Battery Degradation

The performance of lithium-ion batteries deteriorates over time whether they are used or not. This affects the performance and range of EV's.

Aging based on use is called cycle aging and aging based on time is called calendar aging.

The two main consequences of aging are energy capacity reduction and power reduction.

In an EV, the energy capacity determines the range an EV can travel on a single charge, and the power determines the maximum acceleration and top speed.

Energy capacity reduction can be caused by a loss of battery charge capacity or by the rise of impedance.

Loss of the cyclable lithium and deactivation of the active electrode material are the primary sources of capacity reduction.

The rise in impedance is caused by the chemical or physical transformation of the various materials and interfaces.

When the internal impedance increases this affects the power since the battery voltage is reduced.

Calendar Aging

Most cars are parked more than they are driven. Calendar aging is degradation that happens while time passes.

The factors that affect calendar aging are the State of Charge and environmental temperature.

Storage stress

Storage stress

Scientific articles to read to understand more about Calendar aging

Cycle aging

Cycle aging is the reduction of energy capacity or power caused by charging/discharging.

Cycle aging based on SOC area

The below diagram shows how the SOC area used when charging/discharging affect the degradation

Cycle stress

Cycle stress

Charging speed effect on cycle aging

The most important factor for cycle charging is the charging speed. High-speed DC charging increases the effect called lithium plating that converts the lithium ions to lithium metal.

When this happens the available ions in the battery are reduced. Higher speed charging increases the number of lithium ions that are not able to reach the anode. This is again caused by limitations on the chemistry process. If you want to learn more about lithium plating you can read this presentation from Nasa.

Scientific articles to read to understand more about cycle aging

How to preserve battery

  • Don’t park the car with High SOC over a long time
  • Don’t park the car in extreme heat
  • Limit high-speed charging.