SOC estimation of battery using Coulomb counting method

SOC: SOC stands for the state of charge of a battery. SOC represents the amount of energy left inside the battery with respect to the total amount of energy in a battery at the Fully charged condition. SOC can help a user to determine when to charge the battery or stop charging and how much energy he can extract from a battery at that time. There is no direct method to calculate SOC for a Li-ion battery but there are several ways in which we can calculate SOC with the help of some additional parameters.

The easiest method to calculate the SOC of a battery is by using a battery voltage translation method. This method is not accurate but helps to approximately and quickly calculate SOC and can be used in devices like power banks where SOC is shown in range and not the exact percentage. For example, a typical power bank shows SOC in the range from 0-25%, 25-50%, 50-75%, 75-100%.

There are several limitations to this method and hence more accurate methods are needed like the Coulomb counting method, the Fuzzy logic method, the Kalman filter, the RBF Neural network, the Fuzzy neural network, the Hybrid methods, etc.

Each method has its limitations, accuracy, and complexity and hence proper decision should be made while selecting methods for SOC estimation.

Coulomb Counting Method :

The coulomb counting method is also known as the battery current integration method. In this method, a discharging/charging current I(t) is measured and integrated over time to calculate the amount of energy remaining inside the battery. In this method, SOC is taken at the previous time step SOC(t-1) and the amount of energy removed/added at the current time step is subtracted/added from the SOC of the previous time step SOC(t-1) to obtain the amount of energy present in the battery at the present instant.

The SOC estimation in the coulomb counting method is done using the below equation,

`SOC(t) = SOC (t-1) * int_(t-1)^t(I_t*eta)/(Q_n)* dt`

SOC(t-1) = SOC at previous time step

`eta` = efficiency

t = current time step

t-1 = previous time step

Q = Charge capacity of the battery

This method is accurate and reliable than the voltage translational method but does not take into account the discharge rate, temperature, hysteresis, age of the battery, etc.

The above is a generalized form of how a BMS works for SOC estimation. Depending on the method used for SOC estimation various parameters are taken as input by the BMS. These parameters are used further to calculate the SOC using various relations as per the method and SOC is given at the output. In our case temperature is not needed since the Coulomb counting method is independent of the working temperature of the battery.