Some manufacturers have placed trucks with lithium-ion batteries into Ex applications but others, including Pyroban do not believe the technology is appropriate at this time. Steve Noakes, Managing Director of Pyroban and convenor of the EN1755 standards committee, discusses the problem.
We receive a growing number of enquiries for Ex-certified lithium-ion battery options, and for good reason. Lithium-ion batteries allow opportunity charging, require less maintenance, and offer longer operation between charges, so have proven to be popular in many industries.
However, as it stands in June 2021, for the foreseeable future Pyroban declines to quote for the ATEX conversion of lithium-ion forklift trucks.
Simply, it is not clear what the right level of safety is for this technology and, as the Ex standards do not appear to cover traction battery applications, one has to question therefore whether they are safe for use in hazardous areas.
At present, Ex standards for lithium-ion batteries applies mainly to small battery packs, not explicitly traction battery capacities. So, there is no clear guidance on what battery conditions should be considered for Zone 2 and Zone 1 areas (normal operation, foreseeable faults) and what Ex protection concepts are appropriate.
Characteristics of lithium-ion batteries
There are various types of lithium-ion batteries using different chemical compositions. Some are more volatile than others and there is a balance between power density and inherent stability (safety).
A lithium-ion battery comprises far more components than a conventional traction battery. Individual cells are connected together to form modules and each module is connected together to form the overall battery package which is overseen by the Battery Management System (BMS). How each element is designed, produced, and tested is key to achieving the right level of Ex safety.
Cells are produced in very high volumes by factories not always owned by the final battery OEM, so one question posed by the EN1755 committee is how the production of this key component can be controlled and verified for Ex applications.
Which Ex protection concept is appropriate is also a fundamental question which does not have a clear position at the moment as it depends on what conditions are included in the normal operation and foreseeable malfunction requirements.
Even an Ex d flameproof enclosure may not be suitable for a lithium-ion battery despite its ruggedness.
Why are lithium-ion batteries a risk for hazardous areas?
Primarily, if lithium-ion batteries become unstable, they could pose a high risk of ignition if not maintained or operated correctly. This would complete the ignition triangle within a potentially explosive atmosphere.
This is because lithium-ion batteries:
- Are vulnerable to thermal runaway caused by the individual cells breaking down. Lithium-ion cells can overheat under adverse operating / fault conditions which could result in cells catching fire
- The degradation of cells generates hydrogen and oxygen which perpetuates the overheating / fire / risk of explosion
- As oxygen is produced as a result of the chemical reaction when the cells break down, even if sealed from atmosphere (which would be the usual source of oxygen) oxygen would be available. It is this fact that fuel, oxygen, and a heat source will be produced when the cells fail which is dangerous. All sides of the ignition triangle are complete
- Lithium-ion battery fires are also intense and very difficult to put out so a truck on fire may be in the location for hours before it can be extinguished
- There is permanent reduction in capacity, and possible overheating, from over-charging and over-current during charging
- There is reduction in capacity or even instability from internal breakdown from under-voltage or over-discharge
- Unstable battery conditions can occur if the battery is damaged
In other words, all elements in the battery design, construction, and use are key to assuring stable operation and prevention of a significant ignition source developing.
Do battery manufacturers offer EX certified lithium-ion batteries for lift trucks?
The main battery traction manufacturers have resisted producing an Ex lithium-ion battery for lift trucks because of these concerns and a lack of Ex standards to certify against.
Also, is it practical? As the charger cannot be kept in the hazardous area, this makes opportunity charging difficult.
Pyroban’s position on lithium-ion battery powered lift trucks
Pyroban remains active in its technical review of the ever-evolving lithium-ion technology, and in the Ex standardisation work we do to drive clarity in the standards to fork truck applications.
Although Pyroban has pioneered many new technologies in the past, we do not feel confident to unilaterally decide what is safe for Ex lithium-ion traction batteries. Companies which are currently offering Ex lithium-ion batteries, in our view, do so without a clear foundation even if they have involved a third party, as the recent Safety Alert demonstrated.
And the Notified Bodies position?
Notified Bodies are reluctant to take on lithium-ion Ex projects due to lack of clarity regarding the risk assessment requirements for Zone 2 and Zone 1 hazardous areas.
A safety alert has been issued raising concerns about the use of lithium-ion batteries in Ex d enclosures. Why? The alert identified that the explosion energy generated when cells fail can far exceed the mechanical strength of the enclosure and would therefore not prevent the ignition propagating to the open atmosphere. Pyroban also considers that as Ex d enclosures are not fire-proof, an internal fire may distort the enclosure negating the Ex d properties and the surface temperature may exceed the T (temperature) class.
Other new battery technologies suit explosion protection
The current successful ATEX compliant lead acid battery technology is safe and proven for use in zoned hazardous areas. Furthermore, the new generation of batteries, featuring technologies such as thin plate pure lead (tppl), also deliver similar benefits to lithium-ion with opportunity charging during short breaks, reducing the need for battery changes. These are already available as ATEX compliant options.