Lithium-ion batteries: How can thermal runaway be prevented?

09/27/2018 Know-How

A Tesla driver died in Switzerland after his car crashed and burst into flames. In Silicon Valley, a Tesla battery reignited again and again days after catching fire in a crash. In both cases, there was talk of thermal runaway in the lithium-ion battery. What is thermal runaway and how can this risk be reduced?

In the Tesla, as in most other electric cars, e-bikes, e-scooters, electric forklifts, but also in power tools, lithium-ion batteries are used as the energy storage medium. As a rule, they are battery packs. They consist of several battery cells and a battery management system.

The battery management system ensures that the battery remains in its specified working range. This requires extremely precise measurement of the charge and discharge currents, cell voltage, and temperature. With regard to thermal runaway, also known as "venting with flame", the temperature is decisive.

Lithium-ion batteries have a narrow operating temperature range of between +15 and +45°C. The functional safety, service life, and cycle stability of the battery cell - and thus also the functional safety of the battery and the entire electric car or electric tool system - depend to a large extent on the battery cell remaining in this range. If the temperature exceeds a critical level, thermal runaway occurs.

What is thermal runaway?

Thermal runaway of the lithium-ion battery initiates an unstoppable chain reaction. The temperature rises rapidly within milliseconds and the energy stored in the battery is suddenly released. Temperatures of around 400°C are thus created, the battery becomes gaseous, and a fire erupts that can hardly be extinguished by conventional means. The risk of thermal runaway begins at a temperature of 60°C and becomes extremely critical at 100°C. When the battery actually catches fire depends on the specific cause.

How does thermal runaway of a lithium-ion battery occur?

Several factors can lead to thermal runaway of a lithium-ion battery:

  • Internal short circuit: Due to an accident or similar mechanical impact, e.g. if a tool falls down from a great height, the battery is deformed, material penetrates the battery cell and triggers an internal short circuit.
  • External short circuit: Deformation of the battery cell causes an external short circuit.
  • Overcharging the battery beyond the maximum voltage specified in the data sheet, e.g. to increase the range of an electric car. Depending on the degree of overcharging, the battery may be damaged permanently and the service life of the battery decreases.
  • Excessive currents when charging or discharging the battery, e.g. when rapid charging.

How can the risk be reduced?

To minimize the risk of thermal runaway, the mechanical and thermal stability of the battery must be guaranteed. This is ensured by appropriate monitoring mechanisms of the battery cells and the battery pack.

Battery cell

Monitoring of the battery cell is crucial, as thermal runaway is created here and can spread to other battery cells, the entire battery, and finally the entire electric car or electric tool due to a domino effect.

The cylindrical 18650 lithium-ion battery cells come with outstanding mechanical and thermodynamic properties:

  • Their metal casing makes them robust and acts as a heat sink.
  • The wound structure separates the anode and cathode multiple times, thereby increasing the level of safety.

Further advantages: They are relatively cost-efficient and have long been available in the same form factor. High quality cylindrical 18650 lithium-ion battery cells are offered by the world market leader <link _blank external-link-new-window "open internal link">Samsung SDI</link>.

But thermal runaway can also occur with 18650 lithium-ion batteries. A thermal management system is therefore essential. Since thermal runaway is triggered in the shortest possible time and can then no longer be stopped, fast and precise measurements are decisive for the lithium-ion battery structure in addition to comprehensive thermodynamics know-how. Respective sensors are available, e.g., from Rohm, Sensirion, and STMicroelectronics. For example, <link _blank external-link-new-window "open internal link">the STS3x from Sensirion</link> has a response time of just two seconds with an accuracy of up to +/- 0.1°C. Ideally, there are three temperature sensors on each battery cell; one sensor per battery cell plus one or two sensors per battery pack is recommended as minimum.

Battery pack

The number and arrangement of the battery cells play a key role in the battery pack:

  • Twenty-four lithium-ion cells in a row: Relatively uniform temperature distribution
  • 3 x 8 lithium-ion cells: Hotspot inside
  • 5 x 5 lithium-ion cells: Hotspot with higher temperature inside

Liquid cooling systems, fans, thermal conducting plates, and thermal conducting film are available to cool down the battery or to dissipate heat from a hotspot.

Fans are available in sizes from 2cm to 14cm at heights of between 10mm and 38mm, some with integrated pulse width modulation (PWM), speed measurement, speedometer signal or automatic restart. Many models from <link _blank external-link-new-window "open internal link">Jamicon</link> or <link _blank external-link-new-window "open internal link">Delta</link>  are also available with customized connectors.

In addition to the arrangement of the battery cells, the arrangement of the fan is also crucial when using fans. Tests have shown that when discharging the battery with a 1-fold, 2-fold, and 3-fold rated current, surprising temperature distributions occur and hazardous hotspots can arise. Therefore, it is not sufficient to rely on assumptions when constructing a battery with a fan; rather, exact measurements need to be carried out or existing investigations used.

In order to dissipate heat effectively, a thermal conducting film is recommendable. Particularly high thermal conductivity (up to 1950W/mK) is provided by the thin (10µm to 100µm) and lightweight <link _blank external-link-new-window "open internal link">PGS (pyrolytic graphite sheet) from Panasonic</link>.

To also reduce the heat at the hotspots, <link _blank external-link-new-window "open internal link">NASBIS insulating film</link> is available in addition to the PGS. Positioned between the battery cells, the NASBIS film prevents thermal runaway from one battery cell reaching neighboring cells and thus the entire battery. Its thermal conductivity of 0.02W/mK is lower than air and it, therefore, serves as thermal insulation. The NASBIS films are also extremely thin and flexible and can, as a result, also be used in confined spaces.

Expert knowledge of lithium-ion batteries

Developers at Rutronik will find everything they need for a lithium-ion battery - battery cells, temperature sensors, fans, thermal conducting films, insulating films, and all other components for the battery management system - as well as comprehensive expert knowledge of lithium-ion batteries. Rutronik has carried out extensive research into the functioning of battery cells, the design of batteries, and the optimization of the battery management system in its own research and investigations and together with universities and universities of applied science. Our experts are happy to pass this expertise on to our customers.

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