Battery management system solution for lithium battery charge and discharge balance integration
With the development of battery technology, lithium-ion power lithium batteries have become an ideal power source for a new generation of electric vehicles due to their high energy density, good power characteristics, and long life. In order to meet the energy and power requirements of electric vehicles, power lithium battery packs are often formed by single cells in series and parallel for vehicles. Due to the inconsistency of battery performance, the performance of grouped batteries is far less than that of single batteries in terms of utilization, service life, and safety. The battery management system with efficient balanced management function can greatly improve the overall performance of the power lithium battery pack, effectively prolong the service life of the battery pack, and greatly reduce the use and maintenance costs of the entire vehicle. Provide technical support for the promotion of safe, efficient and practical electric vehicles.
At present, the battery management system has made some progress in battery online monitoring, state evaluation, charging management, data communication, control strategy, etc., but the research on high-efficiency balancing technology of battery packs is still in its infancy. The research of equalization technology is divided into two aspects: equalization control strategy, topology design of equalization circuit and hardware implementation. The practical application of online balancing strategy takes the external voltage of the battery as the control object. Since the external voltage cannot effectively reflect the actual internal difference of the battery, the balancing efficiency and effect are not ideal; in terms of hardware, the resistance bypass discharge balance is mainly used, and the balancing current is affected by It is difficult to increase the calorific value.
Battery pack consistency and balance strategy:
The consistency of the battery pack refers to the differences in capacity, internal resistance, SOC, etc. between the single cells connected in series in the battery pack, which directly determines the performance of the entire battery pack, thereby affecting the power of electric vehicles. and cruising range.
The important reasons for the inconsistency of batteries include: inconsistencies in the production process, differences in battery production processes and materials, resulting in differences between batteries in terms of initial capacity, DC internal resistance, self-discharge phenomenon, and charge-discharge efficiency. Differences in performance parameters accumulate during use; the consistency of initial battery performance parameters is amplified during use; differences in battery use environments have a greater impact on the consistency of battery packs.
Since the inconsistency of the battery comes from the internal resistance, capacity and SOC of the battery, and the voltage consistency other than the traditional consistency evaluation method and the equalization method is used as the control target, it does not effectively improve the usable capacity of the battery pack, so it cannot improve the consistency of the battery pack. The adverse effect of the sexual problem on the use of the battery pack.
Since the DC internal resistance, polarization voltage, and maximum usable capacity are specific parameters of the battery, they basically do not change during one or several continuous charging and discharging processes, so the balance of the battery pack is mainly achieved by adjusting the SOC of each single cell. . After research, the SOC is taken as the reference object of the balance, the balance object is relatively fixed, the balance time is fully utilized, and the balance utilization rate is improved to reduce the balance current capacity.
Taking the SOC of the battery as the control object, the SOC difference between the batteries is narrowed by charging and discharging the single battery. The first step is to determine the balancing target. Usually, in order to improve the efficiency of balancing and give full play to the advantages of charge-discharge balancing, the target is set as the average state of charge (SOC) of the battery pack. The balance control band (dSOC) is also set to prevent the fluctuation of the balance, and the cells with high SOC are discharged and balanced, and vice versa. Then, the difference between the SOCs of each battery (ASOC) and the rated capacity can be used to calculate the equilibrium capacity required by each battery, and the equilibrium can be completed by measuring the capacity. The balancing strategy judgment process is shown in Figure 1. The SOC-based equalization strategy can not only achieve the purpose of improving the capacity utilization rate of the battery pack, but also solve the problem of the impact of the consistency problem on the status recognition of the battery pack. Since the SOC of each battery tends to be the same after equalization, the SOC of the battery pack is equal to the SOC of the single battery with the worst capacity. Correcting the SOC in this way can greatly reduce the complexity of estimating the SOC of the battery pack.
Battery pack consistency and balance strategy:
The battery management system is mainly composed of a single battery voltage detection module, an equalization module, a battery temperature detection module, a current detection module, a CAN communication interface, an MCU and other peripheral circuit units. , current acquisition, CAN communication and other functions. The battery management system conducts real-time online analysis of the collected battery voltage, temperature and current data to evaluate the current working status of the battery pack, and informs the vehicle, charger or other controllers through the CAN bus to achieve optimal driving and charging management , and in the charging phase, the battery pack is charged and discharged online equalization according to the equalization algorithm.
The charge equalization circuit transfers the energy of the whole group of batteries to the single battery through the converter, adopts a multi-level transformation structure, and uses a controllable switch to connect the single battery with a lower SOC to the charge equalization circuit to implement charge equalization. In order to be able to measure the equalized capacity, the charge equalization converter operates in constant current mode. The discharge equalization circuit realizes SOC adjustment by transferring or consuming the battery energy with high SOC.
The SOC-based balancing strategy is to achieve battery pack balance by reducing the SOC difference between batteries, thereby improving battery pack capacity utilization.
Aiming at the technical bottleneck of the current battery management system in the battery online balance, this paper proposes a battery balance control strategy based on SOC and a battery management system solution that integrates charge and discharge balance. This method can effectively improve the consistency of the battery pack and improve the battery The capacity utilization rate of the battery pack is improved, and the overall performance of the battery pack is improved. Future research can optimize the size of the battery‘s equalization control band, and do a detailed study on the relationship between the shrinkage of the battery control band and the fluctuation of the battery equalization, so as to optimize the selection of parameters and further improve the equalization performance.
