How to Build a Safe EV Pack (Even Though Cells Sometimes Burn Up)

In recent times, the news has been replete with the kind of stories that strike fear into the hearts of battery engineers: Fires. Recalls. More fires. More recalls.

In October of 2020, Ford recalled approximately 20,500 Kuga plug-in hybrid crossovers and halted sales, due to concerns that the battery packs could overheat and catch fire. The recall cost $400 million. In February of this year, Hyundai’s Kona crossover was recalled due to fire hazards. Eventually, the recall will cost around $900 million. In August, GM announced they’ll spend close to $2 billion to recall Chevy Bolt EVs due to battery fires.

We could go on. These stories stress that – even though the electrification of the world is exciting; even though batteries are playing a greater and greater role in every industry – there are big challenges ahead. And one of the biggest is safety. Batteries are dangerous if not used correctly.

What can engineers do? Root-cause analysis by automotive OEMs has pointed the finger at cell-level defects. Cell quality is important, but the bigger reality is this: there is no such thing as a perfect battery. We will never rid battery packs of bad cells.

The good news is that it is still possible to build a safe EV battery pack. It just requires working from the start with this reality in mind.

The Dangers Caused by Cell-Level Defects

Almost two decades ago, the news began to fill with stories of lithium-ion batteries spontaneously combusting. These batteries resided inside laptops and other electronic devices. This worrying issue – caused by thermal runaway – has led to recalls by companies like Dell, HP, Lenovo, and even Apple. This past August, an airline passenger’s Samsung Galaxy A21 caught fire, causing major disruptions and major headlines. In January of 2019, the US Product and Safety Commission recalled approximately 78,500 HP laptops due to “fire and burn hazards.”

These fires have been a factor ever since the lithium-ion battery was first introduced. Way back at the dawn of the modern EV industry, Tesla began trying to build the biggest lithium battery packs to date. They wanted to see what would happen if one cell in a big pack were to burn. During nighttime experiments in the company parking lot, they discovered how a single burning cell could ignite the cells around it, triggering a cascading inferno. The company immediately began to investigate this worrying phenomenon of “thermal propagation.”

After discussions with battery consultants – including Voltaiq Advisor, Celina Mikolajczak – and some internal calculations, Tesla came to a conclusion: They needed to design a battery pack that would remain safe even when that pack included a bad cell.

This approach to battery design has defined Tesla’s approach ever since. It has been a core reason why their products are market-leading, and why they are the most highly valued automobile company on earth (by market cap).

Investigating the Cause of High-Profile Recalls

Faced with the challenge of cell-level defects and the dangers of thermal propagation, how are OEMs coping?

Let’s assume a worst case scenario, where a modern OEM gets one defective cell in every million from their supplier. (And Six Sigma, the gold standard of quality control, predicts more than that: 3.4 defects per million.) Given that defect rate, what would we expect to see across the fleets of some EV models in the news today?


Expected frequency of packs with a defective battery cell

Of these, it is the Hyundai and Chevy models, built with smaller numbers of much larger that are currently facing billion-dollar recalls. Let’s compare these figures to the frequency of fires reported in these recalled models, relative to the number of vehicles sold:


Frequency of reported fires in vehicle fleets

The takeaway here is this: For these two recalled models, the expected frequency of packs containing bad cells is close to the frequency of fires reported across the fleet.

What differentiates the automobiles that have suffered recalls from those that haven’t? Glance at the first table again. Tesla uses ten to twenty times as many cells per pack as Chevy or Hyundai.

Is Pack Architecture to Blame for Battery Fires?

These numbers raise the possibility that pack architecture plays a role in making catastrophic thermal propagation more likely when a bad cell starts to overheat on its own. From the jump, Tesla has based its pack designs around a larger number of small cylindrical cells. Whereas the OEMs currently experiencing recalls are built with smaller numbers of much larger pouch cells.

