adjacentPairCorrelationModel

Identify worst cell relative to other cells in serially connected lithium-ion battery pack

Since R2023a

Syntax

adjacentPairCorrelationModel(data)

adjacentPairCorrelationModel(data,Name=Value)

[worstcell,corrcf] = adjacentPairCorrelationModel(___)

Description

adjacentPairCorrelationModel(data) analyzes the battery data in data to determine the voltage correlation coefficients between adjacent cells in a serially connected lithium-ion battery pack, and plots the results.

The lower the correlation coefficient is between a pair of adjacent cells, the more likely it is that one of the cells in the pair has a fault condition, such as an internal short circuit [1]. The plot legend indicates the cell pair with the lowest correlation coefficients. Using this syntax also displays the index of the worst cell in the pack.

The algorithm imposes no restrictions on battery activity during the collection of data. The battery can be in any operating sequence, including charging, discharging, and standby phases.

adjacentPairCorrelationModel(data,Name=Value) incorporates additional options specified by one or more name-value arguments. For example, increase the SquarewaveAmp argument to a value greater than the default value of 0.003 to accommodate higher levels of noise when the batteries are at rest.

[worstcell,corrcf] = adjacentPairCorrelationModel(___) returns the index of the worst cell worstcell and the estimated correlation coefficients between each adjacent pair. You can use this syntax with any of the previous input-argument combinations.

Examples

Identify Worst Cell in Serially Connected Battery Pack Using Adjacent-Pair Correlation Model

Load the data, which represents an operating profile for an eight-cell battery pack in which one cell is experiencing an internal short circuit. The profile includes standby, driving, charging, and balancing phases. The data consists of 10 columns that contain the sample time (column 1), battery voltages (columns 2–9), and pack current (column 10).

load internalShortCircuit.mat internalShortCircuit

For the adjacent-pair correlation model, you need only the cell voltages. Extract the voltage columns into the data set data.

data = internalShortCircuit(:,2:end-1);

Plot the battery voltages.

plot(data)
legend('Voltages (V)')
title('InternalShortCircuit Dataset - Voltages')
ylabel('Voltage (V) Values')

The voltages appear to track closely, but at this scale it is hard to differentiate them.

Zoom into the region after t = 4000 to show the individual voltages more clearly.

xlim([4000 4010])
ylim([3.4 3.6])
title('Voltage Separation')
ylabel('Voltages (V)')

The voltages track closely together. There is no cell that is obviously degrading relative to the others.

Identify the worst cell using adjacentPairCorrelationModel.

adjacentPairCorrelationModel(data)

ans = 
5

The correlation model identifies cell pair 5,6 as the worst pair and cell 5 as the worst cell.

Specify Time Variable

Load the data, which is contained in a table, and show the first two rows.

load internalShortCircuitTbl.mat internalShortCircuitTbl
head(internalShortCircuitTbl,2)

    Time    Cell1     Cell2     Cell3    Cell4     Cell5     Cell6     Cell7     Cell8    Current
    ____    ______    ______    _____    ______    ______    ______    ______    _____    _______

     0           0         0        0         0         0         0         0        0       0   
     5      4.0037    4.0018    4.002    4.0012    4.0029    4.0012    4.0042    4.002       0   

The adjacent-pair correlation model does not require the Current values. Extract the time and voltage values into the timetable data.

data = internalShortCircuitTbl(:,1:end-1);
head(data,2)

    Time    Cell1     Cell2     Cell3    Cell4     Cell5     Cell6     Cell7     Cell8
    ____    ______    ______    _____    ______    ______    ______    ______    _____

     0           0         0        0         0         0         0         0        0
     5      4.0037    4.0018    4.002    4.0012    4.0029    4.0012    4.0042    4.002

Use adjacentPairCorrelationModel to identify the worst cell. Use Time to specify the time variable.

adjacentPairCorrelationModel(data(:,2:end),Time=data.Time)

ans = 
5

The function identifies the worst pair and the worst cell within the pair.

Obtain Correlation Results

Load the data, which is contained in a timetable, and extract the cell voltages into data.

load internalShortCircuitTT.mat internalShortCircuitTT
data = internalShortCircuitTT(:,1:end-1);

Run adjacentPairCorrelationModel using output arguments to store the analysis results.

[worstcell,corrcoef] = adjacentPairCorrelationModel(data);

Identify the worst cell.

iwc = worstcell

iwc = 
5

Plot the estimated correlation coefficient curves. For the legend, use the VariableNames property of data to access the cell names. With this plotting approach, the curve for each cell in the legend represents the correlation coefficient curve between that cell and the cell to the right in data.

For example, Cell1 represents the correlation curve between Cell1 and Cell2. Cell8 represents the correlation curve between Cell8 and Cell1.

plot(corrcoef.Time,corrcoef.Variables)
title('Correlation Coefficient Curves')
legend(corrcoef.Properties.VariableNames)
ylabel('corrcoef')

Cell 5, which the function identifies as the worst cell, has the lowest pair of correlation coefficient curves.

