Hangzhou Guheng Energy Technology Co., Ltd.
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With the increasing awareness of environmental protection, new energy vehicles are becoming more and more popular as a green travel mode. Among them, the power battery pack is a key component of new energy vehicles, and its performance is directly related to the vehicle’s mileage and life. The power battery pack is composed of thousands of cells. During the use of new energy electric vehicles, as the number of charge and discharge cycles increases, the difference between each cell will gradually increase. The actual capacity of the battery pack depends on the capacity of the smallest cell, and the difference in cell capacity will directly affect the range.
Because the battery pack’s own balancing system is inefficient and ineffective, it is difficult to meet the needs of battery balancing. Therefore, maintenance personnel tend to use battery equalizers to maintain battery performance and increase the actual capacity of the battery pack.
2. Why do batteries need to be balanced?
The significance of battery balancing is to use power electronics technology to keep the voltage deviation of lithium-ion battery cells or battery packs within the expected range, thereby ensuring that each single cell remains in the same state during normal use to avoid overcharging or over-discharging.
The battery pack is composed of multiple single cells. Due to the differences in the battery manufacturing process, manufacturing materials, and working environment, there are inconsistencies in the single cells inside the battery pack. The inconsistency of power batteries refers to the inconsistency of capacity, resistance, and voltage. The most intuitive manifestation of inconsistency is the different battery voltages.
The charging and discharging of the battery pack is controlled by the BMS. If the SOC of a single cell in the battery pack is higher than that of other cells, this single cell will be fully charged first during the charging process, and the charging of other single cells will stop even if they have not reached the rated capacity.
Similarly, if the SOC of a battery cell is lower than that of other cells during the load process, it will reach the discharge cut-off voltage first during the discharge process, and other cells cannot be released even if they still have residual capacity. The charging and discharging of the cells in the battery pack follows the “barrel theory”, and the highest/lowest cut-off voltage is controlled by the cell that reaches the cut-off voltage first. Therefore, if you want to increase the effective capacity of the battery pack and extend the range of new energy vehicles, you must balance it.
3. Battery balancing method
Battery balancing is generally divided into two types: passive balancing and active balancing.
Active balancing method :
Active balancing technology achieves balancing by precisely controlling the charging and discharging process of each battery cell.
Passive balancing method :
Passive balancing technology uses external resistors or special discharge devices to dissipate excess energy.
4. Battery Balancing Scenarios
Some batteries on the market now have passive balancing function, that is, the balancing function of BMS itself. However, if the voltage difference of the battery cell is too large (0.3-0.5V), the effect of passive balancing is minimal, and maintenance equipment is needed to control the charge and discharge of each single battery to achieve balancing. Active balancing has the advantages of fast balancing speed, good balancing effect, and high energy utilization.
For different balancing scenarios, appropriate maintenance equipment should be used . A battery module consisting of 12 lithium iron phosphate batteries is used as an example to simulate the battery balancing scenario.
Scenario 1
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.295 | 3.296 | 3.295 | 3.293 | 3.294 | 3.187 | 3.293 | 3.294 | 3.296 | 3.295 | 3.294 | 3.292 |
The voltage of the No. 6 cell of this module is much lower than that of other cells. If a cell in a module has a large voltage difference compared to other cells, you can use a single cell charge and discharge maintenance instrument to charge the single cell to a voltage close to that of other cells (3.294V). The effect after maintenance is as follows:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.295 | 3.296 | 3.295 | 3.293 | 3.294 | 3.294 | 3.293 | 3.294 | 3.296 | 3.295 | 3.294 | 3.292 |
At this time, the voltage difference of each battery cell of the module is within 5mv, and the problem of large voltage difference is well solved. If you want a smaller voltage difference, you can use a balancing maintenance instrument for balancing maintenance.
