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Li-ion battery transforms devices into ticking bombs

Posted: 25 Apr 2014  Print Version  Bookmark and Share

Keywords:lithium-ion  battery  electrode  ether electrolyte 

The electrode composition of Li-ion batteries makes it prone to thermal runaway and cell rupture, which may lead to combustion as exemplified in the Boeing 787 incident. Whereas a number of testing standards exist to mitigate this risk, it is said that a Li-ion battery must also find a balance between cell-balancing and temperature accuracy capabilities.

The lithium-ion battery is a fact of modern life. Cell phones, laptops, cameras, tablets, and many other items take advantage of this convenience. Even so, we still read news articles about batteries of this type exploding, catching fire, or otherwise causing concern. There have even been aircraft lost due to lithium-ion battery fires. The news continues to have stories about the 787's batteries.

Each cell in a pack consists of a pair of electrodes, one of which is lithium, separated by a polymer or other separator, and an ether electrolyte (similar to what is used to start a car). Under certain conditions, this potentially volatile combination can overheat, vent, or combust. There is a wide variety of conditions that can cause combustion, including over-charge, over-temperature, decompression, impact or crushing, short circuit, or over-discharge. There are other items of concern, such as cell balance. This wide range of potential faults means a variety of methods for preventing issues needs to be employed.

There is a variety of standards for lithium-ion batteries. There are UL standards, IEEE standards, IEC standards, and a number of others. When it comes to sending batteries by air, only batteries that have passed the IEEE standard, for example, are protected from de-compression. Some laptops and cell phones have batteries that pass this standard. Many others have batteries in which that capability does not exist or has not been tested, so their status is unknown.

Semiconductor vendors have created a variety of special integrated circuits to manage and charge lithium-ion batteries. Some have good temperature accuracy, but no cell-balancing capability. Others offer good cell-balancing (bleeding off charge to match the cells in a stack), but poor temperature accuracy. Lack of cell-balancing can cause some cells in a multi-cell pack to over-charge and others to under-charge. One example of the reason that temperature accuracy is important is that, although a large number of today's laptops use low-end, relatively cool-running CPUs, a number of laptops feature warm-running (70°C) high-end processors and graphics accelerators.

If you take one of these latter units, discharge the battery at 70°C by using just battery power during the day, and then let it sit turned off in a hotel room at 25°C while charging the battery overnight, then a sizable error between the apparent charge energy and discharge energy can occur. This can potentially result in over-charge or over-discharge, either of which can cause the pack to swell or even leak ether.

My older Toshiba has these issues, with a hot GPU and hot CPU, coupled with no cell-balancing. This means that in a way it's a ticking bomb, I suppose. What are your experiences with this issue?

- William Murray
  EE Times





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