What Is State of Charge?

State of charge (SOC) is a term that describes the percentage of full charge remaining in the batteries of electric or hybrid vehicles. This concept expresses how “full” the battery is as a percentage value with 100% being fully charged and 0% being empty or flat. The SOC of batteries cannot be established directly by simply including a gauge or meter in the circuit. There are four indirect methods used to establish SOC. These methods require access to the battery casing 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 may seem. Simply inserting a volt meter in the battery circuit may well see the user stranded on the 10th hole; the voltage reading of a battery is not a reliable indicator of how long it can still drive its designated equipment. This is due to the fact that most batteries, particularly those designed for electric vehicles, are designed to maintain their rated voltage throughout their effective charge ranges. This means that a 12 volt battery will still return a measurement of 12 volts, or very close to it, even if it lacks the charge capacity to supply the necessary current to drive the vehicle. State of charge may be thought of as a fuel gauge for battery vehicles and, as a measurement method, is a reliable indicator of effective battery strength.

There are four commonly used methods of measuring 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 the insertion of a measurement probe into battery water. This probe measures the pH or specific gravity of the battery water, and the readings are then used to calculate the SOC of the battery.

The second method is current integration which is calculated with metered readings from the battery circuit. Also known as coulomb counting, this method employes battery current readings mathematically integrated over the usage period to calculate SOC values. Although accurate, current integration suffers from several weaknesses such as calculation drift over time and a lack of referencing. This requires battery vehicles with this type of state of charge system to have the metering recalibrated on a regular basis.

The third state of charge measuring system is a pressure test which also requires the installation of a sensor inside the battery casing. This system relies on a battery's tendency to develop a set pressure in the casing when fully charged. This pressure dissipates or drops as the battery is discharged and the reading, in conjunction with charge/discharge calculations based on Peukert's law, can return accurate SOC values. This method is most effective when used on nickel-metal hydride NiMH type batteries.

The fourth state of charge calculation method is voltage based and uses a fairly complex discharge curve calculation to return SOC values. The system provides a number of ambient readings such as the battery's temperature and electrochemical kinetics values to achieve this. While this method is generally fairly accurate, the stable voltage range of the batteries mentioned previously makes it difficult to implement.