Understanding Actual SOC and Usable SOC (Display SOC) in EVs
Electric Vehicles (EVs) rely on batteries to store and deliver energy. However, understanding the State of Charge (SOC) and how it impacts vehicle performance is crucial for EV owners and enthusiasts. This article explains the concept of Actual SOC and Usable SOC, their differences, and why manufacturers use Usable SOC instead of the full capacity.
Before diving into the details of Actual SOC and Usable SOC, let’s first understand what SOC and SOH mean.
What is State of Charge (SOC)?
SOC (State of Charge) refers to the current level of energy stored in an electric vehicle’s battery, expressed as a percentage of its total capacity. It functions similarly to a fuel gauge in a gasoline-powered car, indicating how much charge remains available for use.
What is State of Health (SOH)?
State of Health (SOH) measures the overall condition of the battery compared to when it was new. As the battery ages, its SOH decreases, meaning its maximum energy capacity is reduced over time.
Actual Battery SOC vs. Usable Battery SOC
What is Actual Battery SOC?
Actual SOC represents the real charge level of the battery based on its total energy capacity, from 0% (empty) to 100% (full). This includes the entire energy storage potential of the battery.
What is Usable Battery SOC?
Usable SOC refers to the portion of the battery SOC which is used for vehicle operation. EV manufacturers limit the usable range within the actual SOC to improve battery longevity and performance.
Another common name for Usable SOC is Display SOC, as this is shown to the user on HMI display.
The following example illustrates the difference between Actual SOC and Usable SOC. In this case, 15% Actual SOC corresponds to 0% Usable SOC, while 90% Actual SOC corresponds to 100% Usable SOC.
Why Do EVs Use Usable Battery SOC Instead of Actual SOC?
- Battery Health Preservation:
- Charging a battery from 0% to 100% Actual SOC repeatedly accelerates degradation.
- To prolong battery life, manufacturers restrict the charge range, often between 10% and 90% Actual SOC.
- Consistent Driving Range Over Time:
- As the battery ages, its SOH declines, reducing the maximum energy capacity.
- To compensate for this, EVs gradually expand the Usable SOC window (shown in above picture), ensuring the vehicle maintains a relatively stable driving range over its lifetime.
- Without this adjustment, an EV’s range would significantly decrease as the battery ages.
How Is SOC Information Presented to EV Users?
All SOC values displayed in an EV (such as the battery SOC on the dashboard or target SOC charge levels) are based on Usable SOC. This ensures a user-friendly experience and aligns with the EV’s operational strategy to optimize battery performance.
By implementing Usable SOC, manufacturers enhance battery longevity and ensure that EVs provide a consistent driving range over time. This approach makes EVs more reliable and efficient for long-term use.
EV Charging Explained – Everything you need to know about Electric Vehicle Charging
