Is a forklift battery charging area a hazardous zone?

Normally not as a whole. A charging area for lead-acid forklift batteries that is ventilated according to IEC 62485-3 (EN 62485-3) is treated as non-hazardous, because the hydrogen is diluted far below its lower explosive limit. The space directly above and around the batteries on charge is different: there, ignition sources must be kept away, and UK HSE guidance treats up to one metre around batteries under charge as a Zone 1.

Short answer

• Lead-acid and nickel-cadmium traction batteries release hydrogen and oxygen when charged, most of it towards the end of charge. Ventilation sized to IEC 62485-3 keeps the charging area as a whole out of the explosive range.
• Near the cell openings dilution is not assured. Industry guidance on EN 62485-3 gives the standard's safety distance of 0.5 m of air from cell openings, free of flames, sparks, arcs and glowing parts, with surfaces at most 300 °C. HSE guidance considers a Zone 1 of up to one metre around batteries under charge.
• Gassing continues after charging. HSE advises letting batteries stand at least 20 minutes after charging; Swiss guidance asks for forced extraction to run at least one hour after the end of charge.
• If the ventilation is not adequate or not interlocked, the area has to be classified under IEC 60079-10-1 and the result can be a larger zone, especially under the roof.
• Lithium-ion forklift batteries are outside IEC 62485-3 and do not gas in normal charging; their main hazard is thermal runaway, which releases flammable gases.

Why the charging area is usually not zoned

A charger for a large traction battery pushes high current for hours, often unattended overnight. As each lead-acid cell approaches full charge, surplus current splits water into hydrogen and oxygen. Hydrogen ignites from 4 % vol, needs very little ignition energy and rises quickly.

IEC 62485-3:2014 (edition 2, covering lead-acid, nickel-cadmium, nickel-metal hydride and other alkaline traction batteries, but not lithium) handles this with a ventilation formula. Industry guidance on the European version writes it as Q = 0.055 x n x Igas, with Q in m³/h, n the number of cells and Igas the gas-producing current in amperes from the charger characteristic. If the real Igas is not known, the guidance says to use at least 40 % of the charger's maximum charging current. With that air flow, the hydrogen leaving the batteries is diluted far below the LEL, and both HSE and Swiss accident insurance guidance accept that the area beyond the immediate vicinity of the batteries can be non-hazardous.

What the formula cannot do is dilute the gas at the moment it leaves a cell. That is why the space just above the battery tray needs its own rule.

Worked example: five trucks on charge

A warehouse charges five trucks at the same time. Each has a 48 V lead-acid battery (24 cells) and a charger whose real gassing current is unknown, with a maximum charging current of 100 A.

1. Igas: at least 40 % of 100 A, so 40 A per battery.
2. Air flow per battery: Q = 0.055 x 24 x 40 = about 53 m³/h.
3. Total: five batteries charging together need about 264 m³/h of air through the charging area.
4. Natural ventilation: the guidance sizes each inlet and outlet at A = 28 x Q cm², about 0.74 m² of free area for the inlet and the same for the outlet, inlets low and outlets as high as possible.

With the charger manufacturer's actual Igas the figure is usually lower. The point of the example is the order of magnitude: a busy charging area needs real, planned ventilation, not just a large hall.

Practical rules for the charging point

• Designate the charging area and keep it for that purpose. US OSHA 29 CFR 1910.178(g) requires designated charging areas, adequate ventilation for dispersal of fumes from gassing batteries, no smoking, and precautions against open flames, sparks and electric arcs.
• Open the battery cover or compartment before charging starts, so the gas is not trapped in the truck.
• Keep ignition sources out of the space above the batteries: chargers, sockets, switches, non-Ex luminaires, welding, grinding and mobile phones. Mount chargers beside, not above, the battery positions.
• Interlock forced ventilation so that batteries are charged only when the extraction is working, and keep it running after charging ends.
• Never disconnect the charger plug under load. Switch the charger off first; the spark at the connector is next to the richest gas.
• Avoid ceiling pockets above charging positions, where hydrogen can collect.

Common mistakes

• Charging in an open hall with no calculation. A large building volume does not replace an air flow calculation, and hydrogen can collect in roof voids.
• Using the float current of a stationary battery standard for traction batteries. Traction charging is governed by the charger characteristic under IEC 62485-3.
• Moving a truck off charge straight into a narrow aisle with hot work going on. Batteries that have just finished charging still release gas.
• Applying lead-acid rules to lithium-ion trucks, or the reverse. Mixed fleets need both sets of measures.

Related on excodex

• Battery rooms and charging areas: hydrogen ventilation and hazardous areas
• Hydrogen safety basics: wide flammable range, low ignition energy and invisible flames
• Is a lead-acid battery room a hazardous area, even with sealed VRLA batteries?
• How is the required ventilation of a battery room calculated?
• Gas card: hydrogen

Standards and further reading

• IEC 62485-3:2014 (edition 2), safety requirements for secondary batteries and battery installations, Part 3: traction batteries: catalogue entry
• IEC 60079-10-1, classification of areas, explosive gas atmospheres
• ZVEI information leaflet 14, ventilation of battery charging rooms for lead traction batteries, guide to EN 62485-3 (May 2020): PDF
• HSE INDG139, Using electric storage batteries safely (rev. 2011): hse.gov.uk (PDF)
• US OSHA 29 CFR 1910.178(g), changing and charging storage batteries of powered industrial trucks: osha.gov
• Suva, ventilation measures when charging lead batteries (in French): suva.ch

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.