Why balancing is important for lithium battery and what is the difference between active battery cell balancing and passive battery cell balan

When we design the battery packs, there are several basic principles:

· Once the first cell is fully charged, charging must stop.

· Once the first cell is dead, discharging must end.

· Weak cells age faster than strong cells.

· The weakest cells will eventually limit the available charge in the pack (the weakest link).

As we can see, without balancing, the voltage difference between the weakest and strongest cells will increase with each charge and discharge cycle. Eventually, one of the cells will always be close to the maximum voltage, while the other is close to the minimum voltage and hampering the pack's ability to charge and discharge.

The same applies to energy storage batteries, normally the battery life and number of cycles are of great concern to everyone, but the cell voltage difference during use is a very important influencing factors, especially for large energy storage products. Therefore, balancing the cell voltage difference is an important function that BMS needs to have.

Why is voltage balancing required for the battery energy storage system?

Different applications need lithium batteries with different voltages and capacities, which need to connect single lithium battery cells in series and in parallel to produce the battery packs with different voltages and capacities for use. During the use of batteries, due to the difference in production, aging, temperature effects and load conditions, the voltage and capacity of each battery are different. This difference will become larger and larger during the charging and discharging process of the battery pack, and the overall life of the battery pack will be shortened over time. Therefore, to extend the life of the battery pack, the inherent differences between the battery cells create demand for battery balancing.

Generally, the balancing function is added to the battery BMS.

The main principle of balancing is to consume or transfer the power of the pre-charged single battery cell during the charging and discharging process, so that other single battery cells that have not been fully charged or discharged can continue to charge, thereby ensuring that the battery pack as a whole can be charged more fully. The existing balancing solutions are mainly divided into passive balancing and active balancing.

What's the difference between passive balancing and active balancing?

Passive balancing mainly refers to energy dissipation balancing. The working principle is to connect a resistor in parallel to each battery cell. When a single battery cell has been fully charged in advance and the battery pack needs to be charged again, a resistor is connected to discharge it and consume the excess energy.

Advantages of passive balancing:

Simple: Passive balancing requires fewer additional electronic components, so it is relatively easy to design and deploy.

Lower cost: Passive balancing is cost-effective due to its simplicity compared to active balancing methods.

Disadvantages of passive balancing:

Energy loss: A large amount of energy is released in the form of heat, resulting in energy loss, which results in reduced overall efficiency.

Thermal management: In high-power applications, heat dissipation may require additional cooling systems.

Active balancing, which mainly refers to non-energy dissipative balancing, is based on the principle of transferring the energy in the pre-charged cells to the cells that are not yet fully charged in the form of inductance or capacitance, ensuring that each cell in the battery pack can be fully charged. Active balancing can also play a role during the battery discharge process because it does not consume battery capacity.

Compared with passive balancing with the extra energy consumed in the form of heat. Active balancing attempts to redistribute this extra energy to other cells in the battery pack that need to be charged. Therefore, while improving the overall efficiency of the battery system, it also reduces energy losses.

Advantages of active balancing:

Energy efficiency: Active balancing does not consume excess energy, but redistributes it to improve the overall efficiency.

Balancing Speed: Active balancing is usually fast to equalize the charge imbalance between cells.

Flexibility: Active balancing methods can be easily adapted to different applications and battery chemistries.

Disadvantages of active balancing:

Complexity: Additional electronic components and control techniques make active balancing design more complex and create more challenges to the reliability of the products.

Higher Cost: The higher the complexity, the higher the cost, both in components and design work.

To understand it easily, If the battery pack is compared to a wooden barrel, the batteries connected in series are the boards that make up the barrel. The battery with low power is the short board, and the battery with high power is the long board. The work of passive balancing is to "cut the long board without making up for the short board". The energy in the battery with high power is dissipated as heat, and the efficiency of electric energy use is low. Besides, because the electric energy is converted into heat dissipation, it brings a dilemma. That is, if the balancing current is large, the heat is large, and how to dissipate heat becomes a problem in the end; if the balancing current is small, then the power balancing effect in a large-capacity battery pack with a large power difference is very inefficient, and it takes a long time to achieve balance, Weighing the pros and cons, the current of passive balancing is generally at the level of 100 mA.

Due to the limitations of passive balancing, the concept of active balancing has been proposed and developed. Active balancing is to transfer the energy from the high-energy battery to the low-energy battery, which is equivalent to "cutting the long board to make up for the short board".

As we can see, both active and passive balancing methods have advantages and disadvantages, when choosing the balancing methods for a battery energy storage system, in order to make a wise choice between different balancing methods when developing BMS that requires balancing, there are several factors need to be considered including: System efficiency, Complexity and cost, Balancing speed, Energy storage capacity and type etc...

In conclusion

Passive balancing: For smaller battery packs or applications where higher efficiency is not a key factor, passive balancing with balancing current of about 100mA is recommended.

Active balancing: For large-scale energy storage systems that require efficiency and precision control, active balancing with balancing current of about 1A-10A is recommended.

If we want to produce good battery energy storage products, the most important part is to choose good cells, that's why Micergy insists on using grade A cells only, because both active and passive balancing are only auxiliary means.

About Micergy

Micergy started lithium battery manufacturing in 2009 and provides custom lithium battery products for different sectors, including energy storage batteries, lighting electric vehicle batteries, and custom batteries for golf carts, Marine, RV, and Forklifts. Besides, not limiting ourselves to the R&D of BMS to protect the battery and lengthen the lifespan, relying on the competitiveness of the battery, Micergy further embraces the IoT technology and developed comprehensive 'Energy + IoT' solutions for various sectors to solve the affordability problem of product with a high upfront cost.

Welcome to visit www.micergy.com or drop an email to [email protected] to learn more about Micergy custom lithium-ion battery products for the energy storage sector and e-mobility sectors.

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