Why do we need to balance?
Due to the differences in battery use process and materials, as well as the differences in temperature, humidity and other environments during the actual use of the battery, there are differences in the SOC of the single cells in the battery pack. The difference in SOC is intuitively reflected in the different voltages of the batteries.
Assuming that the SOC of a battery in the battery pack is higher than that of other cells, this battery will be fully charged first during the charging process, causing the charging of other cells to stop before reaching the rated capacity; similarly, assuming that the SOC of a battery is lower than that of other cells, it will reach the discharge cut-off voltage first during the discharge process, causing other cells to have residual capacity that cannot be released;
Therefore, we can draw a conclusion that batteries are different.
1. Definition and significance of battery balancing
Definition: Battery balancing refers to the use of specific technologies and methods to make each battery cell in the battery pack reach a relatively consistent state in voltage, capacity and state, thereby improving the performance and life of the entire battery pack.
Significance: Improve battery pack performance: Through balancing, the performance degradation of the entire battery pack caused by the degradation of individual battery performance can be avoided.
Extend battery life: Balancing can reduce the voltage and capacity differences between battery cells, reduce the internal resistance of the battery, and thus extend the battery life.
Improve safety: Balancing can prevent overcharging or over-discharging of battery cells and reduce the risk of safety hazards such as thermal runaway.
2. Battery balancing method
The following mainly introduces the BMS balancing function. Through the BMS balancing function, the inconsistency between each battery cell can be reduced and the available capacity of the battery pack can be increased. At present, the main balancing methods used are passive balancing (energy dissipation balancing) and active balancing (non-energy dissipation balancing, energy transfer balancing).
There are two main methods for battery balancing: active balancing and passive balancing.
Active balancing: Active balancing is a technology that achieves voltage balancing between battery cells by energy transfer. It achieves more accurate balancing by transferring energy from a single cell with a higher capacity to a single cell with a lower capacity.
This transfer can be achieved through technologies such as capacitors and transformers. During the charging process, if a single cell reaches the upper limit of the operating voltage first, the BMS will identify the single cell with a lower capacity and transfer energy from the high-voltage battery to the low-voltage battery through the balancing circuit.
Advantages: high energy utilization, fast balancing speed, and can improve the overall performance of the battery pack.
Disadvantages: complex control algorithm and high production cost.
Passive balancing:
Principle: By consuming energy, the excess energy in high-voltage or high-capacity battery cells is dissipated in the form of heat energy, thereby reducing its voltage and capacity and achieving balance between battery cells.
Passive balancing (energy dissipation balancing) is achieved by shunting the parallel resistance of the single battery. The energy of the battery with a higher state of charge in the battery pack is consumed through the parallel resistance to achieve balance with other batteries in the group.
Typical passive balancing is implemented as follows: the voltage of each single battery is measured at the high or low end of the SOC. When the voltage of some single batteries exceeds the average voltage of the battery pack, the estimated balancing time is calculated based on the voltage difference or the single SOC difference, and then the parallel resistance of these high-energy batteries is turned on, so that part of their energy is consumed on the parallel resistance, and finally the balance of the entire battery group is achieved.
Advantages: simple implementation and low cost.
Disadvantages: large energy loss, slow balancing speed, and heat may be generated to cause the battery pack temperature to rise.
3. Battery balancing process
The battery balancing process usually includes the following steps:
Detection: Detect the voltage, current, temperature and other parameters of each battery cell in the battery pack through the BMS.
Judgment: Determine whether there are differences between battery cells and the degree of difference based on the detection results.
Perform balancing: Select the appropriate balancing method based on the judgment results and perform the balancing operation. For active balancing, it may be necessary to accurately calculate the amount of energy transfer through the control algorithm; for passive balancing, it may be necessary to control the on and off time of the switch to consume excess energy.
Monitoring: Continuously monitor the parameter changes of battery cells during the balancing process to ensure the effectiveness and safety of the balancing operation.
End balancing: When the difference between battery cells reaches the set threshold, the balancing operation is ended.
4. Precautions for battery balancing
Select the appropriate balancing method: Select the appropriate balancing method according to the actual situation and performance requirements of the battery pack.
Control the balancing speed and degree: Avoid damage to the battery cells or performance degradation due to excessive balancing speed or excessive degree.
Monitor battery parameters: Continuously monitor the changes in parameters such as voltage, current, and temperature of battery cells during the balancing process to ensure the safety and effectiveness of the balancing operation.
Preventing heat buildup: For passive balancing methods, measures need to be taken to prevent heat buildup that could cause the battery pack temperature to rise