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Battery safety: Are consumer electronics safety considerations the same for large batteries?

Source: CNN Tech

Thanks to the work we completed in 2016 on battery safety testing (available here), DNV GL is now able to speak publicly about battery safety in a very transparent way. And in 2017, we will be spending a lot of time debunking battery safety myths.

The recent consumer battery fires observed from the Samsung Galaxy Note7 and its subsequent recall have again brought battery safety into the media spotlight. In my previous post on battery safety I discussed why anchoring and confirmation bias skew the perception that batteries are universally dangerous across sectors and the factors that cause their failures are mysterious and unexplained. In today’s post, I’ll describe why this isn’t necessarily true, as well as highlight what lessons other sectors can learn from consumer electronics. In short, consumer batteries should not be considered miniature analogs to larger batteries, as the design and hazard considerations differ.

On January 23, 2017 Samsung issued a press release on the results of their internal analysis of the unfortunate battery fires in relation to the Samsung Galaxy Note7 in 2016.  The transparency that Samsung provided in these interviews was unprecedented and commendable in an industry that has been highly sensitive to discussing issues that affected intellectual property and trade secrets.  Samsung was battling a strong confirmation bias headwind fueled by headlines like “exploding batteries” and “battery calamity”. Such media dramatism does little to clarify what occurred.

Among Samsung’s findings were the following:

  • They had two suppliers that had two separate manufacturing and assembly defects causing two separate sets of problems. These unfortunate coincidences resulted in a series of Note7 fires that looked related, but were independent from one another.
  • They were not able to identify these hazards prior to production and were under pressure to replace the first supplier with a second, which led to quality control errors

Samsung is not the first large company affected by their supply chain. Boeing had a similar manufacturing defect issue in their battery cells. GM and Tesla have also been forced to defend the technology when the reasons for the failure were not entirely their fault.  This demonstrates how liability can spread across a supply chain and affect a brand in a severe way.

The first Samsung defect issue is related to mechanical deformation of the battery that short circuited the electrodes.  The second issue is related to puncture of the separator due to a welding burr on the positive electrode. Quality control processes should examine the structural integrity of the battery and identify any kind of defect or irregularity. There are methods to nondestructively scan for such but they add cost and complexity to the manufacturing process.

The Note 7 fires need not be replicated in other industries if quality control processes are enforced on the supply chain. However, a consumer electronics battery is a different engineering challenge than a marine or stationary storage battery. While the chemistry is similar, the consumer electronics battery is designed to far more stringent power and energy density requirements and is engineered to be much thinner. The space constraints limit the ability to add supplementary safety systems like cooling, added physical protections, and advanced battery management system (BMS) electronics.

We often liken the safety hierarchy to the figure below. The minimum safety consideration can be as simple as keeping the battery small. A small battery limits the heat source and the total energy of a potential safety event. However, the industry keeps advancing; even a small battery has 150% more energy in it than it did 8 years ago.

Consumer electronics, because of their space constraints, may have a basic BMS (more like a charge controller) and some semblance of physical protections. There may be air cooling (such as the fan in your laptop). That is about the extent of safety systems that are possible to engineer into a consumer electronics battery.

All the other sectors, however, have a BMS, physical protection, and some degree of air cooling. Even in 2017, many pack designs still do not implement cascading protections between cells. Cascading protections can be metal plates, air gaps, or other materials such as phase change materials or intumescents to prevent cascading thermal runaway. The Boeing 787 battery lacked cascading protections.

DNV GL also witnessed this during testing last year. For this reason, we recommend that battery manufacturers consider how to design the system to pass the IEC 62619 Propagation Test with the additional provision that cascading be limited to the smallest unit of assembly in the battery, which is the same test we require for battery type approval on vessels. Cascading protections are insurance against internal cell defects, but in the event of an external fire they slow the burning of the battery module similar to the way that residential fire blocking minimizes the damage from a house fire.

The Chevrolet Volt battery is a great example of a highly engineered set of integrated safety systems that include aluminum plates between the cells. Like many pack BMS’s, the Volt’s intelligent BMS monitors activity and shuts down in the event of a severe error. Automotive batteries are also further along in the development cycle to include passive materials that limit heat propagation and contain single battery fires, such as phase change materials, intumescent materials, and ceramics.

Active fire suppression is almost exclusive to marine and stationary systems. There is not a well-known historic precedent for active battery fire suppression on aircraft, and of course there aren’t automated extinguishers on automobiles or smart phones. Automated gas-based suppression systems have the challenge of not only extinguishing a fire, but getting to its “deep seat” and providing cooling. Last year, we discovered through testing that these systems can extinguish a fire, but they don’t offer much cooling and will be overwhelmed if the fire consumes more than a few cells. As a backup, water suppression can be used to rapidly cool a fire if it has progressed to the point that the system is a total loss and nearby property or people are at risk.  We’re eager to see data indicating that physical properties of gas-based agents demonstrate a competitive advantage to water; until that occurs, we will remain skeptical of any agent that claims it can cool a fire better than water.

Everything to the right of passive monitoring in the previous figure is an area of advancement in marine and stationary storage. There are commercial marine battery solutions that are highly engineered with liquid cooling, cascading protections, and active monitoring that can electronically mitigate voltage or current anomalies.  However, there are also air cooled marine battery solutions today. The picture is similar in stationary storage; there are systems offered today offer a limited safety solution with air cooling and passive monitoring, but there are also more advanced systems with some form of cascading protections, active monitoring, and fire suppression.

Obviously, these levels of complexity are impractical to build into a small consumer battery. Samsung and other manufacturers of phones, tablets, and personal electronic devices are competing in a market that values aesthetics in thin, sleek, and lightweight designs. The aesthetics requirements of the personal electronics space are deliberately trying to obscure the existence of a battery altogether, requiring that each generation be smaller, thinner, and more integrated into the case than the last. If this keeps occurring, there won’t be much space to incorporate features like cooling or sensors. For this reason, it is technically inaccurate to state that consumer batteries carry the same risk as automotive, marine, or stationary batteries. The fundamental mechanisms of abuse are the same; the methods to mitigate them are different.

After the fire suppression testing DNV GL conducted in 2016, it became obvious that safety systems should not be a collection of off-the-shelf components thrown into a box and wired together. The furthest right edge of the safety spectrum should include a fully integrated safety system that evaluates BMS data to determine the best course of action, such as when fire suppression should be activated and what type, what electronic mitigation measures to deploy before a situation escalates, and what external signal should communicate the hazard and its severity.  Marine and stationary have made some advancements toward this area, but such intelligence is not yet evident in a BMS, at least from what we’ve seen.

If you’d like to learn more about this topic, please contact me. Later this year I will talk about what we are doing in BMS testing to get ready.

 

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