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State of Charge (SOC) is the percentage of full charge remaining in electric or hybrid vehicle batteries. There are four methods to establish the SOC, including chemical analysis, current integration, pressure testing, and voltage-based calculation. A battery’s voltage reading is not a reliable indicator of its charge level.
State of Charge (SOC) is a term that describes the percentage of full charge remaining in electric or hybrid vehicle batteries. This concept expresses how “full” the battery is as a percentage value with 100% fully charged and 0% empty or depleted. The SOC of batteries cannot be directly established simply by including a gauge or meter in the circuit. There are four indirect methods used to establish the COS. These methods require access to the battery case or external calculations based on battery readings.
Determining the amount of usable charge left in electric and hybrid vehicle batteries is not as simple as it seems. By simply inserting a voltmeter into the battery circuit you can see the user stranded on the 10th hole; A battery’s voltage reading is not a reliable indicator of how long your designated equipment can drive. This is due to the fact that most batteries, particularly those designed for electric vehicles, are designed to maintain their nominal voltage throughout their effective charge ranges. This means that a 12-volt battery will still return a 12-volt measurement, or very close to it, even if it lacks the charging capacity to supply the current needed to drive the vehicle. The state of charge can be considered as a fuel gauge for battery vehicles and as a measurement method it is a reliable indicator of the effective strength of the battery.
There are four commonly used methods to measure the state of charge of a battery. The first is a chemical analysis of the electrolyte. This is only possible on non-sealed batteries and involves inserting a measurement probe into the battery water. This probe measures the pH or specific gravity of the battery water, and the readings are used to calculate the SOC of the battery.
The second method is current integration, which is calculated using measured readings from the battery circuit. Also known as coulomb counting, this method uses mathematically integrated battery current readings over the period of use to calculate SOC values. Although accurate, the current integration suffers from several weaknesses, such as calculation drift over time and lack of references. This requires battery vehicles with this type of state of charge system to have the calibration recalibrated on a regular basis.
The third system of measuring the state of charge is a pressure test that also requires the installation of a sensor inside the battery casing. This system is based on the tendency of a battery to develop a set pressure in the case when fully charged. This pressure dissipates or drops as the battery discharges and the reading, together with charge/discharge calculations based on Peukert’s law, can return accurate SOC values. This method is most effective when used on NiMH-type nickel-metal hydride batteries.
The fourth state of charge calculation method is voltage based and uses a rather complex discharge curve calculation to return the SOC values. The system provides a series of environmental readings such as battery temperature and electrochemical kinetic values to accomplish this. While this method is generally quite accurate, the stable voltage range of the aforementioned batteries makes it difficult to implement.
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