Lithium-ion battery module balancing systems can address the issue of inconsistent voltage within lithium-ion battery packs. Due to these inconsistencies, the performance of battery packs in terms of utilization, lifespan, and safety is far inferior to that of individual cells. Battery management systems with efficient balancing capabilities can significantly improve the overall performance of power lithium-ion battery packs, effectively extend their lifespan, and greatly reduce the overall vehicle's operating and maintenance costs. This provides technical support for the promotion of safe, efficient, and practical electric vehicles.
The lithium-ion battery module balancing system can handle the problem of inconsistent voltage in lithium-ion battery packs.
Inconsistency in lithium-ion batteries affects the lifespan of the battery pack and reduces the performance of the assembled battery assembly. Inconsistency in lithium-ion battery packs refers to differences in parameters such as capacity, voltage, internal resistance, and self-discharge rate among individual cells. This is caused by variations in the battery pack's assembly structure, operating conditions, operating environment, and battery management. Inconsistency in individual cells is a critical factor affecting battery pack performance; it can reduce the usable capacity of the battery pack and shorten its cycle life.
The consistency issue of lithium-ion battery packs refers to the differences in capacity, internal resistance, and state of charge (SOC) between individual cells connected in series within the pack. This directly determines the overall performance of the battery pack, thus affecting the power and driving range of electric vehicles. The main reasons for inconsistencies in lithium-ion battery packs are the initial differences between individual cells and the differences in the structure, operating conditions, and environment after the battery pack is assembled.
To mitigate performance degradation and lifespan reduction after battery assembly, the manufacturing process can be optimized to reduce initial variations in batteries; batteries with minimal inconsistencies can be screened before assembly; the impact of connection methods and structures on inconsistencies can be fully considered when assembling battery pack systems; and reasonable battery management, effective balancing, and thermal management can be implemented during use to reduce inconsistencies caused by different usage conditions.
Reasons for inconsistencies in lithium-ion battery packs
①During the manufacturing process, due to process issues and uneven material distribution, there are very slight differences in the activation degree and thickness of the active material of the battery plates, microporosity, connecting strips, separators, etc., which makes it impossible for the capacity, internal resistance, etc. of the same model of batteries produced in the same batch to be completely consistent.
② When installed in a vehicle, the differences in electrolyte density, temperature, ventilation conditions, self-discharge level, and charging/discharging process among the individual batteries in the lithium-ion battery pack can lead to inconsistencies in parameters such as battery voltage, internal resistance, and capacity.
What are the balancing technologies for lithium-ion batteries?
A critical issue in lithium-ion battery manufacturing is the battery pack assembly process. To address the "battery consistency problem," the industry widely employs balancing technology. Currently, the industry categorizes mainstream battery balancing technologies into three types: passive balancing, active balancing, and internal balancing.
1. Commonly used equalization methods include active equalization and passive equalization, which are implemented through circuits. Passive equalization involves connecting a high-voltage individual cell to a load resistor to discharge it. The circuit compares the voltage of the high-voltage individual cell with that of the other individual cells. If they are the same, the discharge stops. The disadvantages of passive equalization are that it consumes the energy of the individual cells, generates heat, and takes a long time to equalize.
The passive equalization method is characterized by its simple principle and ease of implementation. When the equalization current is small, the device cost is relatively low. However, it has two major problems: ① The resistor can dissipate the discharge, wasting energy and generating heat; ② Since the discharge resistor cannot be chosen too small, at the end of charging, according to the battery characteristics, the voltage of the small capacity battery is often higher. During static equalization, it is precisely the small capacity battery that is discharged, which increases the mutual difference between batteries.
2. Active balancing refers to transferring the energy of the higher-voltage battery to the lower-voltage battery instead of dissipating it through a resistor during charging and discharging, thereby achieving balanced charging and discharging of the lithium-ion battery pack. However, active balancing circuits are complex and cannot balance all batteries simultaneously, so their reliability needs to be improved.
The key features of active balancing are the use of a DC/CD bidirectional active balancing circuit, high balancing efficiency, balancing during charging, discharging and static processes, large balancing current and fast balancing speed. However, it also has two major problems: ① complex technology, high cost and difficult implementation; ② frequent switching of the balancing circuit, which causes great damage to the battery and affects the battery life.
3. Internal balancing utilizes a topology algorithm in the battery management system (bMS) to adjust the charging current and control the charging voltage during the charging process of series-connected batteries, thereby achieving a near-uniform charge level among the individual cells in the battery pack. The advantages of internal balancing technology include a simple algorithm, no energy loss, no additional charging or discharging processes, no impact on battery life, and no additional hardware. However, if the charge levels of the batteries differ significantly, it will take a considerable amount of time to achieve balancing.
Battery inconsistencies stem from internal resistance, capacity, and state of charge (SOC). Traditional consistency evaluation methods and balancing approaches, which use external voltage consistency as a control target, have not effectively improved the usable capacity of the battery pack. Therefore, they cannot mitigate the adverse effects of battery pack consistency issues on the use of assembled batteries.
Since DC internal resistance, polarization voltage, and maximum usable capacity are specific parameters of a battery and remain essentially unchanged during one or several charge-discharge cycles, battery pack balancing is primarily achieved by adjusting the state of charge (SOC) of each individual cell. Research has shown that using SOC as a reference point for balancing, with a relatively fixed balancing target, and fully utilizing the balancing time, can improve balancing efficiency and reduce the balancing current capacity.
From the perspective of the battery management system, individual cell parameters are tested during battery pack use, especially voltage distribution under dynamic and static conditions. This helps to understand the development pattern of inconsistencies among individual cells in the battery pack, allowing for timely adjustment or replacement of cells with extreme parameters to prevent the inconsistencies from increasing over time. Simultaneously, from an energy management and strategy perspective, a practical battery pack energy management and balancing system is introduced to formulate reasonable battery balancing strategies, proactively intervening to reduce battery inconsistencies.
