Battery-related safety issues became one of the hottest news topics over the past winter holiday season, with a multitude of horror stories arising from the latest craze for hoverboards. While not proven yet, the finger of suspicion has pointed to their lithium ion batteries catching fire or exploding. This led to retailers like Amazon suspending sales, airlines banning hoverboards on passenger flights and carriers like the US Postal Service stopping shipments by air.
This is not the first time that lithium batteries have been blamed for electrical fires – remember the “smoke in cabin” incidents that grounded Boeing 787 Dreamliners some three years ago. While these measures to protect life and property are entirely responsible reactions, they must nonetheless be considered as short-term precautions. Identifying the true cause of each problem and then engineering a proper solution is the only real remedy, although procuring batteries that meet relevant transportation safety legislation is another sensible precaution.
As yet even the causes of the Dreamliner battery failures remain unknown and returning aircraft to service only became possible by making changes to their battery systems to contain fires. The hoverboard events are too recent to have been fully investigated, although the mechanism by which a lithium ion battery can catch fire or explode is simple to understand.
Like most battery technologies, a lithium ion battery comprises three key elements: positive and negative electrodes, and an electrolyte. The negative electrode of a conventional lithium-ion cell is generally made from carbon, typically graphite. A metal oxide forms the positive electrode and this could be lithium cobalt oxide, lithium iron phosphate or lithium manganese oxide. The electrolyte will be some form of lithium salt in an organic solvent.
An organic solvent is necessary because pure lithium reacts vigorously with water to produce lithium hydroxide and hydrogen gas. This not only precludes the use of an aqueous electrolyte but also means lithium batteries need to be sealed to exclude moisture. From this, two underlying risks are immediately apparent: firstly, organic solvents are highly flammable even if the electrolyte is a gel rather than a liquid (to avoid leakage); second, any physical damage to a lithium battery that penetrates its sealed case will allow moisture ingress.
Lithium ion batteries have become popular because of their higher energy density, greater charge/discharge efficiency, low self-discharge, and minimal capacity reduction due to memory effects. However, charging needs to be careful managed to limit peak voltage and prevent over-charging and protection circuits may also be used to limit discharge currents, particularly in the event of a short circuit.
It may prove to be that the hoverboard incidents are due to faulty or poorly designed chargers, or shortcomings in the design of the hoverboard itself e.g. not providing sufficient protection against battery overload or output short-circuit in the event a stalled motor. Then there is the risk of physical damage and certainly hoverboards are subject to fairly punishing use. So it is entirely conceivable that the very thin electrolyte layer between the battery’s electrodes could become damaged giving rise to an internal short circuit that could cause the solvent-based electrolyte to heat up so quickly that the battery explodes.
While these possible explanations deal with scenarios when batteries are in use, i.e. charging or discharging, some of the original concerns relate to potential issues during transportation when the battery may not even be connected. The concern then becomes focused on the quality of the battery itself, its construction and the materials used. In the past other incidents with cell phone and laptop batteries have been blamed on cheaply sourced batteries from China and elsewhere or quality branded batteries that prove to be counterfeit.
Fortunately, the safety of lithium ion batteries falls under a United Nations recommendation that deals with the transportation of dangerous goods. UN38.3 provides a manual of tests and criteria specific to lithium metal and lithium ion batteries. These subject batteries to extensive tests that include: the ability to withstand overcharge and forced discharge; their impact, crush and shock resistance; and thermal, altitude and vibration testing.
Securing compliance with UN38.3 at least provides the assurance that the likes of Amazon are insisting on to preserve their reputation in the wake of the hoverboard melee. It is also something that other manufacturers, suppliers and end-users should insist on. For its part, Cyntech Components works exclusively with manufacturing partners to provide custom battery pack designs that are fully compliant with UN38.3.