Week 7 State of charge estimation

The SOC of a battery is defined as the ratio of its current capacity Q(t) to the nominal capacity Qn. The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows:

An accurate onboard State-of-Charge (SOC) estimation is one of the key functions a Battery Management System (BMS) has to perform in order to provide the optimal performance management of the battery system under control. The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC. Coulomb counting method is done to estimate the SOC(t), which is estimated from the discharging current, I(t), and previously estimated SOC values, SOC(t-1). SOC is calculated by the following equation:

There are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life. Coulomb counting is advantageous in terms of a simple structure and its ease of implementation.

To improve the Coulomb counting method, a new technique called modified Coulomb counting method is proposed. The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation. The corrected current is the function of discharging current.

Where, corrected current is:

this is the battery current integration method.

Depending on the battery chemistry, Coulomb Counting can be a very accurate technique.
Coulomb Counting doesn't work as well with Lead Acid batteries, because:

• The significant leakage current within Lead Acid batteries does not go through the battery current sensor and is therefore not taken into account
• Not all the charge in a Lead Acid battery is converted into useful work: some of it is converted in waste heat, an effect that is characterized by the peukert exponent.

On the other side, Coulomb Counting does work as well with Li-Ion batteries, because of their low leakage and good Peukert exponent.

Another limitation of Coulomb Counting is drift. In any integration, any small, constant error in the variable being integrated results in a drift in the result. In the case of Coulomb Counting, any small offset in the measurement of battery current will result in the SOC drifting up (or down) over time.

proposed algorithm: