Views: 2 Author: Site Editor Publish Time: 2024-04-23 Origin: Site
power battery
At present, almost all electric vehicles use lithium-ion batteries as power batteries. According to the choice of polar materials, power batteries can be divided into three types: nickel-cobalt-manganese ternary battery NMC, nickel-cobalt-aluminum ternary battery NCA and lithium iron phosphate battery LFP, the comparison information of the three is as follows:
1. NMC
Nickel-cobalt-manganese ternary battery, or NCM for short, is the abbreviation of the three main constituent metal elements and is named according to different content ratios. There are different numbers depending on the nickel content, the most famous being NCM523 and NCM811.
Nickel manganese cobalt (NMC) batteries have good endurance and charging performance and are currently the most common battery cathode material in electric vehicles. The main advantage of NMC batteries is the high energy density, which can reach about 250 Wh/Kg, which means that it can accommodate more energy in the volume of each battery, thereby providing longer driving range and saving space. In addition, NMC batteries Less sensitive to cold temperatures, it can charge faster in cold climates. Mining of cobalt and nickel pollutes the environment and is expensive, with NMC battery packs costing more than LFP batteries.
Additionally, NMC has a short lifespan, estimated to only last 1000-2000 complete charge cycles (0 to 100%). But after 1000 cycles, the capacity may drop by about 40%. Most car brands recommend a daily charging limit of 80% for NMC battery packs to maintain good battery health.
2.NCA
Nickel-cobalt-aluminum ternary battery, referred to as NCA, is also the three abbreviation of its main elements. The ratio of the three elements is 8:1.5:0.5. Its energy density reaches the peak value of 350Wh/kg required by current national standards, its charge and discharge effect is also first-class, and its long-term battery life is stronger than NCM811.
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Nickel-cobalt-aluminum (NCA) cathode lithium-ion batteries are basically similar to NMC batteries. However, NCA swapped manganese for cheaper aluminum and reduced the amount of cobalt in the cathode. The aluminum contained in NCA batteries is an alkaline metal, which may cause side reactions to release a large amount of gas during battery operation. This will cause the battery to bulge. The higher the nickel content in NCA batteries, the worse the thermal stability.
Compared with NMC batteries, NCA batteries have higher energy density, replace environmentally unfriendly manganese with aluminum, and also increase the service life of the battery. However, the NCA battery pack still has a shorter life cycle and is more expensive than LFP batteries due to its use of cobalt and nickel.
3. Lifepo4
Lithium iron phosphate battery, referred to as LFP, is characterized by low energy density, only 200Wh/kg, and is not cold-resistant. When the outside temperature is below minus 10-20°C, the energy density of lithium iron phosphate batteries will decrease proportionally, resulting in shortened battery life.
In addition, Lifepo4 batteries have the characteristics of high temperature resistance, stable internal structure, and strong battery shape safety. Even if it reaches a maximum temperature of 700-800°C, it will not explode as easily as a ternary battery. Lithium iron phosphate (LFP) is emerging as a lower-cost, more sustainable battery type and is seen as key to lowering the price barrier for entry-level electric vehicles.
Unlike NMC and NCA, LFP cells do not contain nickel, cobalt and magnesium, making them cheaper to manufacture. A key advantage of LFP is its longer life cycle. LFP battery packs are capable of over 3000 full charge cycles, while NMC battery packs are only capable of 1000-2000 cycles. However, LFP batteries not only have lower energy density (about 70% lower than NMC), but also charge slower in low temperature environments and still rely on lithium, which is a limited resource, and the cost of lithium is rising due to high demand. .
2 Electric vehicle battery management system
Regardless of the type of power battery, a battery management system (commonly known as BMS) is required to manage the status of the battery. As an extremely important electronic component of electric vehicles, BMS can control and monitor battery pack voltage, temperature, and charge and discharge status. These are key parameters for the safe operation of electric vehicle batteries, thereby ensuring the performance and safety of electric vehicles.
Under abnormal conditions, lithium-ion batteries may fail due to various reasons such as overcharging/over-discharging, thermal runaway, aging and wear, and even cause fires. This requires a battery management system (BMS) to ensure that the electric vehicle battery is always in the best safety mode.
There are two types of battery management systems: centralized BMS and distributed BMS.
Centralized BMS architecture
Within the battery pack assembly there is a central BMS to which all battery packs are connected directly as shown in the image below. Centralized BMS are more compact and tend to be the most economical because there is only one BMS. However, centralized BMS also has disadvantages. Since all batteries are connected directly to the BMS, the BMS requires a large number of ports to connect all the battery packs. This means that large battery packs require a large number of cables, connectors, etc., which complicates troubleshooting and maintenance.
Additionally, inputs can easily be confused and misconnected, and connections can become loose, increasing the likelihood of failure. Another disadvantage is the lack of scalability and flexibility of the system architecture. In addition, the main controller is the core. Once the main controller fails, the operation of the entire system will be threatened. This is a big drawback.
