Electronic Design

The Fire Next Time?

Last week’s media feeding frenzy over Dell’s recall of 4.1 million laptop battery packs, followed by this week’s recall of 1.8 million Apple battery packs has some worrisome aspects. I wish I knew more than I do at this point. Here’s the short version: if you abuse a lithium battery, it can experience a “discharge with flame” and you can’t put it out. It generates its own oxygen to keep the fire going. Also, as we discovered this week, errors in manufacturing can give them the potential to self-ignite.

Here’s a longer version. You must know about Dell’s and Apple’s recall of the Sony batteries. There were endless news reports about the size of the recall (the what of the story), but few reported the why. When the first story broke, we heard a little about Sony’s problems from CNET and the Financial Times: "The problems Dell is having stem from impurities within the anode and cathode of the battery, said \[Roger Kay, an analyst with Endpoint Technologies Associates\], who was briefed on the problems by Dell executives. Over time, those impurities, usually tiny pieces of metal, can work their way to the edge of the anode or cathode and rupture the isolator that sits between the two, he said. Once that happens, you get a short circuit and possibly a fire."\[1\] However, I have not seen anything in the way of a more substantive update since.

Fires Can’t Be Extinguished
The most authoritative study of the fire dangers of lithium batteries that I could find on the Web was performed by the FAA office of aviation research. \[2\] It concerns not laptop batteries, but single cylindrical CR2 and PL123A cells, which are roughly the diameter of a standard AA cell, but about half the length. They’re commonly used in larger cameras.

The study was triggered by an “…incident \[that\] occurred at Los Angeles International Airport in April 1999. A pallet of batteries caught fire while being handled between flights. There was no known external ignition source.”

The researchers tested batteries from a number of manufacturers by suspending individual batteries over firepans charged with a quantity of 1-propanol. This took place in a 4-by-4-by-4-ft test chamber set up to provide the same halon 1301 concentration used in a standard aircraft cargo compartment for initial fire knockdown.

Here’s the executive summary from the report:

“A relatively small fire source is sufficient to start a primary lithium battery fire. The outer plastic coating easily melts and fuses adjacent batteries together and then ignites, contributing to the fire intensity. This helps raise the battery temperature to the self-ignition temperature of lithium. Once the lithium in a single battery begins to burn, it releases enough energy to ignite adjacent batteries. This propagation continues until all batteries have been consumed.

“Halon 1301, the fire suppression agent installed in transport category aircraft, is ineffective in suppressing or extinguishing a primary lithium battery fire. Halon 1301 appears to chemically interact with the burning lithium and electrolyte, causing a color change in the molten lithium sparks, turning them a deep red instead of the normal white. This chemical interaction has no effect on battery fire duration or intensity.

“The air temperature in a cargo compartment that has had a fire suppressed by Halon 1301 can still be above the autoignition temperature of lithium. Because of this, batteries that were not involved in the initial fire can still ignite and propagate.

“The ignition of a primary lithium battery releases burning electrolyte and a molten lithium spray. The cargo liner material may be vulnerable to perforation by molten lithium, depending on its thickness. This can allow the Halon 1301 fire suppressant agent to leak out of the compartment, reducing the concentration within the cargo compartment and the effectiveness of the agent. Holes in the cargo liner may also allow flames to spread outside the compartment.

“The ignition of primary lithium batteries releases a pressure pulse that can raise the air pressure within the cargo compartment. The ignition of only a few batteries was sufficient to increase the air pressure by more than 1 psi in an airtight 10-meter-cubed pressure vessel. Cargo compartments are only designed to withstand approximately a 1-psi pressure differential. The ignition of a bulk-packed lithium battery shipment may compromise the integrity of the compartment by activating the pressure relief panels. This has the same effect as perforations in the cargo liner, allowing the Halon 1301 fire suppressant to leak out, reducing its effectiveness.”

Extrapolating from CR2s to laptop batteries suggests that a laptop fire is not a trivial event.

Li-Ion ICs: Don’t Leave Home Without ‘Em
For those who don’t follow the world of ICs for battery-specific applications, here’s an overview. These chips fall into three general categories: charging controllers, “gas gauges,” and security. In practice, all three work together. For instance, the charger IC may tailor the current it delivers based on many functions, including the battery’s current state of charge, which it gets from the gas gauge. In addition, the gas gauge may store a record of events, such as overvoltage lockouts, which the battery manufacturer may use to deny warranty coverage. (A history of lockouts may indicate attempts to perform fast recharges outside the laptop, which may be prohibited by the warranty.) Security chips are necessary to assure that the battery being charged isn’t a bootlegged item that has the wrong kind of charging controller chip or no chip at all. Authentication is a non-trivial process. The chip in the consumer product and the chip on the battery use the SHA-1 one-way hash function defined by Federal Information Processing Standards (FIPS) Publication 180-2, but including specific data based on the use of the chip in the battery. The consumer product and the battery use a public message/secret key protocol to produce a secure fingerprint that authenticates the device in the battery.

Sixteen years have elapsed since Sony introduced the first lithium batteries. Roughly a gazillion devices that use them have been produced and there have been no major catastrophes. It’s no time to panic. But I still wish there were some way to put the fire out.

You can e-mail Don Tuite at [email protected].

\[1\] “Dell to recall 4 million batteries,” Tom Krazit and Michael Kanellos
http://news.com.com/Dell+to+recall+4+million+batteries/2100-1044_3-6105486.html#
(That story also says, “Several companies, including Valence Technology and PowerGenix, are working on safer lithium ion batteries or batteries which rely on different chemicals.”)

\[2\] DOT/FAA/AR-04/26, “Flammability Assessment of Bulk-Packed, Nonrechargeable Lithium Primary Batteries in Transport Category Aircraft” (June 2004)
http://www.mobilit.fgov.be/data/aero/FAALiBaFiRe.pdf

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