Analysis of the BMS system structure and key technologies for new energy vehicles

With the shortage of traditional energy and the objective requirement of environmental protection, new energy vehicles have become the development direction of future vehicles. In recent years, the new energy vehicle industry has entered a rapid development track. In 2015, the global sales of new energy vehicles reached 500,000 units, of which 330,000 units were sold in China. In the rapid development of new energy vehicles, the battery management system (BMS) plays a vital role as a core technology.

Why do new energy vehicles need BMS?

Lithium batteries usually have two appearances: cylindrical and square. The battery uses a spiral winding structure inside, with a very fine and highly permeable polyethylene film isolation material between the positive and negative electrodes. The positive electrode includes lithium cobalt oxide (or lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron phosphate, etc.). The negative electrode material is mostly graphite. Titanate may be a better material in the future.

Internal structure of lithium battery

In simple terms, lithium ions move back and forth between the positive and negative electrodes through the electrolyte and the diaphragm during the charging and discharging process. The quality of lithium ions depends on the number of times they move back and forth. Too much or too little will not work. If it is well controlled, it can be charged repeatedly without reducing the capacity. Otherwise, the battery capacity will drop permanently or even explode.

In addition, during the manufacturing process of each battery cell and each batch of battery cells, process problems and material unevenness result in very slight differences in the activation degree and thickness of the active substances in the battery plates, microporosity, connecting strips, separators, etc., leading to incomplete consistency in the internal structure and materials.

In actual use, the differences in electrolyte density, temperature and ventilation conditions, self-discharge degree, and charge and discharge process of each battery in the battery pack cause differences in parameter values ​​of voltage, internal resistance, capacity, etc. for batteries of the same type and specification, so that when used in electric vehicles, their performance indicators often fail to reach the original level of single cells, seriously affecting their application in electric vehicles.

Battery packs are composed of series and parallel connections. The series connection is like a row of people lined up in a row. If one of them walks slowly, it will affect the entire team. The performance degradation of one battery cell will affect the performance of the entire battery pack, and serious problems will require the entire pack to be replaced.

If the lithium battery cell is too large, it will easily generate 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 have the same performance. In addition, the influence of the use environment will cause differences in battery life, which will greatly affect the life and performance of the entire battery pack.

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

Battery Pack and BMS for New Energy Vehicles

We know that electric vehicle power batteries are composed of thousands of small cells. The battery pack mainly consists of cells, modules, electrical systems, thermal management systems, boxes and BMS.

New energy vehicle battery pack

The battery pack is a core component of new energy vehicles, providing driving power for the entire vehicle. It is mainly wrapped in a metal shell to form the main body of the battery pack protection. The battery cell is integrated through a modular structural design, and includes the heat dissipation hardware of the battery cell. The quality of the heat dissipation system design is the prerequisite for BMS to achieve excellent management, which is also an important reflection of the technological advancement of each manufacturer. The thermal management performance of the battery pack is optimized through thermal management design and simulation. The electrical components and wiring harnesses realize the control system's safe protection of the battery and the connection path; the battery cell is managed through BMS, as well as communication and information exchange with the entire vehicle.

A complete battery pack system

Principle and system block diagram of BMS

A battery management system (BMS) is a system that manages batteries. It usually has the function of measuring battery voltage to prevent or avoid abnormal conditions such as 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. It uses sensors to detect the battery voltage, current, and temperature in real time. It also performs leakage detection, thermal management, battery balancing management, alarm reminders, calculates the remaining capacity (SOC), discharge power, reports the battery degradation degree (SOH) and remaining capacity (SOC) status, and uses algorithms to control the maximum output power according to the battery voltage, current, and temperature to obtain the maximum mileage, and uses algorithms to control the charger to charge with the optimal current. It communicates in real time with the on-board master controller, motor controller, energy control system, on-board display system, etc. through the CAN bus interface.

BMS system block diagram

BMS overall function

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

Three key technologies and developments in BMS

SOC estimation

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

The SOC estimation accuracy is high, and for the same amount of battery, a longer driving range can be achieved. 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 SOC informs the owner of the current power level, it also lets the car know its own power level, prevents overcharging and over-discharging, improves balance consistency, increases output power and reduces extra 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 parameter consistency of the battery cells, that is, to charge the single cells evenly so that each battery in the battery pack reaches a balanced and consistent state. Balancing control is divided into active balancing and passive balancing.

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

In traditional energy-consuming BMS systems, the balancing method is mainly passive balancing, which uses single-cell batteries in parallel to shunt energy-consuming resistors, and balancing can only be done during the charging process. Its working principle is to detect the difference between the series-connected single-cell batteries by collecting the voltage, and take the "upper threshold voltage" of the set charging voltage as the benchmark. As long as any single-cell battery reaches the "upper threshold voltage" first during charging and detects the difference with the batteries in the adjacent group, that is, the battery with the highest single-cell voltage in the battery group, discharges the current through the energy-consuming resistor connected in parallel to the single-cell battery, and so on, until the single-cell battery with the lowest voltage reaches the "upper threshold voltage" for a balancing cycle.

Active and passive balancing

Thermal Management

Make the battery work within the appropriate temperature range and reduce the temperature difference between each battery module. Thermal management mainly includes determining the optimal operating temperature range of the battery, battery thermal field calculation and temperature prediction, heat transfer medium selection, thermal management system heat dissipation structure design and fan prediction stable point selection.

The key technologies of thermal management system are:

Determine the optimal operating temperature range of the battery;

Battery thermal field calculation and temperature prediction;