Week 7 State of charge estimation

State of Charge:

The level of un used charge in electric battery is called State of Charge  (SoC). There is no sensors to measure the SoC directly from the battery like voltage current, force, pressure etc.

Methods of SoC estimation:

• chemical
• voltage
• current integration
• Kalman filtering
• pressure

Current Integration method or Coulomb  counting method (CCM)

This method, also known as "coulomb counting", calculates the SoC by measuring the battery current and integration it in time. Since no measurement can be perfect, this method suffers from long-term drift and lack of a reference point: therefore, the SoC must be re-calibrated on a regular basis, such as by resetting the SoC to 100% when a charger determines that the battery is fully charged (using one of the other methods described here).

 Limitations of CCM:

 There are several factors that affect the accuracy of Coulomb counting method including temperature, battery history,
discharge current, and cycle life.
* Coulomb Counting doesn't work as well with Lead Acid batteries, because of 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 Peukerts 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.

Implementation of  coulomb counting for SOC estimation in BMS :

The methods that are used to measure the SoC in BMS are given as follows

1. CCM

2. Extended Kalman Filter (EKF)

3. Untended kalman filter (UKF)

CCM:

The inputs to CCM are current and temperature. The capacity is calculated with respect to input temperature using 1D look up table.  The capacity and current are given ti product and multiplier block which are combinely given to integrator. 

Gain K has 1/3600 value which converts hours in seconds. FInally SoC is measured at output side.

Conclusion:

This report presents methods of SoC measurment. The CCM is discussed in details with matlab/simulink diagram along with BMS circuit.