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Hydrogen Ventilation for VRLA and UPS Batteries

Oct 21
5 min read

Updated: Oct 22

Valve-regulated lead-acid (VRLA) batteries are often called "sealed", but they still need ventilation. This guide covers what C&D Technologies and EnerSys publish about hydrogen gas, ventilation and charging, and the product scope each document applies to.

Where the hydrogen comes from

  • Charging past full. C&D explains that when a lead-acid battery charges, the part of the current beyond what keeps it at 100% state of charge electrolyzes water in the electrolyte, producing oxygen and hydrogen (C&D 41-6739, p. 1).

  • VRLA batteries recombine most of it. C&D says the VRLA battery's oxygen recombination cycle minimizes hydrogen generation. It gives a typical recombination rate of 95 to 99%, so a VRLA battery emits "only a very small fraction" of the hydrogen a vented (wet) lead-acid battery would (C&D 41-6739, p. 1). For its Liberty 2V series, C&D says testing shows recombination at nearly 99% efficiency under normal conditions (C&D Liberty 2V manual RS02250, §6.2, p. 12).

  • The valve opens under pressure. C&D says the VRLA battery's self-resealing one-way valve relieves internal pressure from overcharging above 1 to 2 psig, then closes to keep outside air out (C&D 41-6739, p. 1).

  • Abnormal charging releases more. C&D says VRLA batteries contain potentially explosive gases that may be released under abnormal operating conditions, such as a charger malfunction (C&D Liberty 2V manual RS02250, p. 2). EnerSys warns that in case of accidental overcharge, a flammable gas can leak off the safety vent (EnerSys SBS XL manual, p. 2).

Hydrogen limits: 4% and the recommended ceiling

Both C&D documents put the danger point at 4% hydrogen in air:

The recommended ceiling below that differs by document, so use the figure in your own battery's current documentation:

Document and scope

Recommended ceiling

C&D 41-6739, written for C&D Dynasty 12 Volt VRLA products (larger 2 Volt products are outside its intended scope)

"no more than a 2% level be allowed to accumulate"

C&D Liberty 2V manual RS02250, safety precautions

Customer to provide ventilation so hydrogen accumulation in the battery area does not exceed two percent by volume

C&D Liberty 2V manual RS02250, §6.2 Ventilation

C&D "recommends not allowing hydrogen gasses of greater than one (1%) percent by volume to accumulate"

The Liberty manual also says to contact the local code enforcement officer to find out which codes and levels apply to your battery room (C&D Liberty 2V manual RS02250, §6.2, p. 12). C&D 41-6965 notes that several codes or regulations may apply to managing released gases, "including but not limited to IEEE 1635, National Electric Code, National Fire code and OSHA standards" (C&D 41-6965, p. 2).

Does a VRLA battery need a special battery room?

Not normally, according to both makers, but it does need ventilation:

  • C&D (12V products): the gas volume under normal float and equalization charging is very small, and "special mechanical ventilation would not normally be required". Hydrogen is lighter than air and disperses quickly. Unrestricted air movement around the batteries, plus the normal air circulation and heat removal of an occupied area, is typically more than adequate to prevent an explosive build-up (C&D 41-6739, p. 1).

  • EnerSys (PowerSafe DDmP125-33 LP): "No separate battery room or 'hood' exhaust is required. However, VRLA batteries do require adequate ventilation and should not be installed in 'airtight' locations" (EnerSys DDmP125-33 LP manual, Table 5.1, p. 12).

  • EnerSys (PowerSafe SBS XL): the battery compartment or room "must have adequate ventilation to limit hydrogen accumulation", and batteries must be installed in accordance with IEC 62485-2 and any other local or national laws and regulations (EnerSys SBS XL manual, §3).

Never a sealed cabinet

Every C&D document used here says the same thing:

  • "the VRLA battery should never be charged in a 'sealed' container lacking ventilation" (C&D 41-6739, p. 1).

  • Do not install and charge batteries in a sealed container. If the batteries are in a container, make sure the container or cabinet and the room have adequate ventilation to prevent a build-up of vented gas (C&D 41-6965, p. 2).

  • "Lead acid batteries should never be installed in a sealed, non-ventilated cabinet or enclosure" (C&D Liberty 2V manual RS02250, §6.2, p. 12).

Work out the numbers for your room

C&D says it's advisable to calculate the gas emissions to expect from a battery system and the air exchanges needed to prevent hydrogen accumulation, "if only to provide assurances of the local building inspector". The calculation needs the battery gassing rate, which C&D lists by model in Table 1 of the bulletin, and the room's unoccupied volume (C&D 41-6739, pp. 2–3). For its Liberty 2V series, C&D gives a hydrogen release rate under extreme overcharge voltages of 0.000269 cubic feet per minute per ampere of charging current at 77 °F (25 °C) (C&D Liberty 2V manual RS02250, §6.2, p. 12).

Use your battery maker's gassing data for your models, and have the calculation checked against the codes that apply to your site.

Keep gassing low with correct charging

C&D's recommendations for minimum gassing, written for its 12V VRLA products (C&D 41-6739, pp. 2–3):

  1. Float charge at the recommended 2.25 to 2.30 volts per cell average at 77 °F (25 °C).

  2. Adjust the float voltage for operating temperatures other than 77 °F (25 °C) when the ambient is expected to differ by more than 10 °F (5.6 °C).

  3. Use temperature-compensated charging if frequent wide variations of the ambient are expected.

  4. Do not charge at temperatures above 122 °F (50 °C).

  5. Allow free flow of air around the individual batteries (0.5 inch spacing between units).

  6. Provide natural or forced ventilation of the battery area to avoid gas building up over long periods.

  7. Calculate the expected gas accumulation in the battery area and the safety margin.

C&D also says equalize charging should be limited to 2.4 volts per cell. Above 2.3 volts per cell the gassing rate "increases dramatically", and at 2.5 volts per cell it could increase by a factor of 20 or more compared with 2.3 volts per cell (C&D 41-6739, p. 2). Your own battery's float and equalize settings may differ, so use your maker's figures. See Heat and UPS Battery Life: What Battery Makers Say for temperature compensation.

Basic precautions near the battery

Planning a battery installation or replacement?

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Sources (checked 2026-10-21)

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