Input Arguments

data — Battery data

table | timetable | matrix

Battery cell voltage, specified as a table, a timetable, or a matrix that contains ns rows, where ns is the number of samples, and nc columns or variables, where nc is the number of cells.

If your measurement data also includes an explicit column for time, you must exclude that column from the data argument when you call adjacentPairCorrelationModel. However, you can access the time data for plotting by using the Time name-value argument.

If data is in the form of a timetable, you can use data directly with adjacentPairCorrelationModel. Use timetable properties such as data.Time or data.Variables to extract information for additional analysis or plotting.

Example: adjacentPairCorrelationModel(data(:,2:end),Time=data(:,1)) analyzes the cell voltage correlations using the voltage data in the matrix or table data, and plots the results against the time data in the first column.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: adjacentPairCorrelationModel(data,WindowSize=size(data,1)/2) sets the window size to half the number of samples when the number of samples is even.

Time — Time data

numeric vector | duration vector | datetime vector

Time data to use for plotting, specified as numeric vector, a duration vector, or a datetime vector of length ns, where ns is the number of samples in data. 'Time' can be a column in the data set used for data or a separate vector.

If data is a timetable, the time information in data takes precedence and the software ignores the 'Time' specification.

If data is a table or a matrix and 'Time' is not specified, then the function plots the results against the index vector [1:ns].

Example: adjacentPairCorrelationModel(data,Time=tv) specifies the time vector tv, which has the same number of rows as data but is separate from data.

WindowSize — Length of moving window

positive integer scalar

Length of the moving window for computing the correlation coefficient, specified as a positive integer scalar. The default value is equal to the number of rows, or samples, in data.

• When the window size is odd, each moving window is centered on the current element in data. For instance, if the window size is 49, the window is centered on the 25th element within the window.

• When the window size is even, each window is centered about the current and previous elements.

• When there are not enough points remaining at the end to fill the window, the software truncates the window size so that the final window is full.

Setting the window size is a balance between sensitivity to faults and insensitivity to noise. When WindowSize is small, the correlation coefficients are more sensitive to noises and their impact on voltage changes, which can result in false fault detections. Conversely, when WindowSize is large, the changes in voltage are less detectable and might be missed.

Example: adjacentPairCorrelationModel(data,WindowSize=size(data,1)/2) sets the window size to half the number of samples when the number of samples is even.

SquarewaveAmp — Amplitude of square wave

0.003 (default) | numeric scalar in the range (0,1] | 0

Amplitude of the square wave that is added to data, specified as a non-negative numeric scalar or, to disable the square wave, 0.

When the battery is at rest, noise dominates the correlation coefficient calculations and the coefficients for healthy pairs are close to zero, interfering with the identification of the true worst cell. Adding the square wave raises the coefficient values above the noise floor. A good square wave amplitude is three times the noise level in data. With this amplitude, the correlation coefficients of healthy pairs are around 0.9, allowing truly faulty cells with lower correlation coefficients to be detected correctly.

Example: adjacentPairCorrelationModel(data,SquarewaveAmp=0.1) sets the square wave amplitude to 0.1.

Output Arguments

worstcell — Index of worst cell

integer

Index of the worst cell, returned as an integer that corresponds to the cell that has the lowest corrcf curves with adjacent cells. That is, the worst cell is the cell that deviates the most from its adjacent neighbors.

If the two minimum corrcf curves do not share the same cell, then the software determines the worst cell as follows:

• Identify the adjacent pair with the lowest corrcf curve as the pair that must contain the worst cell.

• Within this pair, compare the raw voltage of the two cells. Lower voltage is an indication of degradation. Identify the cell with the lower raw voltage as the worst cell.

For example, suppose that the cell pair 4,5 has the lowest corrcf curve, but cell pairs 3,4 and 5,6 both have corrcf curves that are higher than the second lowest adjacent-pair corrcf curve. If the raw voltage of cell 4 is less than the raw voltage of cell 5, then the software returns cell 4 as the worst cell.

corrcf — Correlation coefficients

table | timetable | matrix

Correlation coefficients between each adjacent column pair in data, returned as a table, timetable, or matrix. The data type of corrcf is consistent with the data type of data.

For example, corrcf(:,1) represents the correlation coefficient curve between cells 1 and 2. For an eight-cell battery pack, corrcf(:,8) represents the correlation coefficient curve between cells 8 and 1.

References

[1] Xia, Bing, Yunlong Shang, Truong Nguyen, and Chris Mi. “A Correlation Based Fault Detection Method for Short Circuits in Battery Packs.” Journal of Power Sources 337 (January 2017): 1–10. https://doi.org/10.1016/j.jpowsour.2016.11.007.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2023a