New energy vehicle BMS system structure and key technology analysis!

With the shortage of traditional energy sources and the objective requirements for environmental protection, new energy vehicles have become the development direction of future vehicles. In the rapid development of new energy vehicles, battery management system (BMS) plays a pivotal role as the core technology.

Why do new energy vehicles need BMS?

Lithium batteries usually have two appearances: cylindrical and square. The inside of the battery adopts a spiral wound structure, and a very fine and highly permeable polyethylene film separator is used to separate the positive and negative electrodes. The positive electrode includes lithium cobalt oxide (or lithium nickel cobalt manganate, lithium manganate, lithium iron phosphate, etc.). Graphite is mostly used as the negative electrode material. Titanate may be a better material in the future.

In layman's terms, lithium ions are constantly moving back and forth between the positive and negative poles through the electrolyte through the diaphragm during the charging and discharging process. The quality of lithium ions depends on the number of moving back and forth. No, if you control it well, you can charge it repeatedly without reducing the capacity, otherwise it will cause the battery capacity to drop permanently, or even explode.

In addition, for each battery cell and each batch of battery cell manufacturing process, the process problems and material unevenness make the activation degree and thickness of the active material of the battery plate, the microporosity, the connection strip, the separator, etc. exist very much. Small differences lead to incomplete consistency in internal structure and materials.

In actual use, the differences in electrolyte density, temperature and ventilation conditions, self-discharge degree, and charge-discharge process of each battery in the battery pack are affected. The voltage, internal resistance, capacity and other parameter values of batteries of the same type and specification are different. When used in electric vehicles, their performance indicators often fail to reach the original level of single batteries, which seriously affects their performance in electric vehicles. Application in the car.

The battery packs are composed of series and parallel connections. The series connection is like a line of people. If one of them walks slowly, it will affect the entire team, and the performance of one cell will affect the performance of the entire battery pack, which is serious. The result of the overall replacement.

If the lithium battery cell is too large, it is easy to produce high temperature during use, which is not conducive to safety. Large-capacity batteries must be connected in series and parallel to form a battery pack. However, it is impossible for each single battery to achieve the same performance, coupled with the impact of the use environment, will cause the difference in battery life, which greatly affects the life and performance of the entire battery pack.

Therefore, lithium batteries need BMS (Battery Management System) to strictly control the charging and discharging process to avoid overcharging, overdischarging, and overheating. Extend the service life of the battery pack and maximize its performance.

We know that the power battery of an electric vehicle is composed of thousands of small cells. The composition of the battery pack mainly includes cells, modules, electrical systems, thermal management systems, cabinets and BMS.

New energy vehicle battery pack

The battery pack is the core component of a new energy vehicle and provides driving power for the vehicle. It is mainly wrapped in a metal shell to form the main body of the battery pack. The battery core realizes the integration of the battery cell through the modular structure design, and includes the battery heat dissipation hardware. The quality of the heat dissipation system design is the prerequisite for the BMS to achieve good management, and this is also an important manifestation of the advanced technology of each manufacturer. Through thermal management design and simulation, the thermal management performance of the battery pack is optimized. The electrical components and wiring harness realize the safety protection and connection path of the control system to the battery; realize the management of the battery cell, as well as the communication and information exchange with the whole vehicle through BMS.

The battery management system (English: Battery Management System, abbreviated BMS) is a system for battery management. It usually has the function of measuring battery voltage to prevent or avoid abnormal conditions such as battery over-discharge, over-charge, and over-temperature. With the development of technology, many functions have been gradually added.

The battery management system is closely integrated with the power battery of the electric vehicle. The voltage, current, and temperature of the battery are detected in real time through sensors. At the same time, it also performs leakage detection, thermal management, battery balance management, alarm reminders, and calculation of remaining capacity (SOC) , Discharge power, report the battery deterioration degree (SOH) and remaining capacity (SOC) status, also use the algorithm to control the maximum output power according to the battery voltage, current and temperature to obtain the maximum mileage, and use the algorithm to control the charger for the best current For charging, real-time communication with on-board master controller, motor controller, energy control system, on-board display system, etc. is carried out through the CAN bus interface.

The functions of the battery management system (BMS) should include basic battery protection functions, battery balancing functions, battery reserve energy calculation functions, and network communication functions.

Three key technologies and development in BMS

SOC estimation

That is to accurately estimate the remaining battery power, ensure that the SOC is maintained within a reasonable range, and prevent damage to the battery due to overcharging or overdischarging, so as to predict how much energy is left in the energy storage battery of the hybrid electric vehicle or the state of charge of the energy storage battery at any time.

The estimation accuracy of SOC is high, and for the same amount of battery, it can have a higher cruising range. Therefore, high-precision SOC estimation can effectively reduce the required battery cost.

SOC is the transmission information calculated based on the monitored external characteristic information. While the SOC informs the owner of the current power level, it also allows the car to understand its own power level, prevent overcharge and overdischarge, improve balance consistency, and increase output power to reduce additional redundancy. The bottom layer of the system is calculated by complex algorithms to ensure the safe, continuous and stable operation of the car and improve safety. Therefore, it is very important to accurately estimate the SOC value, and its algorithm is one of the core competitiveness of related companies.

Balance control

To ensure the consistency of the parameters of the battery cells is to charge the cells in a balanced manner, so that each battery in the battery pack reaches a balanced and consistent state. Balance control is divided into active balance and passive balance.

Active balancing is to balance the capacity or voltage differences between battery cells during the process of charging, discharging, or placing the battery pack to eliminate various inconsistencies generated inside the battery. In this process, energy transfer is involved. There are generally two methods for energy transfer. One is to balance the energy of the high-energy single battery to the low-energy battery, and the other is to increase the voltage (capacity). The energy of the single battery is transferred to a backup battery, and then transferred from the backup battery to other batteries with lower voltage (capacity).

In the traditional energy-consuming BMS system, the balancing method is mainly based on passive balancing, and the parallel shunt energy consumption resistance of the single battery is adopted, and the balancing can only be done during the charging process. Its working principle is to find the difference between the series-connected single cells through the collection of voltages. Based on the set "upper threshold voltage" of the charging voltage, any single battery only needs to reach the "upper threshold voltage" when charging. When the upper threshold voltage" and the difference between the battery in the adjacent group are detected, that is, the battery with the highest single cell voltage in the battery pack is discharged through the energy consumption resistance of the single cell in parallel, and so on, until The single battery with the lowest voltage reaches the "upper threshold voltage" as a balance cycle.

Thermal management

Make the battery work in an appropriate temperature range and reduce the temperature difference between each battery module. Thermal management mainly includes determining the battery's optimal operating temperature range, battery thermal field calculation and temperature prediction, heat transfer medium selection, thermal management system heat dissipation structure design, and fan prediction stability selection.

The key technologies of the thermal management system are:

Determine the battery's best operating temperature range;

Battery thermal field calculation and temperature prediction;

Selection of heat transfer medium;

Thermal management system heat dissipation structure design;

Fan and temperature measurement point selection.