Maximising safety in Li-ion battery implementations

Author : Paul Edwards, Steatite

05 September 2016

Projections from the analysts at Allied Market Research suggest that the worldwide lithium-ion (Li-ion) battery market will experience a compound annual growth rate (CAGR) of approximately 11% between now and 2022.

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This means that it will be generating an annual revenue of over $46.2 billion by the end of that period.

The broad spectrum of applications into which Li-ion technology can be deployed are destined to help drive this incredible growth - covering portable consumer electronics goods, hybrid electric vehicles, industrial control hardware, environmental monitoring systems, medical implants, freight rail and avionics. Regardless of where these batteries are utilised, safety is always going to be paramount.  

The first commercially available Li-ion batteries began to emerge about 25 years ago and since then they have gradually supplanted other battery technologies in many tasks and seen increasingly widespread uptake. Among the key qualities that have made Li-ion so attractive to design engineers as a modern energy storage medium are the following:

1. It is rechargeable. 
2. It delivers competitive energy density levels (in the region of 260Whrs/kg).
3. It is thus very compact in form.    
4. It can hold its charge for very lengthy periods of time.
5. It can cope with protracted recharge intervals.
6. It has a prolonged operational lifespan.

From this it is clear that Li-ion has the ability to deliver strong levels of performance across a wide range of different parameters. As a result of its high energy density, Li-ion technology proves considerably lighter and less bulky than other rechargeable technologies of equivalent capacity, thereby furnishing design engineers with greater scope by which to optimise their system. A direct consequence of this is that they can take into account space or weight constraints (even if they are quite exacting ones) and tailor the battery’s dimensions accordingly, while still being able to adequately attend to the system’s power requirements.  

Li-ion batteries do, it should be noted, exhibit a reasonably high degree of robustness. They can support a broad operational temperature range. In addition, they possess strong resilience to mechanical stresses caused by pressure, shock and vibration. Also, as the batteries are sealed, they can be used in any orientation - which mitigates the possibility of accidental leakages occurring (something that is more commonplace in other battery types, such as lead-acid). However, despite all this there is no avoiding the fact that in some situations Li-ion batteries can potentially be dangerous and precautions should be taken. They may, under certain circumstances, be caused to short circuit and might potentially start fires. There have in recent years been examples publicised of batteries catching fire in context of everything from laptop computers right through to electric cars, and even (on very rare occasions) the batteries melting in commercial aircraft. Such high profile incidents (though still reasonably rare given the number of Li-ion cells now in operation) can have a huge detrimental effect on the OEM involved. They can damage the company’s reputation, as well as requiring costly product recalls.  

Taken purely from a safety perspective, the main difference that separates Li-ion batteries and conventional primary batteries using an aqueous electrolyte is that they contain unstable materials that are inherently flammable - this stems from the fact that alkaline metals like Li (and any compounds based upon it) are very reactive in nature. As a result, there is a risk of fire (or even explosion) if a battery is either overcharged or allowed to overheat, since the energy will then escape in an uncontrolled manner. Likewise, such events can arise if the casing in which the battery cells are housed is damaged in some way that allows the flammable material to escape. As density levels have been heightened to meet intensifying commercial demands, there has been a trade-off made in terms of battery stability - with cells prospectively becoming more susceptible to igniting. 

Once a single damaged Li-ion cell has caught fire, it is possible for it to produce enough heat to subsequently ignite neighbouring cells that were not damaged - thereby rapidly escalating the seriousness of the problem by causing a ‘domino effect’. Hence, it is vitally important to consider safety during each and every stage of a Li-ion battery’s life cycle - design, production, distribution, use and finally disposal. This is especially important for custom-built battery solutions. 

Though the Li-ion batteries intended for a multitude of different end applications (such as those within the consumer space) are available off the shelf in a variety of sizes and package format, more specialised application scenarios will generally call for the construction of custom battery packs. It is here therefore that the greatest care needs to be taken. Before embarking on development of such battery packs themselves, OEMs must ensure they have a complete comprehension of the characteristics that define Li-ion battery technology and, most importantly, its particular technical limitations. One aspect of this is a good grasp of how environmental conditions (extremes of temperatures, or exposure to harsh external forces) will have an effect on the battery over time.

Equally, if not more, important is an appreciation of the current legislative measures that are associated with this type of battery chemistry (or any other chemistry being employed). There are an extensive array of both general-purpose and industry-specific safety standards now in place. These standards, which are published by bodies such as the UL, IEEE and IEC, concern not only the design and handling of Li-ion batteries during production, they also act as a safeguard to those operating equipment incorporating such batteries, or those simply in close proximity to them, so that they are not put at risk of harm. Compliancy will thus need to be sought for those ones that are relevant. Furthermore, given that these standards are constantly evolving (particularly in relation to rapidly developing application areas, like hybrid electric vehicles and handheld consumer devices), engineers need to ensure they are fully up to date with the latest versions.

Battery technology is a complex and often laborious business, with many different technical considerations involved. It requires an in-depth understanding of the battery chemistries available, what are the plus and minus points of each, where they are most applicable, which technologies are suited to installation in particular environments/settings, correct assembly techniques, the battery management that will be needed to accompany them, what is the relevant safety legislation, etc. 

It is very difficult for any OEM to have access to the wealth of knowledge that is expected. What is more its means that a sizeable chunk of the OEM’s time and engineering resources need to be allocated onto activities that are not part of their core competence. This is why many OEMs are now looking to offload the battery system portion of their product/equipment design projects to dedicated specialists with a proven track record in this field. Following such an approach will enable the OEM’s engineering team to then concentrate fully on their application (where they have their expertise) and not be tied up with battery issues.

Steatite has developed best practices and made investments so that it has the capabilities needed to execute comprehensive and rigorous test procedures. The company’s customers are able to meet the battery performance criteria they require while having confidence in ongoing reliability and operational safety.    

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