Battery rooms and battery energy storage: do they need ATEX zoning?

Battery charging rooms with vented lead-acid or nickel-cadmium batteries must be assessed, because these batteries release hydrogen while charging. In most cases ventilation designed to IEC 62485-2 (stationary batteries) or IEC 62485-3 (traction batteries) keeps the hydrogen well below its 4 % lower explosive limit, so the room itself is non-hazardous and only a small area close to the cell vents needs control of ignition sources. Lithium-ion battery energy storage (BESS) does not release gas in normal operation and is not normally zoned; its explosion hazard comes from thermal runaway and is handled by NFPA 855 explosion control, gas detection and ventilation.

Quick answer by battery type

The hazard: hydrogen from charging

During charging, part of the current electrolyzes water in the electrolyte into hydrogen and oxygen in a 2:1 ratio. The ZVEI leaflet on traction battery rooms notes that gassing rises with state of charge, and that near the end of charge almost all of the charging current goes into gas production. It gives a figure of 0.450 liters of hydrogen per hour for each ampere of final charging current (at 25 °C).

HSE guidance INDG139 warns that gas bubbles trapped in a vented battery can be released when the battery is moved, and that even valve-regulated batteries can vent gas through their relief valves if charged too fast or too long.

How IEC 62485 handles it: ventilation first

The IEC 62485 series does not start from zoning. It requires enough airflow to dilute the hydrogen, and treats a battery location as safe from explosions when ventilation keeps the concentration below the safe limit. For stationary batteries, IEC 62485-2:2010 (adopted in Europe as EN IEC 62485-2:2018) gives the required airflow as:

Q = 0.05 × n × Igas × Crt × 10⁻³  [m³/h]

where n is the number of cells, I_gas the gas-producing current in mA per Ah of capacity (different values for float and boost charge), and C_rt the rated capacity in Ah. The factor 0.05 combines the hydrogen generation rate, the dilution needed to stay below 4 % and a safety factor of 5. For traction batteries, the ZVEI leaflet on IEC 62485-3 uses Q = 0.055 × n × I_gas (I_gas in A) and, where the charger maker cannot give I_gas, tells designers to use at least 40 % of the charger's maximum current.

The same documents set practical rules for the room:

• Air inlets low, outlets high. Hydrogen is buoyant, so air should enter near the floor, pass over the batteries and leave as high as possible.
• Natural ventilation where possible. ZVEI gives an opening area of A = 28 × Q (cm²) for inlets and outlets, with conditions on room volume and air speed. Where natural ventilation cannot be guaranteed under all conditions, forced ventilation is needed.
• All batteries at once. If several batteries can charge in the same room, the airflow is the sum for all of them at maximum gassing, unless the chargers are controlled to prevent this.

When a zone or safety distance still applies

Room ventilation cannot guarantee dilution right at the cell vents. IEC 62485 therefore sets a safety distance, measured through air from any cell opening, inside which there must be no sparks, arcs, glowing objects or flames (surface temperature limited to 300 °C). IEC 62485-2 gives a formula for this distance based on the gassing current and cell capacity. For traction batteries, ZVEI cites 0.5 m under EN 62485-3.

HSE's INDG139 takes a hazardous area classification view of the same area. It says that calculated ventilation may allow all but the immediate vicinity of the battery to be classed as non-hazardous, and that in most situations a Zone 1 should be considered to exist for up to one meter in all directions around batteries under charge. It also sets dilution targets: no more than 0.4 % hydrogen (10 % of LEL) in charging areas people enter, and no more than 1 % (25 % of LEL) in enclosures people do not enter.

A larger or more severe zone is justified when:

• batteries charge inside cabinets, vehicle battery compartments or rooms with no reliable airflow;
• forced ventilation has no airflow monitoring or charger interlock, so its availability cannot be relied on;
• boost, equalizing or fast charging raises gassing well above the design case;
• the ceiling has beams, pockets or a false ceiling where hydrogen can collect out of the air path.

In the US, fire codes set a similar ventilation target for batteries that produce hydrogen during normal charging: the concentration in the room must be limited to 25 % of the LFL (1 % by volume). NEC Article 480 also requires provisions for ventilation and diffusion of battery gases. A battery room is not a classified location by default; it becomes one only if the assessment finds a flammable concentration can occur.

Equipment in and near the charging area

• Inside the safety distance or zone: no ordinary switches, sockets, chargers or luminaires. If equipment must be there, it must suit the zone (Zone 1: EPL Gb, Category 2G) and hydrogen: gas group IIC or IIB+H2. T1 is enough for hydrogen alone.
• Chargers: place them outside the safety distance, and preferably away from the high-level air path.
• Extract fans: the fan handles air leaving the room. Whether its motor must be Ex depends on the concentration it can see if charging continues after a failure. Interlocking the chargers with airflow monitoring reduces that case.
• High-level equipment: check lighting, detectors and cable routes at ceiling level where hydrogen collects if ventilation fails.

