The ageing of traction batteries

Traction batteries, which store the energy needed to operate electric cars, are complex energy storage systems that are the scene of numerous electrochemical reactions. As a result of these reactions, the materials that make up the battery wear out over time, affecting the performance of electric vehicles. This phenomenon of degradation of the packs is called “ageing”.

What is an ageing Battery?

Like all lithium batteries, electric car batteries degrade over time. The older an electric vehicle gets and the more kilometres it travels, the more its performance diminishes. After a few years and tens of thousands of kilometres, an electric car sees its range and power decrease, and its recharging time increase. Worse still, severely degraded batteries can present significant risks of thermal runaway. In this case, the BMS (cf traction batteries) will break down the vehicle to protect the users, but a software failure can lead to an accident.

While lithium-ion batteries were previously mainly used in smartphones or computers, with a life span of two or three years, they are now found in electric cars. As the lifespan of a vehicle is around ten years, it is becoming crucial to understand, control and apprehend this ageing phenomenon. Indeed, who would want to buy a new car whose maximum range would be halved after a few years?

In fact, ageing is due to electro-chemical reactions and physical damage to the cells that make up the traction batteries. There is not just one type of ageing, but several, due to complex mechanisms, many of which are still being studied in the laboratory. We can categorize the aging mechanisms into two categories: calendar aging and cyclic aging.

Calendar ageing refers to the “resting” degradation of the battery. When an electric vehicle is not in use, its battery degrades anyway. This phenomenon is particularly important to take into account because, on average, cars spend 90% of their life in the garage! Calendar ageing is highly dependent on the way the battery is stored: the outside temperature and the battery’s charge level during storage play a major role in this type of ageing.

Cyclic ageing refers to the degradation of batteries during a recharge or discharge cycle, i.e. when the car is used. Depending on the use of the vehicle, cyclic ageing can play a significant part in the degradation of battery packs.

Of course, these two types of ageing combine and interact throughout the lifetime of electric car batteries in real use, which makes the study of the overall ageing of the packs much more complex: we cannot simply juxtapose the effects of cyclic and calendar ageing.

By the way, Batteries can be reconditioned. You can try the New Battery reconditioning method to bring back your dead old battery.

The ageing mechanisms of electric car batteries :

In order to understand the aging mechanism of electric car packs, we must first understand how a traction battery works. If you haven’t already done so, please read our article explaining how traction batteries work.

When batteries are charged and discharged, Li+ ions pass from the anode to the cathode and from the cathode to the anode, respectively, all the way through the electrolyte. The performance of a battery depends directly on the number of lithium Li atoms capable of yielding Li+ that can migrate from one electrode to the other. These lithium atoms are called cycle lithiums: roughly speaking, for a battery to work well, as many cycle lithiums as possible must move.

Two factors are responsible for battery degradation: the loss of cycling lithiums (due to parasitic chemical reactions) and obstacles to their movement (due to the increase in the internal resistance of the battery).

One of the main mechanisms responsible for the ageing of a traction battery is the expansion of the Solid Electrolyte Interphase (SEI). This solid interface is formed by contact between the electrodes (mainly the cathode) and the electrolyte. The SEI is created naturally during the very first charge of the battery and protects the electrodes. This interface is an insulator and provides a safety guarantee (if unwanted contact is made between the electrodes, the battery may catch fire). However, the SEI is not stable and develops over time, which has a double effect :

  • First, SEI expands by reducing the cyclic lithium atoms, which is the primary factor in the aging of the packs,
  • then, SEI increases the internal resistance of cells, the second degradation factor.

Another cause of pack aging is the loss of positive or negative active ingredient. The active ingredient is the substance where energy is stored. Unlike, for example, the electrolyte, which is entirely made up of passive material, the active material is found in the electrodes. Graphite, which is a mineral that is found in many lithium battery technologies at the anode, is part of the active material. Parasitic and detrimental chemical reactions can occur between the anode and the solvents that make up the electrolyte, triggering an exfoliation of the graphite.

How to limit the degradation of electric car batteries?

As you will have understood, it is the environment and the use of the battery that greatly impact the performance of the packs and accelerate the ageing of the batteries. It is therefore advisable to specify here the right reflexes to adopt in order to preserve the health of your battery.

First of all, storage conditions obviously play a significant role in the calendar ageing of lithium-ion batteries. Numerous studies (see sources) carried out on cells of the same technology stored at different temperatures and State of Charge (SOC) levels, have highlighted the link between these two factors and service life. Also, high storage temperatures accelerate the parasitic chemical reactions described above as well as the expansion of the SEI. Similarly, battery storage at high SOC levels leads to premature cell degradation. The combined effects of SOC level and storage temperatures are still under investigation: for example, a 1000-day storage at 60°C and 50% SOC degrades the cells more than a 1000-day storage at 40°C and 100% SOC.

In addition, the mode of use of the vehicle has, by definition, a direct influence on the cyclic ageing of the battery packs. There are many factors involved in this type of ageing, such as the internal temperature of the batteries, which increases when the loads are very high (e.g. on motorways or during rapid charging). In addition to increasing the temperature, rapid charging or discharging also increases the voltage at the cell terminals, causing damage. A study shows that the life of a pack is greatly reduced when the charging voltage increases by 0.1V.

Conclusion :

The batteries in our electric vehicles are fragile and complex objects involving chemical and physical reactions, some of which are desired and others harmful (parasitic reactions). This chemistry causes irreversible degradation of the cells, during their use (charge and discharge) or their storage (90% of the life of a car is spent in the garage). These degradations have direct consequences on the use of an electric vehicle (autonomy, power, recharging time …) and are unique for each car, since ageing is extremely linked to the use of the vehicle, which depends on the owners. There is therefore no general law that would link the age or mileage of a vehicle with the state of health of its battery.

The traction battery represents almost half of the value of an electric car, so you have to be very careful about its condition when buying a second-hand vehicle. To avoid any unpleasant surprises, we strongly advise buyers of used electric cars to find out about the performance level of the battery of the vehicle they are interested in.

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