Practically every manufacturing plant requires that energy reserves be available in the form of accumulators. Accumulators provide backup power to machines in the event of a mains outage, enable the use of electrically-powered vehicles such as forklifts, trolleys and cleaning machines.
Cheap but prone to exploding
Lead-acid batteries are among the most popular types of accumulators used for industrial applications. The main advantage of using this type of battery is its low price – lead-acid batteries are the cheapest battery type on the market. Despite their popularity, some users are not aware of the fact that these batteries pose a genuine explosion hazard.
Lead-acid batteries used for industrial applications can be broadly divided into two groups: traction batteries and stationary batteries. The principle of operation for both types is identical. Lead-acid cells contain lead electrodes. The electrolyte is an aqueous solution of sulphuric acid.
Both stationary and traction lead-acid batteries can be further divided into the following types:
- vented cell batteries,
- VRLA batteries, also known as maintenance-free batteries,
- sealed cell batteries.
Hydrogen explosion hazard
Using batteries requires them to be charged (cyclic or standby charging – table 1). This seemingly safe operation can cause an explosive atmosphere to be produced, resulting from the release of hydrogen from the cells.
| APPLICATION | CHARGING METHOD | STANDARD SPECIFYING SAFETY REQUIREMENTS | |
|---|---|---|---|
| Traction batteries | In electrically powered vehicles: - forklifts, - trolleys, - cleaning machines, - electric on-road vehicles. | Cyclic charging the batteries are alternately charged and discharged. | PN-EN 62485-3:2014 Safety requirements for batteries and their installation - Part 3: Traction batteries |
| Stationary batteries | In uninterruptible power supply devices, including: - UPS devices, - power supply systems in server rooms. | Standby operation - batteries are constantly connected to a charger and are constantly ready to supply energy in the event of a power outage. | PN-EN:62485-2:2018 Safety requirements for secondary batteries and battery installations - Part 2: Stationary batteries |
How is hydrogen produced during charging?
Hydrogen, an explosive gas, is produced in the process of charging stationary and traction batteries as a result of the electrolysis of water by the charging current. When the cell is fully charged, electrolysis of water occurs in accordance with Faraday’s law.
Under normal conditions:
- 1 Ah decomposes 0.336 g of H2O into 0,42 l H2 and 0,21 l O2,
- 3 Ah decomposes 1 cm3 (1 g) H2O,
- 26,8 Ah decomposes 9 g H2O into 1 g H2 and 8 g O2.
Explosive properties of hydrogen
Hydrogen is the lightest of all gases; it is also extremely flammable and explosive. It is characterized by a low minimum ignition energy and a very wide range of explosive limits.
Due to the fact that the density of the gas is much lower than that of air (relative density of 0.07), hydrogen will accumulate in the upper parts of the room when released in unventilated or poorly ventilated rooms. Gas may continue to be released from the cells for up to one hour after the end of the charging process.
How to assess the level of risk of explosive atmosphere formation?
The first step in determining the risk of formation of an explosive atmosphere in a battery charging room is to identify the type of batteries on hand, as the amounts of hydrogen released into the room will differ depending on the type.
Type of cell used is of key importance
Open cells (in both stationary and traction batteries) will release more hydrogen than valve regulated cells.
Hydrogen is released from open cells via openings in the lids/caps that allow gases to be released freely from inside the batteries.
In normal conditions, valve regulated VRLA cells are tightly sealed, but are equipped with valves that allow gas to escape when the pressure inside the cell exceeds a set value. There are also sealed cells available on the market that are guaranteed by the manufacturer not to release gases when operating in specified working conditions. Such cells may be equipped with devices that prevent excessive pressurization.
What should be done if there is no way of preventing hydrogen release?
Due to the inherent nature of the process, hydrogen release during battery charging cannot be completely eliminated. Providing effective ventilation, preferably mechanical, is of paramount importance for ensuring adequate safety in battery charging rooms and locations. A well designed system will reduce the likelihood of formation of an explosive atmosphere inside a room.
