Safety

Safety is a fundamental requirement and we have to look at this from a chemical, electrical, mechanical, thermal and complete system viewpoint.

VW ID4 safety with warning labels

Safety Concept

When designing a battery pack / system it is important to think about and describe the safety concept. This will allow you to understand and show the layers of safety designed in physically or into the control system.

The first thing is to look at the specification of the individual battery cell as this will specify the limits of safe operation:

  • Maximum and minimum operating voltage
    • the voltage needs to be measured for most applications to ensure you do not go beyond these limits
  • Electrical shorts
    • external shorts could result in very high currents that can then result in cells getting very hot very quickly, a fusing strategy is necessary to protect at this basic level
  • Maximum and minimum temperature range
    • depending on the application it is likely that you will need to monitor the temperature of the cells and restrict performance in charge and discharge
    • at maximum temperature the battery pack operation should be stopped, ideally by bringing the demands to zero or in extreme by opening contactors
  • Maximum current
    • the BMS needs to manage and restrict operation of the pack within the maximum current limits, these will be available as a map versus temperature and SoC from the cell supplier
  • Mechanical Limits
    • maximum load that can be applied on each axis or cell face
    • loads required for long term operation of the cell
    • shock limits
    • maximum and minimum ambient pressure

The causes of lithium-ion cell failure is captured by the Faraday Insights:

  • Mechanical – An internal defect introduced during manufacture or damage caused to the cell by crushing or penetration (for example, from an EV collision or penetration by a sharp object) can lead to an internal short circuit. Shorting results in an excessive current flowing through the circuit, causing heating above safe limits, cell damage and, in some circumstances, may ignite a fire.
  • Thermal – Overheating the cell (for example, if exposed to an external fire) causes the cell components, such as the cathode and electrolyte, to break down. These processes are heat generating (i.e. exothermic), resulting in a feedback loop where the cell temperature spirals upwards and eventually results in a fire. This is referred to as ‘thermal runaway’. Overheating can also result in the softening or melting of the separator between the two electrodes (generally a polymer such as polypropylene), which can lead to an internal short circuit.
  • Electrical – Shorting a LiB externally (for example, if a metal object is placed across the terminals or by cells in a battery shorting to one another) creates a low resistance pathway for a very high current to flow, which causes the temperature of the cell to rise above safe limits. Overcharging the cell by using, for example, an incorrect or incompatible charger, leads to internal heating and cathode breakdown, both of which contribute to thermal runaway. Poor pack design or poor battery management system (BMS) design can also result in some cells being overcharged. Overcharging is the most dangerous electrical abuse scenario, as it results in far more energy being pumped into the cell than the cell is designed to accommodate. If this leads to cell failure, the extra energy is released through a fire or explosion.
Causes of lithium-ion cell failure [4]

Knee Point – describes a sudden change in the gradient of a cell cycling curve.

Battery Capacity versus number of cycles showing the Knee Point

Also called:

  • rollover failure
  • nonlinear aging
  • sudden death
  • saturation
  • second-stage degradation
  • two-phase degradation
  • capacity plunge
  • drop-off

This sudden change in capacity and resistance can ultimately lead to the thermal runaway of the cell.

Thermal Runaway

If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal runaway.

EUCAR Hazard Levels

EUCAR Hazard Levels

The EUCAR Hazard Levels define the outcome of cell level safety testing. These levels are normally used to describe the outcome of tests such as overcharge as part of the cell specification.

Billy Wu when batteries go wrong

What happens when batteries are abused?

Lithium-ion batteries are an essential component in electric vehicles, however their safety remains a key challenge. This video explores the science behind what happens when batteries are abused and when they fail.

A great introductory presentation by Billy Wu, Dyson School of Engineering, Imperial College.

What Happens if a Cell goes into Thermal Runaway? A significant abuse condition is the pan fire test: All EVs which are on European roads today (unless they are older than 2015) must have passed the regulatory requirements listed in ECE 324 UN R100. Specifically, annexe 9E mentions the test procedure to test the fire safety of energy storage devices used for vehicular applications. The test is often referred as the pan-fire/bonfire test by industry professionals and it is currently in its 3rd revised form.

Fire Mitigation Strategies

The choice of thermal barrier materials (TBMs) for a battery pack depends on number of factors such as; type of cell chemistry, form factor, pack architecture (modular or cell to pack) and most importantly the legislative requirements. It is possible to pass the ECE 324 UN R100 regulation with very little in the way of thermal barrier material but to pass the Chinese GB38031-2020 a robust design factoring passive fire safety is vital. 

Thermal Runaway vs Electrical Energy

The energy released during Thermal Runaway versus the electrical energy stored in a battery.

The energy released during Thermal Runaway (TR) versus the stored electrical energy. A number has been bandied around for a long time that the energy released in a TR event was 2 to 6 times the electrical energy stored in the cell.

There are a lot of papers published where the heat release from a cell has been measured or estimated during thermal runaway. There have also been measurements of numerous chemistries and states of charge etc.

Thermal Runaway Energy vs Electrical Energy

What is Calorimetry?

Calorimetry is a branch of thermodynamics that deals with heat transfer quantification. Heat can be transferred during a variety of processes, such as chemical reactions, phase changes, or dissolution of solutes in a solvent. Calorimetry follows the first law of thermodynamics, which states that energy cannot be created or destroyed, only transferred. The direction that the heat takes will determine the type of process: if heat is absorbed, the process will be endothermic, whereas if heat is released, the process will be exothermic.

References

  1. UN/DOT 38.3 6th Edition – Recommendations on the Transport of Dangerous Goods
  2. IEC 62133-2:2017 – Safety requirements for portable sealed secondary lithium cells, and for batteries made from them, for use in portable applications – Part 2: Lithium systems
  3. UL 2054 2nd Edition – Household and Commercial Batteries
  4. Improving the Safety of Lithium-ion Battery Cells, Faraday Insights – Issue 17: July 2023, The Faraday Institution