Batteries do not age gracefully – they like to be treated well
Battery treatment is the most important consideration in the assessment of used batteries, as it has a major impact on their degradation. Many of us are familiar with the cell-phone scenario. A new phone performs well initially, but after a while, depending on the model, this performance drops. The phone’s owner notices that they need to charge it more frequently, which has an impact on usage and will eventually mean they start thinking about buying a new phone. For many people, this will not hit their pocket too hard.
The same cannot be said for automotive batteries, as they can cost between €10,000 and €30,000 per pack – approximately 30 to 50% of the total vehicle cost. A battery is the most expensive singular component in a vehicle, whether it is new or used. As it operates and gets older, two major effects can be observed: firstly, the battery’s capacity gradually declines (limiting a vehicle’s range) and secondly, the inner resistance increases (causing a loss in performance).
A typical automotive lithium-ion battery, or battery pack, normally comprises serially connected modules, which are made up of cells. The cells consist of differing materials that need to interact with each other in a specific way for optimum battery performance. Over time, the impact of physical-chemical effects in the cells will affect the ageing process. Negative changes in the lithium-ion cells can occur on the anode, cathode, electrolyte, current collector, and the separator.
Not getting any younger
Lithium-ion BEV batteries are no exception to the rule. Huge amounts of time and effort are being invested into assessing the battery-ageing process. The goals of transparency for users and manufacturers and optimised service life drive this mission. Information on the battery’s state of health (SoH) is only available in very generic terms. The battery management system (BMS) will provide you with a number, such as 89%. At first glance this tells you that 89% of the initial capacity is still usable. But is this a ‘good’ performance for a battery? This type of information alone is not a reliable indicator of battery ageing. This is like going to the doctor and being told that your current health is 89%. What does that really mean for you? 11% of the missing ‘performance’ can translate into having a broken arm, or it could refer to a cardiac defect. Whilst the former will most likely not impact your overall life expectancy, the latter might be significantly reducing it. Without knowing any details about your habits, behaviours, and environment, the medic cannot make any specific prognosis about your life expectancy. More specific information is essential. Similarly, a reduced battery capacity can have different causes, and some will impact the remaining useful lifetime more than others. The state of health prognosis provided by the BMS falls far short of delivering the information required for decision-making.
Vehicle and battery manufacturers alike have recognised the need to better understand the battery degradation process. Two kinds have been identified: cyclic (age resulting from battery use) and calendar (age resulting from chemical processes when the battery is not in use). Cyclic ageing is influenced by operating conditions (stress factors such as charging throughput, temperature, rate of power, charging/discharging hub and state of charge (SoC)) and the way in which the battery is charged. Calendar ageing is driven by the SoC and the storage temperature.
In the BEV, battery degradation manifests itself in two ways:
- The battery capacity starts to decrease, which reduces the range of a BEV; and
- Internal resistance within the cell increases, which weakens performance e.g. acceleration and charging.
Some of these degradation mechanisms can be influenced by the user, others cannot.
Oversizing of battery packs
Manufacturers of electric vehicles commonly oversize their battery packs to compensate for the gradual capacity loss. In order to improve customer satisfaction and account for different treatment and operating conditions (e.g. climate), they include additional capacity in their battery systems, which they gradually make available across the lifetime or warranty period of the battery. So whilst the usable battery capacity (the so-called depth of discharge) may only be 90% at the beginning, this might be increased to 95% towards the end of the warranty period. The user only notices marginal reductions in the range and enjoys a perceived higher product quality. This also prevents warranty claims.
Since not all manufacturers are using such measures and the degree varies, oversizing of batteries is not considered in the simulation.
