Cell Balancing: A technique to improve battery performance and life cycle

It is difficult for any battery manufacturer to predict the exact capacity of a battery and its performance. It is largely dependent on the performance of cells of the battery.

High battery performance and good lifetime are very essential factors to ensure the growth and adoption of devices running on batteries. Quality performance and a good life cycle ensure the durability of the device and develop confidence among consumers.

Cell balancing is a core function of a battery management system (BMS) that can help in increasing battery capacity and performance. With this process, the voltage of cells is equalized at different state of charge (SoC) levels. Also, it is used to monitor temperature, charging, and different features that help in increasing the battery life.

A Lithium-ion battery pack contains a series of non-identical cells. Minor differences among them can be found in the SoC, self-discharge rate, impedance, capacity, and temperature. These differences are the reason behind their imbalances, capacity and performance.

When a battery is used, the voltage of cells starts dropping, and in some cells, it reaches below the lowest level, known as deep discharge. If the energy keeps flowing even at this level through these cells, they get damaged beyond repair and become unable to store energy. If the battery is charged at this level, the healthy cells get overcharged due to the inability of damaged cells to store energy and get damaged. This can be solved with the help of cell balancing techniques.

Why cell balancing

Lithium-ion battery cells are sensitive to overcharging, discharging and cell voltage imbalances. These batteries are susceptible to chemical damage with slight over-voltage or low-voltage during charging. It affects the overall performance of the battery. If the cells are not in a balanced condition, the battery will have a lower capacity and higher cut-off voltage. And, the imbalance in the battery will increase with the charging and discharging. Cell balancing ensures the constant flow of energy, helps in maintaining battery design capacity and ensures better performance.

How imbalance affects performance

In case of cell imbalance, premature cell degradation can occur due to the exposure of the battery to overvoltage. This degradation is auto-accelerating. When a cell with low-voltage will come in contact with a high-voltage cell during charge, it will result in faster degradation of the cell.

Li-ion batteries are high-energy concentration packs. If their internal heat generation rate exceeds the release rate, they may pose a safety threat. A reaction of active substances with electrolytes will start in overcharging or overheating conditions that may result in an explosion or fire.?

Types of cell balancing

Active cell balancing- In this balancing technique, current enters the battery and moves quickly towards the lowest charged cell and charges it to the level of the second-lowest. When all cells come to the same level of charge, then only current is directed equally into all cells. This is the charge shifting method of the balancing technique.

In another category of active cell balancing, energy is transferred from the highest voltage cells to the lowest voltage cells in the pack. This transfer of energy continues until the cells come to equilibrium. The process improves capacity usage, saves energy by transferring the excess energy to a lower cell and increases the life expectancy of cells.

However, there are some disadvantages of active cell balancing. In this method, approximately 10-20% of the energy is lost during the heat transfer among different cells. It is a huge amount of energy and the loss cannot be ignored. There is a one-way flow of energy from the high-voltage cell to the low-voltage cell in this process. Also, this technique is expensive and its algorithm makes it complex.

Passive cell balancing- In this balancing process, the excess energy of the highest voltage cell is released until the charge matches the lowest voltage cell. A resistor is attached to the highest cell and energy is released as heat, thereby making the battery pack equal in terms of charge.

As far as the advantages of this method are concerned, it is a low-cost method by which the SoC of each cell can be maintained. It is also helpful in correcting long-term mismatches. But there are some disadvantages too. Energy transmission efficiency in the process is low which causes a high amount of energy loss. If the cell capacity varies, an excess amount of energy is lost. Most notably, the process doesn’t improve the battery performance and the running time remains almost the same.

Cell balancing is necessary not only to increase the life cycle of batteries and their performance, but safety feature as well. While the battery performance will help the growth of industries that manufacture battery-supported devices, safety features will boost their adoption. As the safety concern has increased in recent times, the significance of cell balancing has increased manifold.