Multi cell Battery Pack

OBJECTIVE-1:

How does the weakest cell limit the usable capacity of the battery pack?

The weakest cell limits the usable capacity of the battery pack. This is due to the fact that any capacity mismatch between cells and SOC levels in a series connection of cells results in a reduction of overall pack capacity. Cell to cell variation more likely occurs between cells, due to more extreme mechanical stresses resulting in this problem.

Because of the weakest cell in the series, the battery pack cannot be fully charged and fully discharged.

Case-1:Discharging

In the case of discharging the 4th in series is fully discharged, so no more discharge can happen in the 4th cell. In the case of other cells, there are some charges left resulting in the wastage of energy.

Case-2:Charging

In the case of charging the 2nd in the series is fully charged, so no more charge can be added in the 2nd cell, also to the battery pack. In the case of other cells, they are not fully charged resulting in the battery cannot be fully utilized.

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OBJECTIVE-2:

What is the solution?

To overcome this problem, cell balancing can be done. There are two types of cell balancing 

1. Active cell balancing
2. Passive cell balancing

In active balancing, the higher energy cell dissipates the charge to the lower level cell. There are several methods like the inductor method, capacitor method, and dc to dc converter method, etc.

In passive balancing, the higher energy cell dissipates the energy in the resistor that is connected parallel to it. There are several methods like the fixed shunt method, switching shunt resistor method.

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