Singapore, 8th October 2016

Behind the scenes of exploding smartphones


Recently, a test conducted at Applied Energy Hub attracted a great deal of attention in global press. From Asia to Europe (http://www.dailymail.co.uk/sciencetech/article-3825286/Shocking-images-Samsung-Galaxy-Note-7-burst-flames-pressure-applied-battery.html) and Americas (https://www.thestar.com/business/2016/10/06/has-the-note-7s-future-gone-up-in-smoke.html), journalists rushed to post the photos from our laboratory, obtained from Reuters. While this may sound as great publicity for a lab like ours, we were bitterly disappointed about the way these photos were interpreted: completely unprofessionally and out of context. Most of these articles shed completely wrong light on our lab and what we do.

To clarify what we actually did on 5th October 2016, we will here present the context of our demonstration and quantitative results of mechanical stress tests on phones.

Background

Lithium-ion batteries are known to cause safety issues. They are storing relatively large quantities of energy. Their inside components are also soaked with a flammable electrolyte. Each battery consists of layers of positive and negative electrodes. These materials are responsible for storing electric charge (and energy). The electrodes are separated by a layer of non-conductive polymer, called separator. Its role is to prevent them from contacting each other – a phenomenon known as internal short circuit. In such event, the battery will release a huge part of its stored energy in form of heat, leading to fire or explosion.

Internal short circuit can be induced by several causes. One of them is mechanical stress, which causes displacement of layers within a battery. Given the sufficient pressure load, any lithium-ion battery will deform and short-circuit itself internally.

Demonstration of internal short circuit

The sole purpose of test is to demonstrate mechanically induced internal short circuit in batteries of two different smartphones:

  • New model smartphone (2016), with laminate pouch battery of 3500 mAh capacity (non-replaceable)
  • Older model smartphone (2012), with pouch cell battery enclosed in plastic casing, capacity 2100 mAh (replaceable)

Phones were placed in the Instron mechanical press under angle of 30 degrees. The press would push the blunt (hemispherical shape, diameter 5 mm) nail into the battery at a rate of 1 millimeter per second. Angle between the phone/battery and nail is therefore 60 degrees. Batteries were fully charged with original chargers of respective phones.

Figure 1: Setup for mechanical stress test

Rear side casing of the phones was removed to expose battery directly to nail impact. Phone was positioned to ensure that the nail indents the battery at the side of its connection to phone's electronics. The mechanical press would drive the nail into the battery until the battery breaks and internal short circuit occurs.

It is needless to say that such test doesn’t even remotely represent a real life usage case scenario.

Results

This type of test always results in battery failure and fire. No exception was anticipated this time, and indeed both phones caught fire.

Figure 2: Both phones at the moment they’ve caught fire

So what is the difference between the two tested phones? Visually, it is obvious that the new model phone catches fire more aggressively. This is hardly surprising, since its battery has 67% higher capacity compared to old model. Higher capacity means more energy to release, and also more electrolyte to fuel the fire.

Another important distinction is the level of mechanical stress required to short-circuit the battery. The graph on top shows imposed load (in newton) over the course of test. Old replaceable battery has plastic housing, which is broken first. The blunt nail faces more resistance thrusting through the battery cell with hard casing compared to laminated pouch cell of a new smartphone. It is also evident that larger battery, which exploded with bigger flame, heated the up the nail to a higher temperature (see bottom graph). The hard casing of the smaller battery from older phone apparently absorbed some of the heat.

Figure 3: Load, displacement, and temperature during mechanical stress test

Conclusion

Demonstration of mechanical stress on smartphone battery shows effects of internal short circuit on lithium-ion battery. The test is purposely designed to provoke such event in any tested battery cells.

Batteries are short-circuited internally at very high loads applied over a small area. We can see that a non-replaceable battery in a newer smartphone catches fire at more than 50% lower mechanical stress than older replaceable batteries. Such difference is normal for all batteries and phone brands.

Larger battery without solid casing also reacts more violently in an event of internal short circuit.

An important message of these tests is that with development of ever-thinner smartphones and increasing battery capacity requirements, the consequences of unlikely safety incidents become more serious, even though the batteries are based on same lithium-ion technology.

Final remarks

Here at Applied Energy Hub, we treat safety of Li-ion batteries as a highly serious topic. We are committed to perform accurate tests and research, and our tests on 5th October 2016 were no different than that. However, no amount of engineering professionalism can offset the damage of sensationalist hype-chasing "journalism".

Disclaimer:
The above mentioned demonstration tests as well as this article were not paid for by any individual or organization.


posted by Jan Geder