State of Charge (SoC) Definition & Estimation

Optimizing Battery Performance: The Importance of State of Charge (SoC)

State-of-Charge (SOC), also known as the state of charge of the battery, refers to the current level of energy stored in a battery relative to its maximum capacity, usually expressed as a percentage, and reflects the actual available power of the battery.

The SOC value of the battery is expressed in one byte, which is a two-digit hexadecimal number (the value range is 0~100), where 0% means that the battery is fully discharged, and 100% means that the battery is fully charged. As the battery’s state of charge can reflect the actual available power of the battery, therefore, SOC is a crucial parameter in battery management systems (BMS) and plays a significant role in determining the available energy for use in various applications.

Since the available electrical energy changes with charge and discharge current, temperature and aging phenomena, the definition of state of charge is also divided into two types: Absolute State of Charge (ASOC) and Relative State of Charge (RSOC). A new, fully charged battery has an Absolute State-Of-Charge (ASOC) of 100%; an aged battery, even if fully charged, will not reach 100% under different charge and discharge conditions.

The following graph shows the relationship between voltage and battery capacity at different discharge rates. The higher the discharge rate, the lower the battery capacity. Meanwhile, when the temperature is low, the battery capacity will also decrease.

Figure 1 Battery Capacity vs Discharge Rate

Figure 1: Battery Capacity vs Discharge Rate

Figure 2 Battery Capacity vs Temperature

Figure 2: Battery Capacity vs Temperature

1. Importance of State-of-Charge (SOC)

* Battery Management

  • SOC provides essential information to manage and control battery charging and discharging.
  • It helps prevent overcharging or over-discharging, which can damage the battery and shorten its lifespan.

* User Information

  • SOC gives users an indication of how much energy is available in the battery, so allowing them to plan usage accordingly.
  • In electric vehicles (EVs) and portable devices, SOC meters provide users with real-time feedback on battery status.

* Performance Optimization

  • Monitoring SOC optimizes battery usageto maximize performance and efficiency.
  • It helps balance power demands with available energy, ensuring optimal operation of devices and systems.

2. Methods for Determining State-of-Charge (SOC)

There are two typical methods for estimating the state of charge of a battery: the open circuit voltage (OCV) method and the Coulometry method. Each method has its strengths and weaknesses, and the choice depends on factors like the application requirements, battery chemistry, and available resources for implementation.

  • Open Circuit Voltage (OCV) Method


  • The OCV method relies on the fact that there is a predictable relationship between the battery’s voltage and its state of charge (SOC) when no current is flowing through it.
  • As the battery discharges or charges, its voltage changes due to chemical reactions occurring within the battery.
  • When a battery is at rest and disconnected from any load or charging source, its voltage stabilizes to a value known as the open circuit voltage (OCV).
  • The OCV represents the equilibrium voltage of the battery’s electrochemical system in the absence of any external influences.

Process: Measure the battery’s voltage when it’s not supplying or receiving any current. Compare this voltage to a lookup table or a mathematical model to estimate SOC.

Advantages: The OCV method is relatively simple to implement and requires only a voltage measurement device. Therefore, it doesn’t require additional hardware such as current sensors or integration algorithms. What’s more, since it only involves voltage measurement, the OCV method is cost-effective compared to some other SOC estimation techniques.


  1. The accuracy of SOC estimation using the OCV method may be affected by factors such as temperature, battery age, and state of health (SOH).Therefore, a fixed open-circuit voltmeter cannot fully represent the state of charge; you cannot estimate the state of charge by checking the meter alone.
  2. Calibration and periodic recalibration may be necessary to maintain accuracy, especially as the battery ages.
  3. The OCV method may not capture the dynamic behavior of the battery during charging and discharging as accurately as other methods such as coulomb counting.

The figure below shows that the state of charge of the same battery voltage under charging and discharging is very different from the state of charge found by the open circuit voltage method.

