Lithium-Ion state-of-charge (SoC) measurement

There are several ways to measure the Lithium-Ion state of charge (SoC) or depth of discharge (DOD) of a Lithium-Ion battery. Some methods are quite complex to implement and require complex equipment (impedance spectroscopy or lead-acid battery hydrometer)

There are two relatively simple and common approaches following to the battery state-of-charge estimation: Open Circuit Voltage (OCV) or voltage method and coulomb counting method.

1.Open Circuit Voltage(OCV) Method to estimate SoC 

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

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

From the diagram it can be concluded that the lead-acid battery has a relatively linear curve which allows a good estimation of the state of charge: for the measured voltage， the value of the associated SoC can be estimated quite accurately.

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 !

2. Coulomb Counting method to estimate SoC 

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 This integration directly gives the quantity of electrical charges injected or withdrawn from the battery, thus making it possible to precisely quantify the SoC of the battery.

This method is different from the OCV method, which determines the evolution of the state of charge of the battery over the course of its use. It does not need to require the battery to be at rest to perform an accurate measurements.

Although current measurement is performed by a precision resistor, small measurement errors may occur, related to the sampling frequency. In order to correct these marginal errors, the coulomb counter is recalibrated at each load cycle.

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

LITHIUM-ION STATE-OF-CHARGE (SOC) MEASUREMENT