Week 7 State of charge estimation

1).

we have the Closed Loop Harness Dashboard model by Mathworks is as shown below.

now we have to select one of the 3 condition. For this we have atest sequence varient block which facilited to selcect the condition what we have need.

first we have select the intial condition and go for simulation.

After simulating the model for 20000 seconds as given default.

Here we can see all the lights have turned green as there is no fault or errors in the model.

It can be observed from the plot that the vehicle is initially in the driving state and charging was done till 90%. After the charging mode it reaches to the standby mode which can be seen from the BMS state plot. From the Cell temperatures plot it is observed that the battery temperature rises in driving and charging. The battery voltage was increased to 4.2 V while charging. The current plot shows the current value supplied to the vehicle while driving. The SOC was observed to be decreased around 50% in driving condition, which increased while charging.

Coming to the second variant condition to simulate.

plot

In this condition , a fault condition occured in second case. So even after charging was started, the soc didnt increase. It continue to decrease.

Coming to the third condition.

plot

• Here we can see that the soc reached 100% in charging and remained in stand by mode. Cell temperature also decrased when stsndby mode was acheived.

From the 3 cases we observer that only charging takes place thus SOC was 80% at first , then SOC increases to 100% in 2000 seconds then SOC remains constant at 100%.

What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?

coulomb counting:-

Coulomb counting is a technique used to track the State of Charge of a battery pack. It works by integrating the active flowing current (measured in amps) over time to derive the total sum of energy entering or leaving the battery pack. This produces a capacity that is typically measured in Amp-hours. which is estimated from the discharging current, and previously estimated SOC values,

SoC Estimation by Coulomb Counting is based on the measurement of the current and integration of that current over time.

Coulomb counting gives a relative change in SoC and not an absolute SoC. If you measure the current over a given time step you have a measure of the number of Ah that have left or been received by the battery.

where:

• SoC(t) = estimated State of Charge at time, t
• SoC(t-1) = previous State of Charge at time t-1
• I(t) = charging or discharging current at time, t
• Q_n = battery cell capacity
• Δt = time step between t-1 and t

If you want to know the absolute SoC you need to know the starting SoC of the cell, SoC(t-1) as given in the equation. One option is to fully charge the cell to a known voltage. Charging to maximum cell voltage and SoC can be used as a reset and known point.

There are a number of sources of error with the coulomb counting method:

• Starting SoC accuracy
• Current measurement accuracy
• Current integration error
• Uncertainty in battery capacity
• Timing error

In order to define the accuracy you need it is important that you firstly look at what the SoC is used for.

• capacity of the cell that is remaining (fuel gauge on your phone or electric vehicle)
• an input to estimate the available charge and discharge power capability of the battery
• used in the calculation of State of Health

The accuracy required for these values will determine the accuracy that is required for the inputs to the SoC algorithm. This will impact the selection of:

• components for cell voltage measurement
• voltage sense wires, connectors and connection locations
• current sensor or sensors
• voltage and current measurement channel accuracy
• timing circuit

The references below give a good overview of the errors associated with Coulomb Counting along with methods to improve them.

Modified Coulomb Counting Method

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. There is a quadratic relationship between the corrected current and discharging current of battery. By practice of experimental data, corrected current is calculated by the following form:

where k2,k1and k0 are constant values obtained from the practice experimental data.

In modified Coulomb counting method, SOC is calculated by the following equation:

The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.

But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life .

For example we can look into the project file we downloaded. A BMS system in the file is represented below.

BMS algorithm models , protects , reports the measurement from battery pack.

By Clicking on the BMS ECU we get the Subsystem

By clicking on the SOC estimation block we get

From above figure we observe the Coulomb Counting block.

A subsystem opens when we click on the Coulomb Counting

This system does the estimation by coulomb counting equation mentioned above.

• Advantage of coulomb counting method is its simplicity and low computational cost.
• Temperature and current from the battery is given as inputs.
• Current signal is given to a gain block and temperature is given to capacity since it varies according to temperature.

disadvantage

one of the main problems in coulomb counting is that it is necessary to calibrate the initial battery SOC. In several applications, the initialization involves rest and the initial SOC is estimated based on the voltage inversion method.