Manifestation of battery ageing
One key physical-chemical effect involves changes to the anode/electrolyte interface. As soon as the anode comes into contact with the electrolyte solvent, a layer of film begins to form. This film is referred to as the Solid Electrolyte Interphase (SEI) and comprises the products of electrolyte decomposition. The SEI is still one of the least understood components of the battery cell. Although having an SEI is crucial for the battery’s safe operation, it can have negative effects on battery-ageing. Essentially, the growth of this layer weakens lithium ions’ ability to react electrochemically. As the layer gets thicker, the mobility of lithium ions in the electrolyte reduces and the internal resistance increases. Further, lithium bound in the SEI cannot be used for storing energy any longer and thus leads to decreasing capacity.
Lithium (Li-) plating is one of the most critical degradation mechanisms. Also known as lithium deposition, it concerns the build-up of metallic lithium around the anode of lithium-ion batteries mainly during (fast-) charging. This has two effects: an increase of the inner resistance and a loss of lithium inventory and thus capacity. It therefore adversely impacts both, performance and capacity. Ultimately, Li-plating can have a deadly effect on the battery. With an increasing amount of Li-plating, the capacity of a battery can rapidly decreases. This so-called non-linear ageing occurs towards the end of a battery’s life before it becomes unusable. The decline is such that the battery cannot even be qualified as suitable for second life, i.e. the application in stationary storages for which lower currents and depth of discharges are required. This undesirable effect can be avoided by charging with moderate currents and avoiding very low temperatures. This is also the reason why some batteries with 80% state of health might already be a couple of cycles away from their end of life whilst others might still have hundreds of cycles to go. The potential buyer needs to consider the end-of-life factor – how many years does this battery have left? The range, i.e. the capacity, of the battery is not the only indicator.
The feel-good environment for batteries
The cell temperature is one of the most important influences on battery degradation. A lithium-ion battery’s feel-good area is very much like a human’s and lies between 20°C and 40°C for most of those commercially available. A temperature increase of about 10°C leads to chemical processes running twice as fast as before. If the temperature rises above 40°C, the rate of ageing increases incrementally, leading to a significantly reduced expected lifetime. The opposite situation can be even worse news for batteries. Charging or discharging lithium-ion battery packs at cold temperatures may cause Li-plating and effect severe capacity loss. Ambient temperature is not the only culprit; battery cells themselves warm up during charging and discharging and must be effectively tempered.
In this paper, we do not consider the impact of ambient temperature on batteries. The stage in our case is set in Germany in a moderate climate. More extreme conditions – e.g. tropical or arctic – also have a significant impact.
Manufacturers address the temperature impact with the appropriate thermal management systems. The quality of the thermal management varies, and the correct setup is very complex, but generally the BEV user has minimal influence on this.
The use and misuse of batteries
This is different when it comes to the use of the vehicle. When using a BEV, the two degradation modes should be kept in mind: cyclic and calendar ageing. The former is significantly impacted by user behaviour.
Obviously, a higher energy consumption due to speeding, rapid acceleration or mechanical braking instead of recuperation will ultimately cause the range of a BEV to reduce – as it does in a regular petrol car. Batteries must then be recharged more frequently, which drives up the overall cycle count. This means that the degradation occurs more quickly. Depending on the operating conditions, e.g. how much capacity of the batteries is used – the depth of discharge – or the ambient temperature, the cycles can also differ in their impact on the remaining useful lifetime of the battery.
As fast charging is often the biggest stress factor on a BEV nowadays, it is worth examining this element as well. BEV drivers who cover long distances and are under time pressure – think sales rep racing up and down the country to visit clients – may need to use fast charging regularly.
On the other hand, a BEV driver who takes their car out once or twice a week to visit their relatives down the road can easily use regular charging at home without putting any excess stress on the battery.
Again, the depth of discharge plays a role here. But it is also relevant to look at the target state of charge. Many manufacturers will, by default, only charge the vehicle to 80%. The reason is that the extremes – either completely discharged or completely charged – stress the battery significantly more than use in the middle of charge window (e.g. 30-60%).
Another effect comes into play here too – the calendar ageing. Parking a BEV fully charged is not ideal. When storing a battery or parking a BEV, lower states of charge and temperatures are better to prevent this degradation mode.