Modular BMS topology
A modular BMS is characterized by multiple identical modules, each interconnected by a wiring harness, and the modules are connected by cables to a single battery or battery unit, similar to a centralized BMS. The BMS module provides data acquisition (single cell voltage, current, temperature) and communication interfaces with other BMS modules. Typically, one of the modules is designated as the master module, or a separate module serves as the master module. The main control module controls the entire battery pack and communicates with other parts of the system, while other modules simply record measurement data and transmit it to the main control module.
Troubleshooting and maintenance are easier due to the repetitive modular design, and scalability is straightforward to larger battery packs. The disadvantage is that the overall cost is slightly higher, and depending on the application, there may be cases where the interface is not fully used.
In contrast to a centralized BMS, the failure of one BMS module does not jeopardize the operation of the entire battery. A defective battery cell or battery pack can be removed from the system, reducing capacity but still maintaining operation.
Master/slave BMS
Conceptually similar to a module topology, but in this case the slave devices are more limited to forwarding measurement information, while the master device is dedicated to calculation and control as well as external communication. Therefore, although similar to the module type, the cost is lower because the functions of the slave devices tend to be simpler.
The maximum number of batteries in the master-slave BMS is set in advance. During system development, the number of batteries in use is fixed. If all input connectors are used, the number of batteries cannot be increased. Likewise, in some cases, some input connectors may be unused, resulting in a waste of resources.
Distributed BMS architecture
A distributed BMS integrates all electronic hardware on a control board that is mounted directly on the battery or module being monitored. This leaves only a few sensor and communication lines for wiring between adjacent BMS modules. As a result, each BMS is more independent and can compute and communicate as needed.
A distributed BMS provides both high reliability and robustness, as well as a cost-effective development process, significantly reducing the cost of the final battery pack. The advantage of a distributed BMS is scalability and flexibility compared to centralized and modular topologies. There is no maximum number of inputs specified and battery cells can be added or removed even after installation. No changes to the module's hardware or software are required. Additionally, the single point of failure of a centralized approach is avoided. Local control of each battery cell also improves safety. Another advantage is the high measurement accuracy. In addition, shorter connection lines allow for more accurate voltage measurements and better immunity to interference. The modular distributed structure facilitates maintenance or replacement of faulty components.
The disadvantage is the increased cost of the BMS, as each battery cell requires a separate BMS module and in most applications an additional main control module.
BMS functions
1. Battery operating status monitoring
The BMS acquires real-time data on basic battery parameters such as voltage, temperature and current. Using these metrics, the BMS can closely monitor important performance parameters such as state of charge (SoC), which represents the remaining charge to the EV battery's maximum capacity, and state of health (SoH), which shows the overall health of the battery pack. SoC monitoring helps electric vehicle users assess the driving range at their disposal and plan stops at charging stations without worrying about range anxiety. With SoH monitoring, manufacturers can help customers perform preventive maintenance to maintain healthy battery status and extend performance.
2.Thermal management
Electric vehicle batteries are very sensitive to temperature changes, which can affect their performance and service life. In this regard, BMS continuously monitors and controls battery temperature values to maintain optimal operation. For example, heating-cooling mechanisms can be used to keep the battery within an ideal temperature range to maximize its performance and service life.
3.Battery pack charge and discharge balance
BMS can control the battery in a dynamic balancing process to maintain consistent performance between battery packs. In the charging state, it balances the voltage between cells through two different methods:
Active balancing--transfers energy from overcharge to undercharge;
Passive balancing - excess energy is consumed through a dissipative bypass mechanism.
It ensures that no battery is overcharged or undercharged, which improves the efficiency and life of the electric vehicle battery pack.
4. Battery abnormal status protection
Because battery cells naturally age and lose their stability over time, the BMS monitors multiple parameters of the battery pack. It has built-in overvoltage, undervoltage, overcurrent, thermal management and external overcharge/overdischarge protection functions. When an abnormality occurs, the system will automatically execute predefined protection programs, such as optimizing low-voltage charging for batteries with degraded performance, and balancing the voltage drop caused by aging batteries to maintain optimal battery performance.
Current trends in BMS development
By employing advanced algorithms and machine learning technology, BMS can optimize battery performance based on battery usage patterns, environmental conditions and other dynamic scenarios.
2.OTA upgrade
Wireless communication protocols are increasingly used in conjunction with BMS, enabling them to update/upgrade the system via OTA.
3. Predictive maintenance algorithm
Electric vehicle battery management systems are being integrated with advanced predictive maintenance systems. These algorithms rely on real-time data to predict when battery components will need repair or replacement, thereby reducing customer maintenance costs and improving vehicle reliability.
We can see that with the continuous innovation of new technologies, the functions of BMS are also advancing by leaps and bounds, which will also promote electric vehicles to enter everyone's life more efficiently and safely.
