Regarding battery pack balance, it refers to the process in which the physical parameters of the cells in a battery pack tend to the average of multiple cells. This physical parameter refers more to voltage or SOC, specifically manifested as the voltage between multiple cells. Alternatively, the consistency of SOC can be achieved through BMS management. But why do battery cells need to be balanced?
Is cell balance necessary:
Because there are inevitably inconsistencies in the manufacturing process of the battery cell, such as the area of the metal foil, the uniformity of the coating, the filling amount of the electrolyte, and the operation of the equipment, which cannot be completely consistent, this leads to differences in the DC internal resistance (DCR) and battery capacity of the battery cell.
Even if this difference is small if left unchecked, considering that the cells in a battery pack are usually connected in series by dozens or hundreds (to ensure a sufficiently high output voltage, the cells are usually connected in series). Ultimately, due to the "short board effect", the capacity and output voltage of the entire battery pack will be significantly reduced.
This is why it is necessary to balance the battery cells to achieve the same power output between each cell, not only to ensure the capacity of the battery pack but also to ensure the consistency of the output voltage.
When charging, as the cells are connected in series, cells with higher DCR will reach the charging cutoff voltage faster (4.2V for ternary lithium batteries). If charging is terminated, cells with smaller DCR will have smaller SOC because they have not reached the cut-off voltage, which means that failure to fully charge will result in capacity waste, which is extremely unsatisfactory for electric vehicles.
What are the types of battery cell balances?
The balance of battery cells is generally divided into active balance and passive balance.
Passive battery balancing:
Passive cell balancing refers to the use of resistors to consume energy from high-voltage or high-charging cells during the charging process, in order to reduce the gap between different cells. This is energy consumption. When the voltage of each individual cell is close to the same, continue charging. This process is repeated until the voltage of each individual cell is close to the same.
Active battery balancing:
Active cell balancing, utilizing energy storage devices, etc., transfers a portion of the energy from the cells with more energy to the cells with less energy, which is called energy transfer. During the balanced charging process, the control switch alternately connects the capacitor to two adjacent electric cores, accepts the charging of the high-voltage electric core, and puts the power into the Low voltage until the voltage of the two electric cores is the same.
Does BMS balance battery cells:
Under normal circumstances, the actual SOC upper limit of the battery pack will be set between 95% -97%, but it will be displayed as 100% on the user interface, which is designed for safety reasons. Cell unit balancing is to balance the voltage or SOC inconsistency between cell units, and specific means can be used to adjust the charging inconsistency caused by physical property inconsistency between cell units.
For example, when the SOC of one cell reaches 97%, the SOC of the other cell is only 90%, so BMS needs to monitor the voltage or SOC data of each cell in real time for precise control and set the SOC to 97%. The electrical energy inside the cell is appropriately released or directly charged into the cell with a SOC of 90%. These are the two methods of passive cell balancing and active cell balancing.
Although cell balance can lead to an increase in charging time, after achieving consistency between cells, the working state of each cell tends to be consistent, and the capacity of the entire battery pack will not be damaged due to premature decay of the lifespan of individual cells. Reduce. Due to the balance of the battery cells, when each cell reaches 97% SOC, the entire battery pack is fully charged, thus achieving the optimal capacity for a single charge of the battery pack.
In the same way, it is also necessary to balance the battery cells during the discharge process. The purpose is to maintain consistency in the voltage or SOC of each cell, avoiding situations where one cell is excessively discharged and the other cell has not yet reached the discharge cut-off voltage.
That is to say, a battery pack with a cell balance function can achieve the optimal SOC range during the charging and discharging process, ensuring that each cell can operate within the SOC range of 5% -97%. Instead of the parameters of the cell with the highest internal resistance and the lowest capacity, the available SOC range of the entire battery pack is determined.
Therefore, it can be imagined how much data collection and processing power BMS needs to complete the seemingly simple task of battery cell balancing. If you want to know what BMS manufacturers are, you can refer to the top ten Battery management system suppliers.
How is cell balance completed:
It is generally believed that pressure difference is a manifestation of cell consistency. BMS balances the battery pack in the hope of reducing pressure difference; There is only one feasible method to adjust the voltage of the battery cell unit, which is to adjust the SOC of the battery cell. To adjust the voltage difference, it is necessary to adjust the SOC difference (charging or discharging a certain string separately).
In addition, if the characterization parameters of the first part (capacity difference, internal resistance difference, and residual power difference) are considered, the capacity and internal resistance of the cell are the characteristics of the cell itself, and it is currently difficult to adjust through external methods, making it difficult to control; however, if a single cell in the battery pack is charged or discharged separately, the remaining power difference of the cells can be changed, thereby achieving the goal of adjusting cell consistency.
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