Of course, this is a touchy topic! Battery engineers all have their own opinions on cell form. We aren’t here to take sides. EV battery design is an optimization between cost, performance, and safety, and there are likely many valid approaches. However, we do want to highlight a few things we can learn.

Most auto OEMs have not taken Tesla’s small-cell approach. Instead, they have used much larger pouch cells, packed together into modules. Why? Because of certain perceived advantages to do with range, cost, and reliability. However, there are a number of reasons to think that certain features of larger format cells could create issues around safety and reliability:

  • There are more places for things to go wrong. The types of defects that lead to thermal runaway tend to scale with the total amount of materials inside the cell. For example, a fold or tear in the separator material, or at the edge of one of the electrodes, will generally occur at a very low but relatively steady rate per length of electrode material. A large-format cell with ten times the capacity of a smaller cell will have approximately ten times the length (and area) of weld, separator, electrode, etc. For this reason, it will be ten times more likely to contain a defect.
  • There is more stuff to burn. By definition, a larger format cell contains more energy. Once thermal runaway begins in a cell, all of the energy within that cell is pretty much certain to be released, even if the incident is confined to that single cell. Ergo, larger cells release proportionally more energy, increasing the severity of a single thermal runaway event.
  • Fire is more likely to spread. Part of the appeal of large-format pouch or prismatic cells is that they can be tightly packed together, increasing energy density and vehicle range. However, when large, highly energy-dense cells are packed together, a single thermal runaway incident is much more likely to spread to adjacent cells, ultimately igniting the whole pack.

How Should Engineering Departments Optimize Their EV Packs?

None of this is to say that OEMs using larger format cells are condemned to balance-sheet-busting product recalls.
However, OEMs do need to do everything in their power to mitigate the consequences of thermal runaway events – which
are guaranteed to occur in some fraction of vehicles
. It would be wonderful to simply eliminate bad cells from
packs. But this is impossible. Packs must be engineered with this reality taken into account.

With this in mind, what steps should engineers take, across the full lifecycle of their EV battery packs, to minimize
the likelihood of a catastrophic fire?

  1. Tidy up cell supply chains. Unfortunately, many cell manufacturers cut corners. They under-invest in data collection and quality control. They don’t have the equipment to properly scan for poor welds, tears, or misaligned electrodes. OEMs should seek out suppliers that demonstrate a commitment to cell quality up front. In supply contracts, they should require that they receive a full “digital thread” of data, describing each cell through manufacturing and cell formation. If the ongoing shortage in cell supply makes these demands difficult to implement, OEMs should adopt a “trust but verify” mindset. This means rigorous incoming QA testing on every cell, prior to it being built into a production vehicle.
  2. Design packs that limit thermal propagation. Once again taking the lead from Tesla: Packs should be designed to incorporate physical spacing between individual cells, with padding to mitigate wear and tear from vibrations. Packs can further be designed to electrically disconnect individual cells from the overall pack upon cell failure.
  3. Monitor the battery throughout its life. This is crucial. Battery cells expand and contract with every charge-discharge cycle, leading to mechanical wear and tear inside the cell. Vibrations from normal driving can have a cumulative effect as well. This means that many issues develop over the course of a pack’s service life, even if it passed QA as it left the plant. These problems can often be detected in situ, however. How? With the appropriate investment in instrumentation inside the pack; and with telemetry to analyze operational data at scale and recognize patterns of early failure across fleets.
  4. Close the loop. When problems are detected in the field, perform root-cause analysis, identify the cause, implement a selective (not fleet-wide) recall if appropriate, and remediate upstream to ensure it doesn’t happen again.

For electric vehicles, the future is bright. However, to guarantee safety, we need to see substantial investments in engineering battery packs across the full lifecycle. To achieve this manufacturers will need full Enterprise Battery Intelligence capabilities.

Get in touch to discuss how Voltaiq can help you help you engineer safety and reliability in from the start.