Electric car batteries - what is their capacity, cell degradation, SoC and DoD concepts

Electric car batteries - what is their capacity, cell degradation, SoC and DoD concepts

Car battery - gross and net battery capacity?

In the context of electric cars, the terms "gross capacity" and "net capacity" of the battery play an important role. They help vehicle owners understand how much energy is available for use and how the battery is managed to optimize safety and performance.

Gross Battery Capacity:

Gross battery capacity refers to the total physical capacity of the battery, or the maximum amount of energy the battery can store, expressed in kilowatt-hours (kWh). This is the value measured before any energy losses are taken into account. Some manufacturers quote this value, which can be misleading to potential customers, implying that all this energy is available for use.

Net Battery Capacity:

Net battery capacity is the amount of energy available for use after taking into account some energy reserves for managing the battery and the vehicle's operating system. It is usually 2-5% lower compared to gross capacity. Here are some reasons why you should pay attention to net and gross battery values:

  • Safety and Longevity: Energy reserves are used to extend battery life and prevent deep discharges (high DoD levels - and more on that later) that could damage the battery.
  • Thermal Management: Batteries operate most effectively within a certain temperature range. An additional capacity buffer is used to manage battery temperature to avoid overheating or undercooling.
  • Failure Protection: The Battery Management System (BMS) may require a certain amount of energy reserves to prevent unexpected failures or to compensate for normal battery consumption.

Concepts - DoD (Depth of discharge) and SoC (State of Charge)

  • State of Charge (SoC): State of Charge (SoC) is the level of charge of an electric battery relative to its total capacity. It is usually expressed as a percentage, where 0% means the battery is empty and 100% means the battery is fully charged.
  • Depth of Discharge (DoD): Depth of Discharge (DoD) is a parameter that determines how much energy has been used from the battery between charging cycles. For example, if you have a 100 kWh battery and use 80 kWh of energy before charging, the depth of discharge is 80%.

In theory, they are simply opposite terms, but due to different methods of calculating the actual state of charge/discharge of batteries, these terms function side by side.


Meaning of SoC and DoD for Electric Car Owners:

Charge Management: Understanding the SoC and DoD is key to effective battery charge management. For example, many studies suggest that maintaining the battery's SoC (i.e. state of charge) of an electric car between 20% and 80% can significantly extend battery life.

Battery degradation in an electric car

The battery in an electric car is like the heart of the vehicle, providing the energy that powers everything from the engine to the on-board electronics. But over time, like other vehicle components, the battery can lose its original ability to store energy. This process, known as battery degradation, can affect the range, performance and overall driving experience of an electric car. That is, over time, the battery's net capacity will decrease.

Battery Degradation Factors:

Battery degradation can be caused by various factors, such as natural aging of chemical components, frequent deep discharges and recharges, and exposure to extreme temperatures. For example, batteries can degrade faster in very hot or very cold conditions. In addition, frequent rapid charging can also accelerate the degradation process.

Effect of Degradation on Battery Capacity:

As battery cells degrade, they lose their ability to store and release energy, leading to a reduction in battery capacity. This process is gradual and can be noticeable after several years of use. Reduced battery capacity means that the vehicle will have less range on a full charge, which can be particularly troublesome for electric car owners who rely on longer routes.

Best practices to keep degradation to a minimum:

  1. Avoid Frequent Fast Charging: Fast charging is convenient when you're on the road, but regular use can accelerate battery degradation. Whenever possible, use standard charging stations, especially when charging daily.
  2. Avoid Deep Discharge: Try to avoid letting the battery discharge completely. Instead, if possible, charge the vehicle when the charge level drops below 10-30%.
  3. Manage Temperature: Batteries work best at moderate temperatures. If you live in a very hot or cold climate, consider garaging your car in a temperature-controlled environment to protect the battery from extreme temperatures. Remember, however, that electric autos have been operating for years in Scandinavia, and despite the extremes of temperature, users have not complained of extreme battery degradation.
  4. Reduce the Maximum Charge Level: Many electric cars allow you to define a maximum charge level, such as up to 80%. Setting a lower maximum charge level can help minimize stress on the battery and extend its life.
  5. Regular Inspection/Verification of Battery Statistics: Regular inspection/analysis can help detect and resolve battery problems before they become more serious. Keep an eye on auto power consumption during prolonged stops, and which systems consume the most power.
  6. Avoid Unnecessary Load: Turning off unnecessary electrical and electronic functions, such as air conditioning or heating, when they are not needed can also help minimize the load on the battery.


Back to blog