Comprehensive information: Battery fires and battery defects

Knowledge about battery fires and defects

Nikolaus Mayerhofer, Patrick Schabus, Peter Bednarik

When talking to people who have had little experience with electric vehicles, sooner or later we always come across the question of the dangers of batteries, especially explosions or non-extinguishable fires. But in our blog post, you can find out how realistic such a scenario really is, apart from Hollywood movies. We also explain what safety risks can actually occur and what steps every EV driver can take to minimize the risk of battery defects. First things first: much more important than the question: "Will my battery burn down?" is the question: "How can I prevent damage to my battery to reduce all potential safety risks?" Because the probability of a fire is significantly lower than with a combustion engine.

How can a fire occur?

Before a battery can catch fire, it must always be preceded by a "thermal runaway", i.e. a cell overheating out of control. This can be caused by various triggers, such as short circuits, overcharging and thermal overloads. Short circuits can be triggered by mechanical damage (e.g. metal spike through the cell), manufacturing defects and chemical processes (e.g. dendrite formation). Thermal overload can occur in the event of missing or damaged cooling or extreme loads at high ambient temperatures. During this "thermal runaway", the components of the cell begin to decompose exothermically, i.e. more and more heat is generated. As soon as this can no longer be dissipated, the pressure in the cell exceeds the load limit and so-called "venting" occurs. The vaporized electrolyte escapes and if the temperature exceeds its flash point, it can ignite. In reality, however, the batteries are very well protected against such a "thermal runaway". The "Battery Management System" (BMS) reliably protects the battery against overcharging and thermal overload (e.g. reducing performance during fast charging). Thanks to the most precise final checks during production, the defect rate is low and the battery is so well protected by appropriate structural reinforcements that no threatening mechanical damage occurs even in the event of an accident. But the older batteries and their control units become, the more important it is to check them regularly.

An explosion like in an action movie - the biggest fear
Everyone knows the scene - the car is blown up in a gigantic explosion. In reality, however, there is no such detonation - when the electrolytes in the cell starts to burn, there is initially a lot of smoke, and only minutes later do the first flames appear, which can develop into a strong fire, but the vehicle is not blown up.

What measures can be taken to counteract this problem?

The BMS has a number of tasks, including ensuring that the cells have the same state of charge at all times. If this is not the case, the range will be reduced, but it may also indicate a problem with the control unit or the battery. If one cell ages faster than the others, at some point the differences can no longer be compensated for by the BMS. Reduced range or lower charging power are the first signs that something is wrong with the battery. And what other signs are there?

Clear indications of a cell defect
There are three ways in which cell or battery defects can be detected without the battery having to be cut open in a laboratory.

  • Spread in the cell voltages: Depending on the temperature, state of charge and SoH of the battery, we can detect abnormalities in the battery or the BMS based on the spread of the cell voltages.
  • SoH deviations:
    • Low SoH: a noticeable loss of SoH can also be a sign of a cell defect (e.g. lithium plating). Cells whose state of health are too far below the manufacturer's specification (e.g. manufacturer guarantees 70%, but SoH is below 60%) can pose a risk to driving, especially if the SoH does not correspond to the age or mileage of the vehicle.
    • SoH variance: If the condition of the cells differs greatly, this may indicate manufacturing defects in new vehicles. In older vehicles, on the other hand, this may indicate an unbalanced load on the cells and thus an increased risk of overloading the aged cells.
  • Deviations in internal resistance:
    • Increased internal resistance: Depending on the temperature, state of charge and
      age of the battery, cell defects can be identified by determining the internal cell resistance.
    • Spread in internal resistance: When the cells age optimally, they age evenly in terms of capacity loss and internal resistance increase. If a significantly greater increase in internal resistance can be detected in individual cells, this is an indication of manufacturing defects.

However, cell defects that cannot be detected with electrical signals also occur. These can only be identified in a laboratory by splitting the cell and analyzing it under a microscope.

Safety risk with new EVs
It is extremely unlikely that a new cell will start to burn. However, there is a possibility that a cell defect could occur during production and not be detected during the manufacturer's end-of-line testing. If the defective cell is then subjected to an extreme load, such as fast charging, the enormous heat can cause a short circuit within the cell.

"And what should I do now?"

FLASH Test as required, even several times a year - to rule out a variety of battery anomalies within the cell pack.
AVILOO FLASH Test, currently the fastest detailed battery test on the market, enables the rapid screening of numerous vehicles within a short time frame. In just three minutes, the test provides an analysis of the battery condition and presents the results in the form of numerical values in the independent AVILOO report. The FLASH test can detect battery faults down to cell level, which is invaluable for the marketing of a used vehicle and transparency when reselling. Its exceptional quality has been confirmed by the European Remarketing Association (CARA).

PREMIUM Test 1x a year - to thoroughly examine the battery
The PREMIUM test enables a more comprehensive battery analysis, as the battery is measured in detail over a longer period of time. This means that significantly more data is available. During the PREMIUM test, the dynamics are used to analyze the internal resistance down to cell level. This test has TUV certification.
Analysis down to cell level is of crucial importance for battery diagnostics, as the overall performance depends largely on the weakest link in the chain. A low level in a single cell leads to a lower overall battery performance. Both the PREMIUM test and the FLASH test from AVILOO include detailed analyses down to cell level, in addition to other categories, and detect battery defects extremely effectively.

Extinguishing electric battery fires - a massive problem
If the fire has already been extinguished, it can reignite itself. This is because there is still enough energy stored in the cell for the process to take on a life of its own and start all over again. In the event of a fire, vehicles must therefore be stored in a water tank, for example, until all cells are completely discharged and no more energy is available.
Another method is to deliberately allow the battery to burn down in a controlled but complete manner. The decisive advantage of this is that it prevents the battery from reigniting and the burnt-out vehicle does not have to be permanently monitored.

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