Week 8 Multi cell Battery Pack

How weakest cell limits the usable capacity of the battery pack? What is the solution?

The battery configuration in series or parallel connection can have a different voltage on each cell. The weakest cell in the battery pack could have more stress than other cells so that it will become the fastest damage. The battery pack is limited in performance by the lowest capacity cell in the pack since once the weakest cell is depleted, the entire pack is effectively depleted. The health of each individual battery cell in the pack is determined based on its state of charge (SOC) measurement, which measures the ratio of its remaining charge to its cell capacity.

The weakest cell may affect the battery pack by the following criteria:

1. It will carry out a limit on complete operation of the battery pack.
2. Due to weakest cell reaching the limit earlier than others, the battery pack may be under-utilized.
3. During weakest cell condition it is important to protect an individual cell, over voltage / under voltage limits apply to all series-connected cells.

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The issue of charging batteries is of the utmost importance for maintaining batteries in good order and preventing premature failure. Charging a modern vehicle battery is not a simple matter of providing a constant voltage or current through the battery, but requires very careful control of current and voltage.

An important point that applies to all battery types relates to the process of charge equalisation that must be done in all batteries at regular intervals if serious damage is not to result. Since a battery is a collection of cells connected in series, if nominally 50% of the charge is taken from a battery, then some cells will have lost only a little more than this, it maybe 52%, while some may have lost considerably more, or up to 60%. This is because of manufacturing variations, and also because of changes in temperature; the cells in a battery will not all be at exactly the same temperature.

If the battery is recharged with enough for the good cell, then the cells more prone to self-discharge will not be fully re-charged.

Figure highlight the three cell balancing modes of the equalization charger. The cell charging balancing mode happens when the balancer charger transfers the pack energy to the cell with little energy thus operating on a pack-to-cell mode, while the cell discharging balancing mode operates when the balancer charger transfers the extra cell energy back to the pack operating in the cell-to-pack mode.

Solution

A cell balancing is a technique to sort out the problem of the weakest cell.

• A balanced battery is one in which, at some point in its cycle, all the cells are exactly at the same SOC.
• Cell balancing is required for the reason of battery capacity, SOC, leakage, Internal resistance.

Cell balancing technique maximizes the capacity of the multi-cell battery pack. Cell balancing is the process of equalizing the voltages and state of charge among the cells when they are at a full charge. The fundamental solution of cell balancing equalizes the voltage and state of charge among the cells when they are at a fully charged state. Cell balancing is typically categorized into two types:

• Passive
• Active

 

• Passive Cell Balancing

The passive cell balancing method is somewhat simple and straight forward. Discharge the cells through a dissipative bypass route. This bypass can be either integrated or external to the integrated circuit (IC). Such an approach is favourable in the low-cost system application.

• Active Cell Balancing

Active cell balancing, which utilizes capacitive or inductive charge shuttling to transfer charge between battery cells, is significantly more efficient because energy is transferred to where it is needed instead of being bled off.

Charging and discharging processes are carried out to identify the weakest cells in a battery pack, identification is carried out in the discharging process and stopped when one of the batteries has reached the minimum voltage limit. Batteries that experience the lowest limit are indicated as the weakest cells in the battery pack. It was concluded that the weakest cell is in the cell in the middle position of the circuit when the battery is connected in series.

The battery cell balancing process is a key issue in the electric vehicular industry as it enhances the performance of the battery pack while increasing its life-cycle, reduced maintenance, and ensuring safe operation at all times.

The weakest cell identification is done by reading the discharging data based on the changes of value in the voltage value of each cell.

MATLAB Simulink for Lithium-ion battery pack for Fault tracing in 20 cells.

Lithium-ion battery pack with 20 cells example displays how to simulate a battery pack involving of multiple series-connected cells in a proper way. It also provides details how a fault can be presented into one of the cells to see the impact on battery performance and cell temperatures. The fault is represented by changing the parameters for the Cell 10 Fault subsystem, reducing both capacity and open-circuit voltage, and increasing the resistance values.

 

Output Result

The results show the 10-cell simulation which consists overheating of the cell due to high temperature indicated by yellow plot whereas another plot the 20-cell normal temperature of the battery pack.

Here is the given plot, we will see three different types of plot

1. Cell 1-9
2. Cell 10 (fault) i.e 0.5
3. Cell 11-20

     

        

Battery Cell Temperature                                             Battery Cell SOC

 

Parameter and Results:

• Cell Area: 0.101908
• Cell-Specific heat (Cp): 810.53 (K)
• Cell Height: 0.220 m
• Cell Mass: 1
• Cell rho Cp: 2040000
• Cell Thickness: 0.0048 m
• Cell Volume: 0.00039732
• Cell Width: 0.2150 m

 

SOC:    0

           0.100

           0.250

           0.500

           0.750

           0.900

           1

Initial Temperature: 293.15 (K)

Temperature Time: 7200s

Temperature Variance: 278.15 (K), 293.15 (K), 313.15 (K)

 

References:

1. The Weakest Cell Identification in Li-Ion battery packs using Discharging Technique Performance by Didi Istardi and Irwanto Zarma Putra
2. Review of Battery Cell Balancing Methodologies for Optimizing Battery Pack Performance in Electric Vehicles by Zachary Bosire Omariba, Lijun Zhang, and Dongbai Sun
3. Electric vehicle technology explained by James Larminine, John Lowry
4. https://www.batterypoweronline.com/blogs/why-proper-cell-balancing-is-necessary-in-battery-packs/