See Gas Groups and EPL for the marking to look for.

Lithium-ion BESS: a different problem

Lithium-ion cells are sealed and do not gas in normal operation, so there is no routine source of release to classify. IEC 62485-5:2020 covers stationary lithium-ion installations and addresses gas emission, fire and explosion hazards under normal operation and expected fault conditions. The explosion risk comes from thermal runaway: a failing cell vents a flammable mixture that typically includes hydrogen, carbon monoxide and hydrocarbons, and in a container this can build up to a deflagration. IEC 60079-10-1 excludes catastrophic failures and rare malfunctions from area classification, so this case is not handled by drawing Zone 1 or Zone 2 around racks.

Instead, the US standard NFPA 855 (Installation of Stationary Energy Storage Systems) requires explosion control for lithium-ion systems:

• 2023 edition: deflagration venting to NFPA 68 or explosion prevention to NFPA 69 (for example gas detection that triggers exhaust ventilation to keep gas below 25 % of the LFL).
• 2026 edition: industry summaries report that the explosion-control requirements were tightened, with more weight on NFPA 69 prevention (listed gas detection plus ventilation that keeps working during a thermal runaway event) and on performance-based designs backed by testing. Check the edition your authority having jurisdiction has adopted and read its explosion-control section before designing. Gas composition and volume data from UL 9540A testing are used in either case.

In Europe, the 1999/92/EC or DSEAR risk assessment must still consider the thermal runaway scenario, even though it usually does not lead to zones. NFPA 855, NFPA 68 and NFPA 69 can serve as the engineering basis there too. Equipment that must keep working in that scenario, such as exhaust fans and gas detectors, has to suit the gas it may handle and the temperatures it may see.

Common mistakes

• Marking the whole battery room as Zone 1 instead of calculating ventilation, which pushes all lighting and power into Ex equipment for no benefit.
• Doing the opposite: calling the room safe with no airflow calculation and no check of the ceiling.
• Taking I_gas for float charge only and ignoring boost or equalizing charge.
• Fitting a charger or socket inside the safety distance around the cells.
• Assuming VRLA or "maintenance-free" batteries need no ventilation.
• Applying lead-acid zoning logic to lithium-ion racks, or assuming "no zone" means "no explosion hazard" for lithium-ion.

What else applies

• Hydrogen detection at high level, alarming and stopping charging, where ventilation cannot be relied on.
• Eye-wash, acid-resistant floors and PPE for electrolyte (INDG139).
• For BESS: NFPA 855 spacing, fire detection and suppression, UL 9540A test data, and emergency response planning.
• For hydrogen-producing systems, background on the gas itself: Hydrogen explosion protection.

This page describes typical practice. The classification for a given battery room belongs in the site's own risk assessment, made by a competent person.

Frequently asked questions

Does a battery charging room need to be ATEX rated?

Usually not as a whole. Vented lead-acid and NiCd batteries release hydrogen while charging, but ventilation sized to IEC 62485-2 or -3 normally keeps the room non-hazardous. A small area close to the cells still needs control of ignition sources, and HSE guidance suggests treating up to one meter around batteries on charge as Zone 1 in most situations.

What is the safety distance around batteries on charge?

IEC 62485 requires a safety distance from cell openings inside which no sparks, arcs, flames or glowing objects are allowed. IEC 62485-2 calculates it from the gassing current and capacity; for traction batteries the ZVEI guidance on EN 62485-3 gives 0.5 m.

How much ventilation does a battery room need?

IEC 62485-2 gives Q = 0.05 x n x Igas x Crt x 10^-3 m3/h for stationary batteries, which builds in dilution below the 4 % hydrogen LEL and a safety factor of 5. Air should enter low and leave high because hydrogen rises.

Do lithium-ion battery rooms need ATEX zoning?

Not normally, because lithium-ion cells do not gas in normal operation. The explosion hazard comes from thermal runaway, which NFPA 855 handles through explosion control to NFPA 69 (and, before the 2026 edition, NFPA 68 venting), gas detection and ventilation.

Sources

1. IEC: IEC 62485-2:2010 Safety requirements for secondary batteries and battery installations - Part 2: Stationary batteries
2. ZVEI: Information leaflet No. 14, Ventilation of battery charging rooms for lead traction batteries (May 2020)
3. HSE: INDG139 Using electric storage batteries safely
4. IEC: IEC 62485-5:2020 Safe operation of stationary lithium ion batteries
5. Exponent: NFPA 855 expands safety guidelines for battery energy storage systems (2026 edition)
6. IEC: IEC 60079-10-1:2020 Classification of areas - Explosive gas atmospheres

Related Topics

• Does It Need ATEX Zoning?
• Hydrogen Explosion Protection
• Zone Classification
• Gas Groups
• EV Charging and Hazardous Areas