Effect of a Weak cell in a Battery Pack

AIM:

To understand how the weakest cell limits the usable capacity of the cell and how to resolve the same.

EXPLANATION:
The weakest cell in a battery pack can affect the overall performance of the pack.The pack that is taken into consideration is made up of 10 cells that are connected in series out of which 9 are in very good condition while the last cell is not. This can cause complications during charging and discharging.The factors that cause a cell to become unbalanced include change in capacity,state of charge, internal resistance and leakage of chemicals.

It is to be noticed that even though the abnormal cell can charge to a higher SoC, the performance of the other cells are affected as they can charge to around 80% only.

It can be observed that the abnormal cell charges at a higher voltage than the rest of the battery pack. Since the cell charges at a higher voltage than rest of the pack the temperature of the abnormal cell tends to be slightly higher than the rest.

The unbalanced cell cannot be completely discharged before charging while the normal cells cannot be charged without discharging thus affecting the usable amount of energy available.

Lithium ion batteries are very sensitive to over voltage, over current and temperature. Even a slight change beyond the maximum voltage can cause damage to the cell. Temperature also has got to be kept under control as high temperatures can also damage the cell in the long run.

The situation can be rectified by using balancing circuits. Cell balancing can classified into:

• Passive Balancing

In this method of cell balancing the overcharged cell is made to loose energy in the form of heat which means there is a wastage of energy. It is recommended that this kind of balancing is done while the cell is charging. The method is not very efficient.

• Active Balancing

In this method of balancing the overcharged cell is made to charge the other cells in the battery pack.There is no wastage of energy. It does require additional circuits like DC-DC converters.The system is complex and costly but it is much more efficient than passive balancing.

RESULT:

The effects that the weakest cell has on the battery pack and ways to rectify the same are understood.