Analysis of basic knowledge and industrialization of electric vehicle BMS (battery management system)

2021-11-25 14:26:36 0

1. BMS system overview

Three electric vehicles on electric vehicles: batteries, motors, and electronic control technology are the core technologies of electric vehicles. Because these three technologies are closely related to the cruising range and acceleration performance of electric vehicles. Just like the barrel principle, the shortcomings of any of these three technologies will directly affect the performance of the vehicle.

In the three-electric technology, the impact of batteries and motors on the performance of electric vehicles is more obvious. For example, the power of the motor affects the power performance of the vehicle, while the cruising range of a pure electric vehicle mainly depends on the storage capacity of the power battery. But what is the specific technical application of the electronic control technology in the same three-electric system in the electric vehicle, and why can it occupy a place in the three-electric system on the same level as the battery and the motor?

The core function of electronic control is the battery management system (Battery management system), referred to as BMS. Without this system, the charging and discharging and service life of the power battery will be greatly reduced. If the battery is regarded as a group of soldiers participating in the war, the BMS system is the staff and general of this group of soldiers, allowing electric vehicles to achieve twice the result with half the effort in practical applications. Effect.

Electric vehicles can control and manage batteries more efficiently through BMS. Each battery works within the range of operation to avoid battery overcharge, overdischarge and thermal runaway problems. The capacity of a single battery cell is relatively low, requiring a lot of battery cells to be integrated into a module, and a battery system contains multiple modules. Usually a battery system contains hundreds or even thousands of cells. BMS plays an important role in how to keep the battery cells working in the proper range.

Since 2013, electric vehicles, especially pure electric vehicles, have caught fire frequently, causing consumers to have safety concerns about electric vehicles. Compared with HEV, the battery system structure of PHEV and BEV is more complicated, and it has higher requirements on battery life and safety, and must be equipped with a more mature and reliable BMS. Therefore, the battery management system industry will benefit as the electric vehicle market expands.

2. The role of the BMS system

BMS is part of the battery pack. The battery pack is the core energy source of new energy vehicles and provides driving power for the vehicle. It mainly forms the main body of the battery pack through a metal shell envelope. The modular structure design realizes the integration of the battery cells, and optimizes the thermal management performance of the battery pack through thermal management design and simulation. The electrical components and wiring harness realize the safety protection and connection path of the battery by the control system; realize the management of the battery cells through BMS , And communication and information exchange with the vehicle.

The BMS function is to monitor the battery status, establish the battery status, protect the battery, report data, balance, etc. The main tasks of BMS in the vehicle are:

  • Protect cells and battery packs from damage;

  • Make the battery work in a suitable voltage and temperature range;

  • After keeping the battery running under suitable conditions, it can meet the needs of the whole vehicle

3. Working principle of BMS system

Regardless of the type of lithium-ion battery used in the vehicle, the power battery is composed of small battery cells in series and parallel to form a battery pack, and then the battery pack finally forms the power battery unit of the vehicle.

And what really plays the role of energy storage in the battery pack is each small battery cell in the battery pack, such as the 18650 lithium ion battery used by Tesla. The number represents the specification of each battery cell: the diameter is 18mm, the length is 65mm. The power battery unit of an 85kW‧h version of Tesla Model S is composed of nearly 7000 18650 lithium batteries.

There are so many battery cells in a car, and each small battery cell is manufactured separately. And due to the electrochemical characteristics of the battery, the consistency of the energy storage of the secondary lithium ion battery after leaving the factory is different. When charging, all batteries are charged from one charging port. How to ensure that each battery is fully charged and will not cause damage to the battery due to overcharging? This is the problem to be solved by the BMS system.

Under normal circumstances, the BMS system must determine how to manage the battery pack through two parts, namely the detection module and the control module.

The realization of the detection module is relatively simple. It mainly collects the parameter information of the battery during use through the sensor, such as temperature, voltage and current of each battery cell, and voltage and current of the battery pack. These data will play a vital role in the subsequent battery pack management. It can be said that without these battery status data as support, battery system management will be impossible.

According to the collected data, the BMS system will allocate how to charge the battery according to the actual situation of each battery cell, and which battery cell is fully charged can stop charging it. And in the process of use, the state of each battery is determined through state estimation, and the control is achieved through SOC (State Of Charge), SOP (State Of Power), SOH (State of Health), and balance and thermal management. Reasonable use of batteries.

This process is simple to say, but these are the essence of the BMS system, and they are also the technical difficulties that BMS manufacturers most hope to overcome.

The current battery management system has two management modes: active balancing and passive balancing. The two management modes have their own advantages and disadvantages. The methods used are generally to collect the voltage of the single battery, the series current, and the temperature and the voltage of the battery pack, and then transmit these signals to the computing module for processing and issuing instructions, and finally the entire processing The information command is transmitted to the automobile central control unit or the vehicle VMS system through the CAN communication system.

Domestic mainstream automotive BMS manufacturers have passive equalization technology, and most of them have active equalization technology reserves. On the configuration sheet given by the manufacturer, active equalization is an "optional" function. The passively balanced BMS has a relatively large installed capacity and occupies a higher share of the new energy vehicle market, which is much higher than the market share of the active balanced BMS. Domestic new energy vehicles are mainly low-end products. Taking into account the cost and configuration requirements, passive equilibrium is relatively easy to accept. With the high-end development of new energy vehicle products, the requirements for BMS are getting higher and higher, and active balancing technology will become the future development trend.

