Are You Playing with Fire When You Play Your Tunes?

Lithium ion batteries have gotten smaller, but some worry this has led to larger risks


Bigger does not always mean better in the world of electronics—unless, of course, we're talking about the width of a flat-screen plasma TV. The latest generation of the iPod shuffle, for example, is just a fraction of the size of the original 2004 iPod mini, yet holds just as many songs. Part of this reduction in size and increase in power for personal electronics is due to improvements in the way that batteries—specifically those that rely on lithium ions—hold and release energy.

But are there drawbacks to this kind of technological miniaturization? Recent reports that some models of the most popular MP3 player can literally burst into flames may suggest so.

An investigative reporter from KIRO-TV in Seattle recently got hold of 800 pages of documents from the U.S. Consumer Product Safety Commission (CPSC), which included descriptions of "15 burn and fire-related incidences blamed by iPod owners on their iPods," according to KIRO. There were cases of damaged homes, an injured child, even an iPod catching fire on a 2,000-passenger cruise ship. Fortunately, none of the reports noted any serious injuries.

Flaming iPods have also lit headlines overseas. The Local (which covers Swedish news in English) reported last month that a car fire was allegedly started by an iPod Nano. The heat-damaged music player was recovered, in this incident, from the front seat of the parked Saab, the newspaper noted. The very next day Apple recalled all first-generation iPod Nanos in South Korea, according to The Korea Herald, which also noted iPod Nano batteries overheating or exploding.

Overheating is not limited to iPods. Laptop computers and cell phones have had their share of problems: Sony recalled more than 400,000 Vaio laptops last September for this reason, and potential iPhone recalls in response to overheating have also been circulating in the past few months.

So, how concerned should consumers be? What can be done to mitigate risks—or are these incidents simply too rare to worry about? And given the possible fire hazard from small lithium ion batteries, should we reconsider using large, heavy-duty versions of them in upcoming fleets of electric cars?

We recently sought answers from Zonghai Chen, an electrochemical engineer at the Argonne National Laboratory in Illinois.

[An edited transcript of the interview follows.]

Why has the lithium ion become the battery of choice for personal electronics?
The lithium ion battery is currently the dominant power source for most consumer electronics due to its high energy density. In other words, in order to store the same amount of energy for electronics, the lithium ion battery is the smallest and lightest compared to other battery types.

And it's getting even more compact. The design of modern electronics has sought more energy while leaving the room for the battery unchanged, or even smaller. In order to meet this demand, battery manufacturers have had to design the battery to load materials in the same size cell. Currently, the same size lithium ion cell stores almost triple the power of its original design.

Are there any trade-offs to this compactness and storage?
The compact design of electronics leaves very little room for the battery, which limits its heat-dissipation efficiency. This makes compact electronics more susceptible to overheating.

The lithium ion battery is especially sensitive to temperature because of the materials used. Generally, this type of battery works well at temperatures below 50 degrees Celsius, and the performance will be significantly impacted when the temperature rises above 50. Some accidents can occur when the temperature passes this threshold, at which point side reactions between different battery components can be triggered—and fire is possible.

Just when, and how, do these fires actually occur?
There are several reasons that could at least partially explain battery fires. The process of charging the battery is one. When you charge a lithium ion battery, lithium will be removed from the positive electrode and inserted into the negative electrode. As a consequence of this process, metal oxide builds up on the positive end and lithium is hosted in carbon on the negative end. Both species can react with electrolytes that are sandwiched in between.

This potentially dangerous reaction occurs very slowly at low temperatures—like room temperature—but it accelerates at higher temperatures. And as it accelerates, it generates heat. If this happens, it can become unstable as increases in temperature trigger the possibility of further runaway reactions, and the possibility of an explosion.

Simple exposure to high temperatures is another trigger. Several cell phone accidents were reported because a phone was left in a car during summertime. And in some rare situations, sunlight is enough to heat the battery to a certain temperature to trigger side reactions.

There are also some reported accidents caused by external shorts. Some people put uncapped lithium ion batteries in their pockets along with their keys—this can connect to both ends of the battery and cause the battery to overheat.

Manufacturing defects can be another factor responsible for the battery accidents.

Can this occur with other types of batteries?

Overheating is a common problem for batteries. However, lithium ion batteries are worse than the others because they store more energy per unit volume or weight than the other technologies. This type of battery is also most sensitive to temperature due to its chemistry.

How concerned should the consumer be, especially after the recent news about Apple iPods overheating?
It's true that 15 out of the 75 million iPods in use is a small proportion, but it is significant enough to impact the confidence of consumers. Moreover, the damage of such an accident can be beyond our imagination. Blowing up a car would be a big shock to all of us.

Can a safer battery be designed?

Lower power design in the electronics industry could ease the engineering design for the battery industry. For example, it is well-known that the mobile CPU for laptops generally operates at a lower voltage compared to the normal desktop CPU. The design of the mobile version is intended to reduce the energy consumption provided by the lithium ion battery. Such low-power design needs less energy from the battery. It also means less reactive materials will be required in the battery. Moreover, low-power design also means smaller currents can be drained from the battery, making the heat generated during the normal operation lower, thereby reducing the risk of overheating. A less compact design for electronics could also leave more room for a battery to have protection devices, like electronics for overcharge protection or a positive temperature coefficient device for overheating protection.

What can a consumer do to minimize the risk of their personal devices overheating?
To reduce the risk of lithium ion batteries, you should limit the exposure of the battery to heat sources. For example, you should not leave electronics in an unventilated car during the summer. Giving the electronics a rest period to release the heat to the environment when the battery is hot will help, as will equipping the device with a cooling fan—like those available for laptops.

Does leaving a device on a charger increase the risk?
Overcharge generally occurs in battery packs with more than one lithium ion cell connected in serial. Lots of portable electronics, like cell phones and iPods, use a single-cell battery, and overcharge on single cells is rare—except in the case of a charger malfunction. It is well known that the battery is generally hot after being charged, so limiting the time of the device on charger can, to a certain degree, extend the life of the battery.

Do the lithium ion batteries used in electric cars have the same potential for overheating and fire?
The nickel–metal hydride battery has been used in hybrid electric vehicles, but the lithium ion battery is still the most promising technology for automobile applications because of its high energy density and power density. Compared to the battery used in portable electronics, the car battery can be more than 1,000 times bigger in size or weight.

Again, in the case of portable electronics, the real issue is that the room provided in the electronics for a battery is small while the energy required is high. Thus, the battery has to be stuffed with as much material as possible, leaving very little room in the electronics to incorporate protection devices. This issue will be even more severe in the car battery, since the energy and power density requirement is even higher than in personal electronics. Therefore, the car battery generally has a bigger potential for overheating and fire.

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