What is cell balancing and its function?

1. The concept of cell balancing
   1. Passive cell balancing
   2. Active cell balancing
2. Why do we need cell balancing?
   1. Small capacity cells directly affect the actual battery life
   2. The characteristics of lithium iron phosphate battery
3. Conditions for cell balancing
4. The role of battery equalization
5. Conclusion

For battery maintenance, in addition to routine inspections, such as battery temperature, airtightness, etc., the most important thing is cell balancing. Because the consistency of the battery is not good, it is the most important factor that causes the battery cruising range to decrease, and it is also the cause of most fire failures.

Therefore, we must pay attention to cell balancing, which is also an issue that the industry needs to pay attention to. In order to better maintain the battery and ensure the battery’s operating life, it is particularly important to understand what cell balancing is.

The concept of cell balancing

Cell balance means that the SOC of the batteries in the battery pack reaches a similar level, so that the capacity of the entire battery pack can be fully utilized. During the use of the battery, the battery cells in series can be activated and charged after equalization, so that the capacity of the battery cells can be maximized and the energy of each battery cell can be ensured.

The pressure difference is generally considered to be the performance of the consistency of the cells. BMS balances battery pack and hopes to reduce the pressure difference. To adjust the voltage of the battery cells, there is only one feasible way to adjust the SOC of the cells, and to adjust the pressure difference, it is necessary to adjust the SOC difference.

In addition, the capacity and internal resistance of the cell are the properties of the cell, and it is currently difficult to adjust by external methods, and the difference is more difficult to control. However, if a certain cell in battery pack is charged or discharged individually, the difference in the remaining power of the cells can be changed, and the purpose of balance adjustment of the cells can be achieved.

Therefore, the balance and maintenance of the consistency of the cells in battery pack can only be accomplished by charging or discharging one or several cells (adjusting the difference in remaining power), and there is currently no other ideal way. However, adjusting the remaining power difference does not necessarily mean reducing the remaining power difference. All current BMS equalization is achieved by charging and discharging individual cells in battery pack.

Passive cell balancing

Passive cell balancing simply means adding a resistive load. When it is detected that the voltage of a certain battery is too high, the resistance is turned on, so that the battery discharges the resistance and consumes energy to make it consistent with other batteries. The advantage is the cost. Low and simple to connect, the disadvantage is that energy is wasted and heat is generated.

Active cell balancing

Active cell balancing, simply put, is to transfer the energy of a high-voltage battery in battery pack to the lower-voltage battery in battery pack through energy conversion. The advantage of this method is less loss.

The disadvantage of active equalization is that the circuit is more complicated, and it is not easy to accurately realize the energy transfer. Many products on the market have clumsy conversion structures and high consumption, and this method has high requirements for voltage sampling, often resulting in excessive transfer, which actually leads to premature aging of the battery.

Why do we need cell balancing?

As the number of charge and discharge increases between the cells, the voltage difference gradually enlarges. During the production process of the cells, there will be slight differences between different cells. When used in a long-term fast charging environment, the voltage difference between the cells will gradually increase.

As the brain and hands of the power battery, the battery management system (BMS) collects various data, calculates the battery state of charge, health and other parameters, evaluates the performance of each cell, and takes emergency control in a timely manner when a dangerous condition occurs in the cell logic to protect the safety of the vehicle.

The BMS calculates the state of charge (SOC: ratio of charging capacity to rated capacity) by collecting various data. Thus, it can help the process of cell balancing.

Small capacity cells directly affect the actual battery life

The battery pack of battery-changing version of the electric vehicle is composed of multiple lithium iron phosphate cells connected in series. Here is a list of top 10 lithium iron phosphate power battery manufacturers in China for your reference. Due to the slight differences in the production process of different cells, the voltage of the cells will inevitably vary gradually under the condition of long-term use of high current.

According to the “short board effect”, the small-capacity cells directly affect the actual battery life. When using the battery, the small-capacity cells are the first to reach the cut-off voltage, which is likely to cause overcharge and overdischarge. This not only affects the user experience, but also has a certain impact on the life and safety of the battery.

In order to avoid the unbalanced trend of batteries due to differences in individual voltages and capacities, a battery management system (BMS) is required to start cell balancing.

The characteristics of lithium iron phosphate battery

If the vehicle is in the fast charging operation state for a long time, it will cause the pressure difference between battery pack cells to be too large, resulting in the reduction of the vehicle cruising range. Therefore, every time the battery-swapped vehicle goes to the swapping station to replace the battery, the swapping station will detect and maintain the battery voltage difference of the replaced battery to ensure that the battery voltage difference is within a reasonable range and prolong the service life of the battery.

So the meaning of cell balancing is:

1. Improve the consistency of single cells, so that each single cell maintains the same state during normal use;
2. Slow down battery deterioration and prolong battery life;
3. Prevent thermal runaway caused by long-term overcharge and overdischarge of individual batteries.

Conditions for cell balancing

The setting of the conditions for the equalization of battery pack BMS to be turned on mainly includes the following aspects:

Battery pack state of charge: most BMS’s equalization turn-on is set in the high SOC segment (related to the target being fully charged), including equalization after charging, equalization in the constant voltage charging phase, and when the cell voltage (SOC) is higher than a certain value balanced.

Current: Balance when there is no current or small current, so as to avoid the influence of current and internal resistance on the voltage.

State of charge and discharge: only in the charging phase and after the end of charging, the reason refers to the state of charge and current.

Differential pressure: Only when the pressure difference between the monomers is large enough, can the balance be turned on.

The role of battery equalization

Cell balancing generally has the functions of improving the consistency of the cells, reducing the pressure difference, making all the cells work in a reasonable range, increasing the capacity of battery pack, and prolonging the life of the battery pack.

Cell balancing can only change the difference in remaining capacity (or the alignment of capacity) and the cell pressure difference (the effect of cell pressure difference on different SOC stages is different, sometimes decreasing, sometimes increasing), which can have less impact.

In terms of battery pack capacity and single DOD and its distribution on SOC, the impact on life is not particularly clear (DOD of the smallest capacity cell may increase and remain at the maximum), and the single cell capacity and internal battery cannot be changed. resistance. Therefore, cell balancing may play a role in improving cell consistency, increasing capacity, and increasing battery life, but these effects cannot be subversive.

In addition, BMS manufacturers of top 10 BMS installed manufacturers in China have said that through active cell balancing during charging and discharging, the capacity of battery pack exceeds the minimum cell capacity and is almost close to the average cell capacity. This is feasible in principle, but for the energy transfer efficiency of active equalization, the calculation amount of the controller in the BMS, the number of actions of the switching device and the reliability requirements are relatively high.

Conclusion

• BMS equalization is only to discharge or charge and discharge a single cell or multiple cells;
• BMS balance can only change the remaining power in the three consistency parameters, which can have a positive impact on consistency;
• The current mainstream equalization goal is full power alignment;
• After the battery is balanced, the cell voltage difference may only decrease in some SOC ranges, and the cell voltage may increase in other SOC ranges;
• BMS balance can make the battery pack capacity equal to but not exceed the minimum cell capacity;
• BMS balance has a certain impact on the consistency and life of the cells in the battery pack, but it will not have a subversive effect unless under specific circumstances.

Related post