State of Charge (SoC) is a characteristic value for the state of charge of batteries. The SoC value indicates the still available capacity of a battery in relation to the nominal value. The state of charge is specified as a percentage of the fully charged state. 30 % thus means that the battery still has a residual charge of 30 % relative to the fully charged state of 100 %.
There are several characteristic values for the state of charge or the state of discharge - State of Charge (SoC) and Depth of Discharge( DoD) - and the related charge efficiency. The characteristic values are directly related to each other. One characteristic value indicates the state of charge, the other the state of discharge, and the third represents the ratio of charged energy to extracted energy. If one value falls, the other increases and vice versa. If a battery is fully charged, the SoC value is 100% and the DoD value is 0%, because no charge has yet been removed. If the battery is discharged, the SoC value is 0% and the DoD value is 100%.
The State-of-Charge value is an important characteristic value for all battery-powered devices and plays an essential role in automotive technology. There are different methods to determine the state-of-charge: the chemical, voltage, current-integrative, pressure-dependent and impedancemeasurement.
The chemical method is based on the measurement of pH and can only be used when the electrolyte is liquid. The voltage measurement method uses predetermined discharge curves to determine the SoC value. Generally, a fuller battery has a higher voltage. The voltage method is relatively inaccurate because temperature and cell material affect the voltage.
In the current integrative method, the SoC value is determined from the Coulomb value, using the battery current during an integration time. In this method, the current flowing into and out of the battery is measured in ampere-seconds( As) and accumulated. And determining the state of charge via pressure only works for certain NiMH batteries, where the internal pressure increases with the state of charge. The state of charge is evaluated by the battery management system and is incorporated into the control mechanism. InSmart Batteries, the state of charge is shown in the state of charge display.