State of Charge, abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.
As an example, for the Samsung SDI 94Ah cell [Ref 1].
However, the capacity remaining needs to be defined against specific test conditions, eg rated capacity at C/3 and 25°C. The maximum capacity will be established at a maximum cell voltage, eg 4.15V for this particular 94Ah cell. The 0% is again set under specific conditions and defined by a minimum cell voltage, eg 2.7V at C/3 and 25°C.
It should be noted that this is not the same as the amount of energy in the cell. Energy = volts x current x time. SoC = (Ah capacity – current x time) / Ah capacity
The SoC estimation of the battery cell is very important as so many other functions depend on the accuracy of this value. It is used to estimate a number of parameters, including: maximum charge and discharge current at any instant, the amount of energy left in the battery pack and State of Health.
In the laboratory the cell temperature can be stabilised over hours and the current can be integrated accurately versus time. Also, it is possible to repeat very defined cycles and so build a more robust dataset. Hence the capacity and SoC can be established very accurately.
SoC is always an estimate as it cannot be directly measured. In an operating system the temperature and current can be changing over a large range and sometimes quite rapidly.
Normally this is estimated using one of the following methods:
- Coulomb counting
- Voltage method
- Kalman filter based
All of these methods depend on:
- cell temperature
- whether the cell is being charged / discharged
- leakage current
- self discharge
- age of the cell
What appears at first as a simple parameter is actually difficult and complex to measure under all operating conditions. Hence, extensive testing is required at cell and pack level to parameterise and validate the State of Charge algorithm.
References:
- Martin Murnane, Adel Ghazel, “A Closer Look at State of Charge (SOC) and State of Health (SOH) Estimation Techniques for Batteries”, Analog Devices
- Samsung SDI 94Ah cell specifications
Usable SoC Window
If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%.