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State of Charge

State of Charge (SOC), is used to describe the percentage of the battery’s full charge. When the battery is fully charged, it is described as 100% SOC.

The below formula describes the relationship of the state of charge and depth of discharge (DOD) (Depth of Discharge describes how deeply a battery is discharged)

 SOC = 100% – DOD


State of charge

State of charge (SOC) describes the level of charge of an electrical battery, in relation to the battery’s full capacity. SOC is a percentage so it is has no unit and is expressed in the range from 0% to 100%. Depth of discharge (DOD) is a different form used to express the level of charge, as DOD is  the inverse of SOC (100% DOD means empty; while 0% means fully charged). SOC is mainly used while discussing the battery’s current state, while DOD is used more when discussing battery’s lifetime after repeated use.

Measuring state of charge and its types

State of charge cannot be directly measured but it is estimated from the direct measurement variables either offline or online. In the offline techniques, the battery requires to be charged and discharged with a constant rate like in Coulomb counting. The offline method provides accurate estimation of battery SOC, but it can be prolonged, have high cost, and can interrupt the performance of the main battery. This is why researchers are searching for other online techniques. The five main methods for indirectly determining the SOC are below:

  1. Chemical method: The electrolyte’s specific gravity or pH can be used for indicating the SOC of the battery. Hydrometers are used for calculating the specific gravity of a battery, then SOC is determined from specific gravity by using a SG vs SOC look-up table.
  2. Voltage method:This method converts the reading of the battery’s voltage to SOC, by using the identified voltage vs. SOC discharge curve of the battery. Yet, the voltage is significantly affected by the current of the battery and temperature.
  3. Current integration method: Referred to as coulomb counting, calculates the SOC by measuring the current and integrating it in time. This method suffers from long term drift thus needs frequent re-calibration.
  4. Kalman filter: can be used to overcome the cons of the voltage and current integration methods, as the battery is modeled with an electrical model,the Kalman filter will be used to predict the over voltage, because of the current. So as to provide precise estimation of the state of charge in combination with coulomb counting.
  5. Pressure method: Used in NiMH batteries, where internal pressure increases quickly when the battery is charged so a pressure switch indicates when the battery is charged.


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