Calculating the State of Charge of a Lithium Ion Battery System using a Battery Management System

The State of Charge (SOC) is like the fuel gauge in your car, telling you how much energy is left in your battery system. Calculating the State of Charge in a lithium-ion battery system requires an understanding of how the battery pack capacity and energy change. This understanding is critical before diving deeper into Coulomb counting.

Topics Covered
State of Charge
State of Charge (Capacity) (SOCcapacity or SOCc)
State of Charge (Energy) (SOCenergy or SOCe)
Fuel Gauge
Coulomb Counting
Depth of Discharge
Open Cell Voltage Lookup

The state of charge is defined as the capacity remaining divided by the total capacity of the battery pack. We look at an example of how the state of charge is calculated. Using a discharge curve, he explains that this curve is not linear. Given this, the true state of charge is often different from the energy remaining. Looking at State of Charge (capacity) and the State of Charge (energy) we can understand the differences between these values and how to calculate the most relevant information.

Coulomb counting is discussed next. Coulomb counting integrates amp hours and time to calculate the capacity removed from the pack. This provides a basis for SOC. The current sensor has drift and measurement error, so it is also important to have an open cell voltage lookup.

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