Figure 3: Battery voltage under charge and discharge

As can be seen in the figure below, the charge state varies greatly under different loads when discharging.

Figure 4: Battery voltage under different loads during discharge

In summary, the Open Circuit Voltage (OCV) method is a simple and cost-effective technique for estimating battery SOC based on its resting voltage. While it has some limitations in terms of accuracy and dynamic behavior, it remains a valuable tool for SOC estimation in various applications.

  • Coulomb Counting/ Coulometry Method

1. Principle: Coulomb counting calculates SOC based on the amount of charge (in Coulombs or Ah) that enters or exits the battery during charging or discharging.

2. Process: Measure the current flowing into or out of the battery and integrate it over time to track the accumulated charge. SOC is then calculated based on the initial capacity.

* Charge and Discharge Coulomb Counters

  • Coulomb counters are used to measure the amount of charge that enters (during charging) or exits (during discharging) the battery.
  • The charge coulomb counter tracks the charge going into the battery during charging, while the discharge coulomb counter monitors the charge leaving the battery during discharging.

* Calculation of Remaining Capacity (RM) and Full Charge Capacity (FCC)

  • By integrating the charge or discharge current over time, the coulomb counters can calculate the remaining capacity (RM) and full charge capacity (FCC) of the battery.
  • RM represents the remaining usable capacity of the battery, while FCC is the total capacity available when the battery is fully charged.

* Estimation of State of Charge (SOC)

  • SOC can be calculated using the formula: SOC = RM / FCC.
  • This calculation provides a real-time indication of the battery’s energy level relative to its maximum capacity.

* Estimation of Remaining Time

  • Based on the rate of charge or discharge and the remaining capacity, coulombometry can estimate the time remaining until the battery is fully charged (TTF) or exhausted (TTE).
  • These estimates help users plan their activities and manage battery usage more effectively.

3. Advantages of Coulometry:

* High Accuracy: Coulometry provides precise SOC estimation by directly measuring the amount of charge transferred.

* Chemistry Agnostic: It can be applied to various battery chemistries with appropriate calibration.

* Less Dependent on Battery History: It’s less influenced by factors like temperature, aging, or past charging/discharging history, providing more consistent results.

* Versatile: It can be used for various applications, including research, development, and quality control of batteries.

4. Limitations of Coulometry:

* Complex Implementation: It requires sophisticated instrumentation and control systems, making it more complex and expensive to implement compared to other SOC estimation methods.

* Offline Measurement: Coulometry often requires interrupting battery operation to conduct measurements, limiting its suitability for real-time monitoring applications.

* Calibration Requirements: Proper calibration is essential for accurate Coulometry measurements, requiring additional and continuous battery testing.

* Limited Dynamic Response: It may not provide real-time SOC estimation during rapid changes in battery operation, such as during transient loads.

In summary, coulometry is a versatile and reliable method for estimating battery SOC, remaining capacity, and remaining time during charging and discharging. Its wide range of applications span a variety of industries and help improve the performance, efficiency, and user experience of battery-powered systems.

3. Challenges and Limitations of SoC

  • Complexity

Accurately determining SOC is challenging due to factors like battery chemistry, aging, temperature, and load variations.

Different estimation methods may be required for different battery chemistries and applications.

  • Degradation

SOC estimation accuracy tends to degrade over time as batteries age and undergo capacity loss.

Calibration and periodic recalibration may be necessary to maintain accuracy in long-term use.

  • Safety Concerns

In critical applications like electric vehicles, inaccurate SOC estimation can lead to unexpected battery depletion and potential safety hazards.

Robust SOC estimation methods are essential for ensuring reliable and safe battery operation.

4. Conclusion

In summary, State-of-Charge (SOC) is a critical parameter in battery management, providing valuable information for optimizing performance, prolonging battery life, and ensuring safe and reliable operation in various applications. Advanced estimation methods and ongoing research aim to improve SOC accuracy and reliability, addressing the challenges posed by battery complexity and variability.

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