Ventilation of stationary battery rooms (UPS/telecom) — IEC 62485-2

The forklift battery room guide covers the ventilation requirements for traction batteries in forklifts and similar internal transport, laid down in IEC 62485-3. This article covers the sister standard for a different application: IEC 62485-2 sets the safety requirements for stationary batteries — the lead-acid (vented or VRLA) or NiCd battery banks that serve as backup power for UPS systems, telecom equipment, emergency lighting and grid support, up to a maximum DC voltage of 1500 V.

Hydrogen gas: the same risk, a different application

Just as with traction batteries, charging stationary lead-acid or NiCd batteries produces hydrogen gas through electrolysis of the water in the electrolyte (see the forklift battery room guide for the underlying chemistry). IEC 62485-2 sets as its core requirement that ventilation of the battery room keep the hydrogen concentration in the air below 1 vol.% — well below hydrogen's lower explosion limit (LEL) in air of roughly 4 vol.%, with a substantial safety margin built into that 1% threshold.

The required ventilation capacity is calculated based on:

• the maximum hydrogen gas production during charging (depending on battery type, charging current and charging phase);
• the volume of the battery room;
• the number of air changes per hour needed to stay below the 1% threshold.

Safety distance from ignition sources

Besides the ventilation requirement, the standard sets a concrete safety distance of 0.5 m (a "free air zone") from a cell opening, within which no open flames, sparks, electric arcs or glowing objects may be present — comparable to the ignition-source-free zone around a forklift charging point, but with its own distance value standardised for stationary installations.

Note: the exact ventilation calculation (air changes per hour, minimum and maximum charging current, number of cells) is project-specific and follows from the full standard and the battery/charger documentation — this article gives the principle, not a ready-made formula for every situation.

Difference from IEC 62485-3 (traction batteries)

IEC 62485-2 and IEC 62485-3 are part of the same standard series and share the same underlying hydrogen-gas risk, but target a different application and a different type of professional:

Both standards share the principle of "calculate the ventilation, do not rely on a fixed 'seems airy enough' feeling", but the exact formulas and boundary conditions differ per standard.

Practical relevance

When designing or inspecting a UPS/telecom battery room (for example a data centre, an emergency lighting backup supply, or a telecom shelter), it must be established whether the existing ventilation (natural or mechanical) is actually sufficient for the maximum charging current and number of cells in that room — expanding battery capacity without recalculating ventilation is a common source of a no-longer-compliant situation.

Common mistakes

1. Assuming the same ventilation values as a forklift battery room — IEC 62485-2 and IEC 62485-3 are separate standards with their own calculation bases; not interchangeable without recalculation.
2. Expanding battery capacity without recalculating ventilation — more cells or a higher charging current means higher hydrogen gas production, which can exceed existing ventilation capacity.
3. Allowing ignition sources within the safety distance around a cell opening during charging, even in a room that otherwise "just looks like a server room".

Related

• Forklift battery rooms — IEC 62485-3
• UPS battery types — VRLA/lithium
• UPS — emergency power continuity