The National Transportation Safety Board Sunday added a new twist to the high-profile probes of electrical malfunctions aboard a pair of Boeing Co.'s 787 airliners by disclosing that the battery that caught fire on a parked Japan Airlines Co. jet earlier this month "did not exceed its designed voltage."
That's not good.
Lithium ion batteries, like all batteries, have a positive and negative plate material, an electrolyte that permits the movement of electrons through it, and plate separator material that prevents the two plates from touching each other.
Lithium chemistry batteries get their high power density from a number of factors, among them being the eV potential difference between the two plate materials (quite high) and the mass of Lithium (quite low.) Their high charge and discharge rates come from how the plate material is fashioned and thus the high surface area of each plate and low distance through which the electrons must flow from one to the other. Lithium batteries are desirable in these applications because although they're expensive they store a lot of energy in a small size package of lesser weight than competing technologies -- and both size and weight are at a premium in an aircraft.
But there is no such thing as a free lunch. High current flow means heat production and heat must be dissipated or either the electrolyte will boil (causing the seals on the casing to be violated) or in extreme cases the casing itself will melt. Once this process begins it is very hard to put out such a fire because the heat source that is feeding it remains until the energy stored in the pack has been consumed.
There are several different nuances in the chemistry in these packs but all are subject to various potential risks. Part of the problem is that the reaction in the battery itself (like all batteries) produces heat, but in addition the cathode in many of these produce oxygen. Unfortunately chemical reactions proceed faster the hotter things get, and as a result a thermal runaway is possible, especially if there is a short-circuit that cannot be automatically interrupted before the battery gets hot enough to thermally run away. The high energy density makes this risk relatively unique to Lithium chemistry batteries because there is much more energy available in a given unit of space in these devices than there would be for other competing technologies.
The fact that overcharging is not implicated in at least one of these incidents leads to some uncomfortable places, with the most-serious being a potential problem with the assembly of the battery itself (that is, an internal flaw that produced a short circuit inside the case.) This has occurred with laptop and similar batteries in the past and is extremely dangerous because the usual protective circuits are of course not effective. If this is proved to be the case here then Boeing's supplier of that battery likely has a major problem with future orders, but more importantly Boeing may have an unsolveable problem with the use of these batteries, because it is essentially impossible to insure that no defects in the battery pack's manufacturing will ever occur.
This is one of the issues that arose with the Volt, if you remember, where a couple of them caught fire without apparent cause. This was ultimately traced to the coolant used around the battery pack leaking and then crystallizing in the circuitry as the temperature fell, causing a short circuit. Since the short was inside the protective systems' pathways they could not interrupt the current and the battery overheated, ultimately catching fire.
I do not know the exact chemistry varient used in the 787 (there are several) and some are exposed to unique risks that others are immune from. But if the finger is pointed at the pack itself as a manufacturing defect there's going to be a problem with resolving the issue since this is not something you can inspect for on delivery from suppliers and thus the usual means of checking for tolerances and such will not be effective as a means of detecting faults before the units are placed in service.
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