What’s State of Charge(SOC) in Lithium Battery

SOC in Lithium Batteries
SOC stands for State of Charge. In the context of lithium-ion batteries, Lithium LiFePO4, NMC, LTO, NaB etc.or any other type of batteries like Lead Acid batteries it represents the percentage of remaining charge in the  battery compared to it’s maximum capacity. Essentially, it’s an indication of the battery gauge which tells the battery’s state of charge for example battery has left 80% ,70%,50% etc. Capacity left compared to its original 100% charged capacity.

SOC chart for various Lithium type batteries

Why is SOC Important for battery:
Accurate SOC estimation is crucial for several reasons:
* Battery Capacity: which can give us idea how much backup we can get from the battery or in case of charging how much time required to charge the remaining battery.
* Overcharging and Over discharging Indication: In case the SOC goes to 0% than there may be chance of over discharge of the complete battery which may require professional help to bring the battery to working condition. As there may be BMS which is consuming more energy to discharge the battery in case of standby condition. If the battery is overcharged in case of Charger then also it will indicate the Overcharge condition in terms of voltage and percentage.

Chart for various voltages of each cell and SOC

* Battery Life: Accurate SOC information allows for more efficient energy management, leading to better overall performance and longer usage times between charges. As the battery get old the SOC voltages can be varied by the AI software to increase the battery life.

Charger Quality: It can give clear cut indication of charger quality as it shows the SOC which helps user to check if the charger is overcharging the battery.

Battery Type Identification: Once we know the SOC at charged condition and make the reference voltage we can identify the cell chemistry as each battery type has different voltages and associated SOC.

Used battery identification: If the SOC is not reaching 100% so it can be identified as the battery is used or cell of battery is gone weak. As the used battery clearly shows that 100% SOC is not achievable.

Su-vastika Lithium LifePO4 battery

* User Benefits:
* Accurate Battery Level Indication: Provides users with a reliable estimate of how much battery life remains, allowing them to charge or discharge battery accordingly.
* LCD indication and IOT display: Knowing the true state of charge helps users trust their devices and one can use the battery accordingly. Now a days LCD on the battery or IOT based bluetooth and Wi-Fi based BMS are available which gives the value on the mobile phone or computers etc.

Settable State of Charge: Now a days BMS has the capability to change the values of SOC based on the life of the battery as it helps to increase the life of the battery if we can change the voltages of maximum charge and Low cut off of the Lithium battery according to the chemistry of the battery cell.
There are several methods to estimate SOC, each with its own advantages and drawbacks:
* Coulomb Counting: This method integrates the measured current flowing into or out of the battery over time to determine the amount of charge consumed or supplied. It’s generally accurate but can be only used when battery is being charged or discharge taking in to consideration of BMS self consumption. But when the battery is in idle condition this method becomes ineffective.
* Open-Circuit Voltage (OCV) Method: This method relies on the relationship between the battery’s voltage and its state of charge. Every battery is defined State of charge as per the voltage measured across the terminals. The Lithium LifePO4 and Lithium NMC has different voltages when they are being measured. However, it requires the battery to be in a resting state for accurate measurements.
* Voltage-Based Methods: These methods use real-time voltage measurements to estimate SOC. They are simpler to implement but can be less accurate than other methods, especially under dynamic load conditions.
* Hybrid Methods: These methods combine multiple techniques, such as Coulomb counting and voltage measurements, to improve accuracy and robustness.
Challenges in SOC Estimation
* Battery Aging: As batteries age, their internal chemistry changes, affecting the relationship between voltage and SOC. This can lead to inaccurate estimations.
* Temperature Effects: Temperature variations can significantly impact battery voltage and internal resistance, affecting SOC accuracy.
* Dynamic Load Conditions: Rapid changes in current draw can make it difficult to accurately track the amount of charge consumed or supplied.
Advancements in SOC Estimation
Researchers are continuously developing more sophisticated algorithms and techniques to improve SOC estimation accuracy, including:
* Artificial Intelligence (AI) and Machine Learning: AI-powered models can learn complex relationships between battery parameters and SOC, leading to more accurate and adaptive estimations.
* Latest technology : Improved Sensors and Measurement Techniques which has the standard voltages and current along with the IR measurements which is very accurate and can measure the cycle life based on the changing IR, voltages and current discharge conditions. More precise current sensors and advanced voltage measurement techniques are being developed to improve the accuracy of input data.