Lithium-Ion Thermal Runaway — (2022)

A demonstration of synthesizing research, powered by search engines.
Many of these sources were used while developing safety protocols for an electric endurance racer.
Shorter sources should be given at least a quick read, longer ones at least a quick skim.

A prototype lithium-ion battery pack that has not passed UN38.3 certification is a Class 9 Hazardous Good.
As of 2020, UN38.3 testing and certification costs more than the largest FS teams spend in a year.
Furthermore, UN38.3 testing requires a complete sacrificial battery pack.
In the United States, §173.185(e)(5) covers transport of prototype packs with a strong, impact resistant outer casing (as required by FSAE rules). Teams are unlikely to receive approval to ship by air.

CTRL+F prototype:

0. Safety First

In the event of a battery fire, evacuate the building immediately. Get everyone out fast. Pull the fire alarm or sprinkler on the way out the door. That's it. Only professional fire fighters with the appropriate equipment can fight the fire and handle exposure to the potentially very noxious compounds being released.

This National Fire Prevention Association report shows how a particular sprinkler system, for a particular cell and number of cells, prevented fire from spreading beyond a storage rack.

It is your team's responsibility to handle, assemble, charge, discharge, and store cells and batteries in a way to minimize electrical danger (a topic for another article), minimize the chances of thermal runaway, and minimize the chances of thermal runaway propagation to multiple cells.

1. Protect Cells From Physical Damage

Puncturing or physically crushing lithium ion cells can cause an explosive reaction. Engineer your storage space, pack, car, and physical transport to prevent this.

2. Thermal Runaway Is High Energy And High Power (Explosive)

Cell manufacturers provide temperature limits. Above the maximum temperature, cells suffer permanent damage and degradation. At some high temperature, an exothermic chemical reaction inside a cell becomes self sustaining. This is known as self-heating, and if not cooled back below the self-heating temperature, the cell will continue getting hotter. Generally, cells will vent gasses that are flammable and noxious on their own, and there may be little or no delay between venting and thermal runaway. Thermal runaway is the near instantaneous release of a large amount of energy, which no amount of cooling can stop. See Figure 5, page 491:

3. Keep Cells At A Low State Of Charge -
But Above The Safe Minimum

How much heat? All electrical energy is released, plus chemical energy that is approximately equal to the electrical energy. So a good starting assumption for the amount of energy in a runaway is 2 * (pack kWh capacity) * (state-of-charge (SOC)). See Conclusions, page 16.

Based on the amount of energy needed for a cell to heat itself, then heat one of its neighbors, thermal runaway and propagation to another cell becomes impossible at lower states of charge. Current Federal Aviation Administration (FAA) regulations prohibit lithium-ion cargo on passenger flights, and in addition to UN38.3 certification, require shipping on cargo flights at no more than 30% SOC. Page 7 illustrates the concept of low SOC lacking the energy to initiate thermal runaway.

Without allowing SOC below the safe minimum, your team needs to develop safe discharging procedures (on and off car) for your prototype packs. Maintain a low state of charge, taking self discharge into account over longer periods, and go to high or full charge only when there is an immediate test or run.

Note: For pouch or prismatic cells (pouch in a box), low SOC reduces the energy available in a runaway. However, pouch cell construction is more susceptible to local failure, before the entire cell is involved. Combined with the generally much higher capacity of individual pouch cells (compared to cylindrical cells), a single pouch runaway is larger and more dangerous, and much more difficult to prevent from propagating to adjacent cells.

4. Engineer A Battery Management System (BMS)

Monitor SOC, prevent over-charging, and prevent over-discharging. Over-charging typically happens at the end of a full charge. Adding additional electric energy can lead to overheating and thermal runaway.

Over-discharging can lead to internal shorting, immediately or on a future cycle. Do not allow cells to drop below the safe minimum or zero SOC. This causes immediate and permanent damage and degradation, and increases the risk of future thermal runaway. Engineer your charging and discharging (on and off car) to prevent this.

Unlike lead-acid cells, lithium-ion cells do not have a linear relationship between voltage and state-of-charge. Your team's battery management system needs to take the appropriate voltage curve, voltage sag, and many other factors into account during charging and discharging.

5. Use Cooling Systems On Car, On Test Bench, And Charging

Thermal runaway is a result of cell overheating. Under normal operation, lithium-ion cells generate heat through resistance and the exothermic chemical reaction of discharging. (The charging reaction is endothermic.) Heat is also generated from resistance as electricity is conducted through the rest of the battery pack. Heat must be dissipated through continuous cooling and/or waiting for a cell and pack to cool by stopping any discharging or charging.

Your team's battery cooling system should be engineered to keep the battery well within safety limits at the highest average RMS power (including regeneration), at the slowest speed, on the hottest day. A system that does not these requirements requires a lot of work being careful in other ways.

One estimate of the specific heat of cylindrical lithium cells:

6. Liquid Coolant (In High Volumes) Can Increase Safety

By definition, these are lithium-ion cells, meaning lithium compounds. Unlike lithium-metal batteries, there is functionally zero elemental lithium to create a reaction hazard with water. Though it can cause damage to lithium-ion cells and packs, water is the safest way to extinguish and cool a lithium-ion battery fire or thermal runaway.

This National Fire Prevention Association report shows that sufficient liquid coolant can prevent thermal runaway in one cylindrical cell from propagating to adjacent cylindrical cells.

The FAA makes it clear (Related Tests, page 4) that this is a heat problem. Combustion is just a nasty side effect of so much heat being released so quickly. Do not use ice or sand, which insulate the reaction and can increase propagation to other cells. Results, page 8 shows that water and water-based solutions are an order of magnitude more effective than any other cooling medium. This is a result of high specific heat, high boiling point, and high heat of vaporization. Phase change cooling under normal operation is not legal under FSAE rules, and expanding steam presents its own burn and bursting hazards that need to be planned for, but water boiling can quickly dissipate a lot of energy in an emergency.

For a full pack fire, 4.4kWh and 16kWh packs took hundreds to thousands of gallons to extinguish over multiple tests. See Table 48, page 190 (216 of 233). Elsewhere, note measurements of heat release rate in kW (power = energy/time).

The results were highly dependent on water application technique, and battery access. Formula E has external ports for safety crews to pump water directly into the batteries for this reason, along with burst discs to relieve pressure.

7. Learn From The Professionals

This presentation shows the National Renewable Energy Laboratory and NASA coming up with criteria to minimize thermal runaway hazards in space, and then coming up with an initial design to meet those criteria.

Dig up the full reports and source materials whenever you can, and save them. Bookmarks aren't good enough. Bottom out the search engines, in the library and on the web. Notice how much publicly available basic science as well as advanced detail design is publicly funded. Look for repositories and evolving projects from the same authors. Go to the stacks, go from one end of the shelf to the other, open every book and leaf through. Don't settle for the general statements of my whole article, look for math and data like this presentation by the same authors as above:

And use Unpaywall everywhere else.

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