Forklift battery charging rooms — explosion hazard and ventilation

An in-house transport operation at a large greenhouse horticulture business — forklifts, pallet trucks, AGVs moving between the greenhouse and the processing hall — often runs on lead-acid traction batteries that are charged overnight. That charging process is not without risk: NEN-EN-IEC 62485-3 therefore sets requirements for the design of the charging room, the ventilation, and safe work with industrial batteries.

Why hydrogen gas is the risk

During charging (especially during the final stage and during equalizing/boost charging) part of the charging current decomposes the water in the electrolyte into hydrogen and oxygen — electrolysis. Hydrogen gas forms an explosive mixture with air as soon as the concentration exceeds 4 vol.% (the lower explosion limit, LEL). Because hydrogen is lighter than air, it collects at the top of a room or above the battery itself if ventilation is inadequate.

Ventilation: the core requirement

The standard requires a calculated minimum ventilation capacity, not a fixed "airing is probably enough" feeling. The simplified formula from the standard:

Q = 0,05 × n × I_gas × C_rt × 10⁻³ (Q in m³/h)

where n is the number of cells, I_gas the gassing charging current (mA per Ah of nominal capacity — to be read from the battery/charger documentation, higher during equalizing charge than during trickle charge) and C_rt the nominal capacity in Ah. The 10⁻³ factor converts I_gas from mA to A — for a battery with 12 cells, 256 Ah and I_gas = 10 mA/Ah, this works out to ≈ 1.5 m³/h. In practice this translates into two design rules:

Air inlet low, air outlet high in the room — hydrogen rises.
Natural ventilation is often insufficient at higher charging powers or in smaller rooms; mechanical extraction is then mandatory.

Note: I_gas differs per battery type, charging characteristic, and charging stage — calculate this per project using the actual charger specifications, do not adopt a fixed rule-of-thumb value.

Not automatically an ATEX zone

A common misconception: "hydrogen gas is generated, so this is an ATEX zone". With correctly sized ventilation (per the formula above) the hydrogen concentration stays well below the LEL, so in practice no ATEX zone classification is often required — provided the ventilation requirement has actually been calculated through and is demonstrably being complied with. Without that calculation (or in case of failing ventilation) an explosion-hazard zone does arise around the battery during charging. See the ATEX guide for the general zone classification in case a risk assessment does lead to an ATEX zone.

Other requirements from the standard

A safety zone directly around the battery in which, during charging, no ignition sources may be present (smoking, open flames, spark-producing tools).
Electrical safety of the charging installation itself (cabling, charging plugs, charger).
Eyewash facilities and personal protection against electrolyte splashes (corrosive battery acid) during maintenance/topping up water.

Common mistakes

Considering a charging room "self-evidently safe" without actually calculating the ventilation capacity — natural ventilation that was once sufficient may no longer be so after fleet expansion or a switch to fast chargers.
Allowing ignition sources within the safety zone around the battery during charging, even if the concentration seems (still) low at that moment.
Assuming a battery charging room is by definition an ATEX zone — and conversely: assuming it can never be an ATEX zone without having calculated the ventilation requirement.