Aside from that bottle of wine on your shelf, most things get slowly worse with age and use. The slowing of your electric vehicle performance, called battery degradation, happens based on two things:
- The age of the battery
- How the battery is used, charged, and stored
The first sort is called calendar aging and it is the inevitable, baseline level of degradation.
Batteries do not degrade by the same amount each year. This chart demonstrates that batteries usually see the fastest drop at the beginning and end of their life, creating an ‘S’ curve. (Psst: “SOH” stands for state of health!). Keep reading to find out why this happens.
Batteries are chemical systems that use an “active material” (in this case, lithium) to generate energy. The physical ways that batteries can degrade can be split into two main categories:
- Capacity fade
- Power fade
Both of these will affect your range, but they do so in different ways. Capacity fade is predominantly due to less available lithium for chemical reactions, meaning that there is literally less available energy in the battery. This happens when secondary chemical reactions use up some of the available lithium, such as when you frequently fast charge in cold temperatures.
There is also an expected, normal loss of capacity when a battery is first used. Some of the lithium is used to make a protective layer for the battery. The protective layer is a necessary evil: it uses some of the lithium available to the battery, but also makes the electrolyte stable enough to last and helps protect the anode from corrosion. In our battery degradation plots, the initial drop in capacity is clear, after which the degradation slows.
Power fade is when the rate of energy transfer is reduced, often due to a physical restriction in the battery that prevents lithium ions from moving freely. For instance, very cold temperatures can increase the viscosity of battery materials, slowing the physical transfer of electrons and reducing the amount of energy output per time. There can also be build up or corrosion in the battery that creates a physical barrier that impedes the free flow of lithium. This can also lead to power fade.
Both capacity fade and power fade can be temporary or permanent, and both will inevitably happen with normal use. The goal is to keep them to a minimum. Extreme temperatures, fast charging, and storing your battery at high states of charge can accelerate how much lithium gets added to the protective layer, which can cause both capacity fade and power fade.