Introduction

Understanding the remaining energy in a battery during use, as compared to its fully charged state, helps users predict how much longer the battery will perform before it requires recharging. This process is referred to as the short-term measure of battery capacity. The long-term measure of battery health is called the State of Health (SOH), which will be covered in a separate article. The State of Charge (SOC) is defined as the measure of the battery’s available capacity, expressed as a percentage of its reference capacity. The most common reference is the fully charged capacity of a new battery, as the battery’s capacity degrades over time. This article will explore the accuracy requirements of SOC and the methods used to determine it.

The Accuracy Requirement of SOC

Lithium-ion batteries are highly chemically reactive and require advanced Battery Management Systems (BMS) to ensure they operate safely within their designed limits and achieve their expected cycle life. As such, SOC is crucial in large-scale lithium battery applications. In the automotive industry, SOC is also vital for efficient energy management in electric and hybrid vehicles.

SOC is used to determine the range of electric vehicles and to control when the engine in hybrid electric vehicles (HEVs) should be activated or deactivated.

Methods for Determining State of Charge

There are several methods for determining the State of Charge (SOC) of a battery. Some methods are specific to certain cell chemistries, while others rely on measured cell parameters that change with SOC. These methods include:

• Direct SOC Measurement
• Specific Gravity Measurement
• Voltage-based SOC Estimation
• Current-based SOC Estimation
• Internal Impedance Measurement SOC Estimation

Direct SOC Measurement

Direct SOC measurement is straightforward when the battery is discharged at a constant rate. The charge stored in the battery is the product of the current and the time the current flows during charging. However, this method has two main drawbacks. First, in practice, the discharge current cannot remain constant, as it decreases in a non-linear fashion as the battery discharges. Therefore, the measurement device must be capable of integrating current over time. Second, this method requires the battery to be fully discharged in order to determine the stored charge, which is not practical for most applications. Ideally, users should be able to estimate the remaining charge without fully discharging the battery.

Specific Gravity Measurement

This method has traditionally been used to assess the charge condition of lead-acid batteries by measuring the active chemical weight change. As the battery discharges, the sulfuric acid in the electrolyte is consumed, reducing its concentration in the battery. This leads to a decrease in the specific gravity of the solution, which is directly proportional to the SOC. A suction hydrometer has traditionally been used for specific gravity (SG) measurements, although this method is inconvenient. Modern technology has introduced electronic sensors that provide continuous digital SG readings by connecting directly to the battery cells. However, this method is not suitable for other types of battery chemistries.

Voltage-based SOC Estimation

This method estimates SOC based on the voltage of the battery cell. The voltage varies with temperature, age, and discharge rates, so compensations for these factors are necessary for accurate results. Figure 1 below illustrates the relationship between open circuit voltage and capacity at a constant discharge rate and temperature for a high-capacity lead-acid cell.

Figure 1: Open Circuit Voltage vs Capacity at 25°C (Courtesy of Simon Mugo)

This method is less suitable for other chemistries like lithium-ion, which exhibits minimal voltage change throughout most of the charge/discharge cycle. Figure 2 shows the discharge curve for a lithium-ion battery, where the voltage drops rapidly towards the end of discharge. Lithium-ion batteries should not be fully discharged, as doing so can shorten their lifespan.

Figure 2: Lithium-ion Battery Discharge Curves (Courtesy of Simon Mugo)

Current-based SOC Estimation

This method, also known as Coulomb counting, estimates the energy in a battery by integrating the current over the time it flows into or out of the battery. The unit of measurement is Coulombs. The battery’s energy capacity can be determined by measuring the current and integrating it over time. This method provides the most accurate results compared to other methods, as it directly measures the charge entering or leaving the cell.

Current-based SOC estimation can be implemented using three types of current measurement techniques:

• Current shunt: This involves measuring the voltage drop across precision, low-resistance shunt resistors placed in series between the load and the battery. This method is less accurate at low currents, leading to some power loss.
• GMR (Giant MagnetoResistance): This method uses magneto-resistive sensors, which are more sensitive and offer higher signal levels but tend to be expensive.
• Hall-effect sensors and transducers: These sensors are also costly and can be affected by noise and high current flows.

This Coulomb counting method relies on the electrical energy flowing into and out of the battery, but does not account for self-discharge currents that may impact accuracy.

Internal Impedance Measurement SOC Estimation

During the charging and discharging process, the chemical composition of the battery’s electrolyte changes, which in turn affects the internal impedance of the battery. By measuring the battery’s internal impedance, it is possible to estimate the SOC. However, this method is less commonly used because measuring impedance in active cells is challenging, and interpreting the data can be complex.

Summary

• The State of Charge (SOC) is the measure of a battery’s available capacity, expressed as a percentage of a reference capacity.
• The accuracy requirement for SOC is determined by the manufacturer’s specifications, including voltage, current, and energy ratings.
• There are several methods to estimate SOC: direct and specific gravity measurement, as well as voltage-based, current-based, and internal impedance measurements.
• Direct SOC measurement involves discharging the battery at a constant rate and calculating the charge stored as the product of current and time.
• Specific gravity measurement is primarily used in lead-acid batteries, where SG is measured using a suction hydrometer.
• Voltage-based SOC estimation uses the cell’s voltage to estimate the battery’s SOC.
• Current-based SOC estimation (Coulomb counting) involves measuring the current flow into and out of the cell to estimate the SOC.
• The current-based SOC estimation includes three methods: current shunt, GMR, and Hall-effect sensors.
• Internal impedance measurement estimates SOC based on changes in the battery’s internal impedance during charge and discharge cycles.

 

 

 

 

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