Monomer charge and discharge maintenance instrument
Scenario 2
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.334 | 3.219 | 3.323 | 3.291 | 3.276 | 3.269 | 3.194 | 3.287 | 3.179 | 3.184 | 3.211 | 3.302 |
The minimum voltage of each cell of this module is 3.179V, the maximum voltage is 3.334V, and the voltage difference is 0.155V. At this time, if you use a single-cell charge and discharge maintenance instrument to charge the single cells one by one to a consistent voltage, the working efficiency is very low. You need to use a balancing maintenance instrument to put all the single cells at a consistent voltage (3.000V). The effect after maintenance is as follows:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.000 | 3.001 | 3.000 | 3.002 | 3.001 | 3.000 | 3.000 | 3.001 | 3.000 | 3.001 | 3.000 | 3.002 |
At this time, the voltage difference between each battery cell of the module is within 3mv, and the problem of large voltage difference is well solved.
Balance maintenance instrument
Scene 3
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.645 | 3.639 | 3.640 | 3.637 | 3.638 | 3.587 | 3.625 | 3.608 | 3.613 | 3.597 | 3.585 | 3.622 |
The lowest voltage of each cell in this module is 3.585V, the highest voltage is 3.647V, and the voltage difference is 0.062V. At this time, you need to use a balancing maintenance instrument to charge all the single cells to a consistent voltage (3.650V). The effect after maintenance is as follows:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.649 | 3.650 | 3.650 | 3.648 | 3.650 | 3.650 | 3.649 | 3.650 | 3.650 | 3.648 | 3.649 | 3.649 |
At this time, the voltage difference between each battery cell of the module is within 3mv, and the problem of large voltage difference is well solved.
Scene 4
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.334 | 3.119 | 3.423 | 3.291 | 3.476 | 3.169 | 3.294 | 3.387 | 3.279 | 3.184 | 3.211 | 3.302 |
The minimum voltage of each cell of this module is 3.119V, the maximum voltage is 3.476V, and the voltage difference is above 0.3V. You can use a balancing maintenance instrument . If you want to maintain more efficiently, you can use a multi-channel single-cell charge and discharge maintenance instrument. The speed of battery charge and discharge is related to the size of the charging current. The multi-channel single-cell charge and discharge maintenance instrument uses a large current of 20A to charge these twelve cells at the same time.
In order to facilitate the clamping of the battery cell, the battery cells that need maintenance are led out using the electrode lead-out tool , and they are charged to the same voltage (3.650V) at the same time. The effect after maintenance is as follows:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 3.649 | 3.650 | 3.650 | 3.648 | 3.650 | 3.650 | 3.649 | 3.650 | 3.650 | 3.648 | 3.649 | 3.649 |
At this time, the voltage difference between each battery cell of the module is within 3mv, and the problem of large voltage difference is well solved.
Multi-channel monomer charge and discharge maintenance instrument
Electrode lead-out tooling
The voltage and SOC curve of lithium iron phosphate battery has a very low curvature at 3.00V-3.65V, so the balanced maintenance is generally full charge and discharge (the target voltage for parallel charge maintenance is 3.65V, and the target voltage for parallel discharge maintenance is 3.00V). The voltage and SOC curve of ternary lithium battery are basically linear, so the target voltage value should be reasonably selected according to the battery cell voltage.
5. Battery balancing accuracy
When current passes through a single cell, the potential difference (electric potential difference) generated at both ends is called voltage drop . Simply put, voltage drop refers to the voltage level drop in the circuit when current passes through a single cell. The magnitude of this drop depends on the current passing through and the resistance of the cell.
Due to the existence of voltage drop, when charging a single battery, the final actual voltage will not reach the set target voltage. For example, if you want to charge all the cells in a module to 3.500V, the actual voltage usually cannot reach 3.500V.
In order to solve the problem of voltage drop, our equipment is equipped with a depolarization function. When the cut-off voltage is about to be reached, the current will be automatically reduced. This will reduce the impact of voltage drop on charge and discharge maintenance, making the actual voltage value after maintenance closer to the set target voltage value.