Ventilation assessment system
Given the above, the ventilation system used in the battery charging rooms must be assessed. Polish standards that define requirements for the safe use and installation of batteries (EN 62485-2:2018 for stationary batteries and EN 62485-3:2014 for traction batteries) suggest a methodology to determine the necessary volume of ventilating air inside a room.
How to arrange components of the ventilation system?
Calculations account for the parameters of charged batteries, but not the internal dimensions of the charging room. Standards allow for the use of mechanical and natural ventilation, provided that an adequate fresh air flow rate is ensured. Both standards suggest the following arrangement of air supply and exhaust vents:
- supply and exhaust vents should be located on opposite walls,
- minimum distance between vents located on the same wall should be 2 m,
- air from the battery room or battery charging location should be extracted to the atmosphere outside the building.
Classification of explosion hazard zones
To classify explosion hazard zones present in the battery room, follow the provisions of standard EN 60079-10-1:2016, Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres.
The procedure involves:
- determining the rarefaction of explosive atmosphere in the charging room/location, accounting for the type of cells used and performance of the ventilation system,
- assessing the availability of ventilation, accounting for the locations of air supply and exhaust vents and whether emergency ventilation is used or not,
- determining of the nature of emissions from a given type of cell (continuous, primary or secondary emission),
- determining the range of the explosion hazard zone based on chart D.1 in standard EN 60079-10-1:2016.
Remember to keep a ‘safe distance’…
Standards EN 62485-3:2014, applicable to traction batteries, and EN 62485-2:2018, applicable to stationary batteries, suggest keeping a so-called ‘safe distance’ – a space around the battery free from any effective ignition sources, such as hot surfaces, sparks, arcs, etc. – in the immediate vicinity of the battery, irrespective of the classification of explosion hazard zones.
… because hydrogen is highly explosive
Due to the very low minimum ignition energy of hydrogen, most ignition sources, even those with very low energy, are capable of causing the explosion of a hydrogen-air mixture. Effective ignition sources also include electrostatic discharges, which are often overlooked when adapting a room to serve as a battery room.
Be particularly careful when performing maintenance work
The risk of explosion is particularly high when performing maintenance work in battery rooms, for example when topping up electrolyte in the cell. This may require (depending on the type of battery and the refilling system used) the removal of the cell cap, which may cause a sudden release of a significant amount of hydrogen built up inside the cell into the room. When combined with inadequate worker attire (i.e. clothing that does not dissipate static electricity) and failure to use conductive flooring in the area where cells are accessed, this can result in an explosion.
Recombination plugs are a useful tool
To limit the risk of explosion and reduce battery handling, special recombination plugs are increasingly used in open cells. The plugs bind the hydrogen and oxygen produced during the charging process, creating water that drips back into the cell.
Recombination plugs do not completely eliminate hydrogen emissions, but reduce it to a considerable degree and enable extending the intervals between adding water to the cells, which significantly improves the safety of work in the battery room. Recombination plugs are marketed by multiple companies and can be adapted to the batteries used by the customer.
Summary
In summary, the room used for charging lead acid batteries, especially open cell batteries, must meet a number of requirements to be considered safe.
The basic requirements that should be met in any battery room are:
- a ventilation installation compliant with standards PN-EN 6094762485-3:2014 and PN-EN 62485-2:2018,
- a floor that dissipates static electricity,
- use of anti-static clothing and footwear by employees,
- avoiding the presence of any other sources of ignition within a specified distance of the cells,
- Implementing organizational changes, i.e. limiting the access of unauthorized persons to the room.
An Explosion Risk Assessment must also be performed in the room; a careful analysis of its results will indicate whether any explosion hazard zones will be present in the room. The classification will confirm whether additional measures are needed to ensure safety, including but not limited to:
- adapting electrical equipment and devices to the requirements of the designated zone (e.g. electrical sockets, light fixtures),
- improving the efficiency and availability of ventilation,
- drafting an Explosion Protection Document in respect of the battery room.
