for natural ventilation, mechanical ventilation is more reliable and effective. There are two approaches to the design of the ventilation system: continuous ventilation at 1 cfm/sq-ft or intermittent ventilation that monitors and limits H2 gas concentration from exceeding 25% of H2 LEL. The best approach will depend on the battery room configuration.



  1. EnerSys US-FL-IOM-001, Safety, Storage, Installation, Operation & Maintenance Manual: Flooded Lead-acid Batteries C,D, E, F and G. Reading, PA 2007, p. 45?.
  2. IEEE/ASHRAE Guide for the Ventilation and Thermal Management of Batteries for Stationary Applications. New York: IEEE
  3. NFPA 1, 2011 Edition, Fire Code.
  4. NFPA70, National Electric Code Handbook, 2014 Edition.
  5. International Fire Code (IFC)
  6. International Mechanical Code (UMC)
  7. Code of Federal Regulations Title 29 (29CFR Subtitle B), Section 1926.411(2), Occupational Safety and Health Administration
  8. The Institute of Electrical and Electronics Engineers (IEEE)

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