Due to the development of automobile electrification, the battery management system for passenger cars can start the battery in low voltage (12V&48V) and high voltage HEV battery (1kwh~1.5kwh), PHEV battery (4~18kwh) and BEV battery (20~85kwh) in the future. Wait to see it inside the battery system.

There is a big difference between a low-pressure system and a high-pressure system. The differences in battery systems between various car manufacturers and various application platforms are relatively large. Each company has its own style. It should be said that the evolution of the design concepts of various car manufacturers in the future will make the high-voltage battery system have a certain similarity.

4. BMS system configuration

There are three different configurations of battery management systems, which we can call centralized management systems, semi-distributed management systems, and distributed management systems.

  • Centralized management system (big BMS method)

This kind of management structure is to gather all the units for collecting single voltage & voltage backup and temperature on one BMS board, and the whole vehicle controller directly controls the relay control box. Most low-voltage HEVs have this structure. The advantage of this is that it is relatively simple and the cost is lower. Since the collection and backup are on the same board, the communication between them is also simplified. The shortcomings are of course obvious. The wiring harness for single sampling is relatively long, which leads to more complicated sampling wire design. Long and short wires cause extra voltage drop when equalizing; the wiring harness arrangement of the entire package is also more troublesome. The highest channel that BMS can support is also limited. This method is low in cost, but its applicability is also relatively poor. The performance cannot be guaranteed in some places, and it can only be applied to smaller battery packs.

  • Distributed management system (BMU+multiple CSC mode)

This is to separate the functions of the battery module (module and CSC one in one), and the entire system forms a CSC (cell management unit), BMU (battery management controller), S-Box relay controller and The vehicle controller is in the form of three layers and two networks. Typical applications are German I3, I8, E-Golf and Japanese IMIEV, Outlander and Model S.

The advantage is that the module assembly process can be simplified, the sampling harness is relatively easy to fix, the harness distance is uniform, and there is no problem of uneven voltage drop; as analyzed later, when the battery pack is large, this mode is very advantageous NS. The disadvantage is that the cost is high, an additional MCU is required, the independent CAN bus supports the integration of information of each module to be sent to the BMS, and the voltage information alignment design of the bus is relatively complicated. This solution has the highest system cost, but it is the most convenient to transplant. It is a typical example of high unit price and low development cost. The battery pack can be large or small.

  • Semi-distributed management system (BMU + small amount of large CSC mode)

To put it simply, this is a compromise between the two modes. It is mainly used for packages with peculiar module arrangement, typical applications such as Smart ED and Volt. This is a way to make the battery management subunit larger and collect more single channels. The advantage of this is that the entire system has fewer components, but it should be noted that the advantages of this method are not obvious. The main There are a lot of components and a higher degree of function concentration. It is the costly solution among the three solutions.

5. BMS cost situation

The battery management system (BMS) accounts for a relatively high cost in the battery pack, and the price is still relatively expensive. The price of BMS is related to the type, power, voltage, etc. of the battery cell. Generally speaking, without PACK, the price of BMS per car is 3000-20000 yuan. The battery capacity of passenger cars is large, the voltage level is high, and the BMS is more expensive. Passenger cars and special vehicles have lower voltage levels and relatively cheap prices. According to calculations, in 2016, the market size of new energy vehicle BMS will be about 7 billion yuan, and by 2020, it will exceed 15 billion yuan.

The price of the battery management system (BMS) is positively related to the number of cells in the battery pack, and the battery management system (BMS) accounts for 30% of the total cost of the battery pack. Considering that the circuit board and chip design of BMS is still in its infancy, and the complexity of later products, the speed of replacement and the scale of mass production will gradually increase, it is expected that the mainstream battery management system (BMS) price will show a slow downward trend , Which can be reduced to about 60% in 2020.

6. BMS market situation

With the development of the domestic economy, the development of the battery management system market is facing huge opportunities and challenges. In terms of market competition, the number of battery management system companies is increasing, and the market is facing an asymmetry between supply and demand. The battery management system industry has a strong demand for further reshuffle. However, there are still relatively few battery management system market segments. With a large room for development, information technology will become the core competitiveness.

The battery management system (BMS) market will expand simultaneously with lithium batteries in the power and energy storage fields. In 2013, the global battery management system (BMS) market output grew by more than 10%, and the growth rate from 2014 to 2016 will jump to 25- 35%. It is predicted that the battery management system (BMS) demand market will reach more than 36 billion yuan in 2020, which is 160 times the current market capacity.

At this stage, automakers, battery factories, and related car and electrical components factories are investing in battery management system (BMS) research and development in order to master the key technologies of the electric vehicle industry. Since car manufacturers are users of battery management systems, many car manufacturers Preference to use its own software for processing and control by special factory regulations to maintain operational flexibility.

The development of the battery management system (BMS) industry may be similar to that of lithium batteries. In order to master key technologies, automakers will work closely with long-term cooperative suppliers to develop products, making it difficult for new entrants to enter. Therefore, in the future, new entrants who want to cut into the supply chain of automakers, in addition to strengthening cooperation with related supply chains, can create customized solutions for their needs, so that they have the opportunity to seize the opportunity.