How healthy is your battery?
Battery terminology you need to know
The most important part and at the same time, the most controversial part of an electric vehicle (EV) is the battery. Once you start talking about it, all the tall tales emerge of people having to buy a new battery at a very high price because their new electric car no longer has any range. There is always some truth in this, but it is also important to know and be able to describe the underlying information.
A battery has a certain capacity which determines how much energy can be stored, expressed in kilowatt hours or kWh. The amount of energy a battery contains can be expressed as a percentage of the maximum amount of energy it can hold. This percentage is the state of charge (SoC). Depth of discharge (DoD) then represents how much energy has been discharged. Thus, SoC + DoD = 100%.
A battery stores chemical energy, to supply or absorb power a chemical reaction takes place, the efficiency of these reactions depends on external factors. Some will have already experienced that at cold temperatures a battery is discharged more quickly. In addition, rapid charging and discharging will also have a negative effect. Discharging too deeply will cause damage. Age will also reduce the efficiency of the reaction. And finally, every charge and discharge cycle will reduce battery capacity. So, it is very important to be able to parameterize the health of a battery. The state of health (SoH) is the still usable capacity of the battery expressed as a percentage of the original capacity. The rule of thumb is that once the SoH is lower than 80% a battery is no longer suitable for EV application. They can then be dismantled to reuse the battery modules for stationary applications, e.g. to store solar or wind energy. When the batteries are no longer sufficient for this second application either, they are recycled. In Belgium, players such as Watt4Ever are already working on repurposing EV batteries.
One way to determine the SoH of a battery is to undergo a complete charge and discharge cycle, whereby it is monitored how much energy can be stored in the battery. This will also be necessary in the future for selling used cars, for example. The law of 22 December 2022 requires sellers to state the official range, but experts advocate a SoH certificate. Without knowing the SoH, you pretty much buy a car blind as the battery is so a major part of the already high cost, even for second hand. The SoH is not derivable externally or purely based on kilometers driven as there are so many more factors that have an impact. It is thus important to protect buyers by giving them all the necessary information.
Theoretical models are also good indicators but not always accurate enough. To apply a theoretical model accurately, the battery life cycle data must be known.
Battery life and performance can already be modelled during development based on life-cycle models. But if a particular battery constantly has to deliver and absorb very large currents, as is the case, for example, when one often accelerates extremely fast or during fast charging of EVs, it is not unlikely that the SoH will be inadequate sooner than modelled. However, if one is aware of the determining factors and considers them, when possible, battery life can be longer than predicted.
