Lithium battery State of Charge(SOC)

Lithium-Ion State of Charge (SOC) measurement

The utilization of lithium-ion batteries is widespread across various applications. In order to maximize their efficiency and lifespan, battery management systems (BMS) are utilized. However, it is important to note that recent advancements in BMS technology have led to increased energy consumption, which can have a negative impact on battery performance.

To address this issue, an innovative approach has been developed. The estimated state of charge (SOC) of the battery is calibrated using an event-driven Open Circuit Voltage (OCV) to SOC curve relation. This method ensures accurate SOC estimation while minimizing energy consumption.

To validate the effectiveness of this approach, a comparison was made with traditional BMS systems. The results clearly demonstrate the superiority of the proposed system. It outperforms traditional counterparts by more than a third-order of magnitude in terms of compression gain and computational efficiency. Importantly, this enhanced performance does not compromise the precision of SOC estimation.

In conclusion, the proposed system offers a solution to the challenges posed by sophisticated BMS technology. By utilizing an event-driven OCV to SOC curve relation, it achieves significant improvements in compression gain and computational efficiency. This innovative approach ensures effective battery utilization and longer lifespan, without compromising the accuracy of SOC estimation.

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Definition and Classification of SOC Estimation

SOC is one of the most important parameters for batteries, but its definition presents many different issues. In general, the SOC of a battery is defined as the ratio of its current capacity () to the nominal capacity (). The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows:


State of charge (SOC) is the level of charge of an electric battery relative to its capacity. The units of SOC are percentage points (0% = empty; 100% = full). An alternative form of the same measure is the depth of discharge (DOD), the inverse of SOC (100% = empty; 0% = full).

Lithium ion VS Lead acid

There are several ways to get Lithium-Ion State of Charge (SOC) measurement or Depth of Discharge (DOD) for a lithium battery. Some methods are quite complicated to implement and require complex equipment (impedance spectroscopy or hydrometer gauge for lead-acid batteries).

We will detail here the two most common and simplest methods to estimate the state of charge of a battery: voltage method or Open Circuit Voltage (OCV) and coulomb counting method.

1/ SOC estimation using Open Circuit Voltage Method (OCV)

All types of batteries have one thing in common: the voltage at their terminals decreases or increases depending on their charge level. The voltage will be highest when the battery is fully charged and lowest when it is empty.

This relationship between voltage and SOC depends directly on the battery technology used. As an example, the diagram below compares the discharge curves between a lead battery and a Lithium-Ion battery.

It can be seen that lead-acid batteries have a relatively linear curve, which allows a good estimation of the state of charge: for a measured voltage, it is possible to estimate fairly precisely the value of the associated SOC.

However, Lithium-ion batteries have a much flatter discharge curve, which means that over a wide operating range, the voltage at the battery terminals changes very slightly.

Lithium Iron Phosphate technology has the flattest discharge curve, which makes it very difficult to estimate SOC on a simple voltage measurement. Indeed, the voltage difference between two SOC values may be so small that it is not possible to estimate the state of charge with good precision.

The diagram below shows that the voltage measurement difference between a DOD value of 40% and 80% is about 6.0V for a 48V battery in lead-acid technology, while it is only 0.5V for lithium-iron-phosphate!

Lithium vs AGM Soc estimation by OCV method

However, calibrated charge indicators can be used specifically for lithium-ion batteries in general and lithium iron phosphate batteries in particular. A precise measurement, coupled with a modeled load curve, allows SOC measurements to be obtained with an accuracy of 10 to 15%.


2/ SOC estimation using the Coulomb Counting method

To track the state of charge when using the battery, the most intuitive method is to follow the current by integrating it during cell use. This integration directly gives the number of electrical charges injected or withdrawn from the battery, thus making it possible to precisely quantify the SOC of the battery.

Unlike the OCV method, this method is able to determine the evolution of the state of charge during battery use. It does not require the battery to be at rest to perform an accurate measurement.

Coulomb Counter

To ensure accurate current measurement, it is important to address any potential errors that may arise due to the sampling frequency. While current measurement is typically carried out using a precision resistor, small errors can still occur. These errors can be attributed to the sampling frequency, which may introduce marginal inaccuracies. However, there is a solution to rectify these errors and ensure precise measurements.

To correct any marginal errors caused by the sampling frequency, the coulomb counter undergoes recalibration at each load cycle. This recalibration process is crucial in maintaining the accuracy of current measurement. By recalibrating the coulomb counter, any errors that may have occurred during the previous load cycle are corrected, ensuring that subsequent measurements are precise and reliable.

By implementing this recalibration process, the accuracy of current measurement is significantly improved. It allows for the identification and correction of any marginal errors that may have been introduced due to the sampling frequency. This ensures that the measurements obtained are highly accurate and can be relied upon for various applications, such as in scientific research, industrial processes, or electronic circuit design.

In conclusion, while current measurement using a precision resistor is generally reliable, small errors can still occur due to the sampling frequency. However, by recalibrating the coulomb counter at each load cycle, these marginal errors can be corrected. This ensures that the measurements obtained are highly accurate and can be trusted for a wide range of applications. By implementing this recalibration process, you can have confidence in the precision and reliability of your current measurements.

Lithium-Ion State of Charge (SOC) measurement made by coulomb counting allows a measurement error of less than 1%, which allows a very accurate indication of the energy remaining in the battery. Unlike the OCV method, coulomb counting is independent of battery power fluctuations (which cause battery voltage drops), and accuracy remains constant regardless of battery usage.