The battery equalizer for electric vehicles should have the following characteristics: low standby loss, real-time equalization, high-speed equalization (supporting large current equalization), high-efficiency equalization (low self-heating), so that the battery pack can operate with low voltage difference. Only in this way, Only then can the consistency problem and rapid decay problem of the battery pack be completely solved.
4. How to solve the consistency problem in BMS
Research and experimental data on equal rate charge and discharge issues show that the difference in charge and discharge rates is the main reason for the expansion of battery differences and prominent consistency issues. Since balance control is a shortcoming of BMS, it is necessary to increase research and development efforts on balance control technology. , to overcome this shortcoming.
The endurance of an electric vehicle depends on the available power of the battery pack. For a standard battery pack, the available power is equal to the minimum capacity battery power. The available power refers to the actual discharge capacity, which represents the actual storage capacity of the battery pack. When the available power is significantly reduced, Under such circumstances, the charging capacity will also be seriously attenuated. In the case of severe attenuation, the battery life will inevitably decline rapidly. After one or two years of use, many electric vehicles have serious declines in actual cruising range and charging capacity. This is all due to excessive attenuation of the battery pack. Caused.
Through a large number of scientific research experiments and statistical analysis of measurement data, we came to the conclusion that an efficient, high-speed, transfer-type real-time battery equalizer is the best technology to solve the battery pack consistency problem. This equalization function can be integrated with the BMS. , can also be developed independently, and then linked and controlled through BMS to implement complementation.
There are two ways of this combination. One is that the two are directly integrated together and exist on the same PCB. The advantage is that the size and cost of the equipment are reduced and it is easy to manage. The disadvantage is that the complexity of the system increases and the probability of failure increases. . The second method is that the BMS and balancing equipment are designed independently, and the startup and shutdown of the balancing function are controlled by the BMS. The advantages of this method are: flexible system design and application, and improved system balancing efficiency and reliability. The disadvantage is that the equipment is independently designed and arranged. Yes, it takes up slightly more space.
5.Examples
The experimental battery pack is a set of 2 parallel and 4 series attenuated lithium battery packs. Due to severe attenuation, the actual remaining capacity and internal resistance are very different. Among them, the actual remaining capacity of the 3# battery is the smallest. Below the battery pack is the lithium battery equalizer experiment prototype. The discharge standard is: 2A constant current discharge, and the set end voltage of the entire discharge discharge is 11.8V. The comparison curve between ordinary discharge and balanced discharge is shown in Figure 2. In the figure, the ordinate represents the voltage in volts, and the abscissa represents the discharge. Time, the unit is seconds, the blue curve is the ordinary discharge curve, and the red curve is the equilibrium discharge curve.
It can be clearly seen from the discharge curve comparison chart that the discharge time, discharge capacity and discharge platform stability of balanced discharge are significantly better than ordinary discharge. Real-time measurement data shows. In the normal discharge mode, when the discharge is terminated, the 3# battery has entered a serious over-discharge state, and the remaining voltage has dropped to 1.4V. However, the other three batteries still have a lot of power that has not been released, and the remaining voltage is between 3.4 and 3.6V. time; in the balanced discharge mode, when reaching the discharge end voltage, the average voltage is between 2.94 and 2.97V. The worst 3# battery has not been discharged, and the other three batteries have also fully released their power, and almost all of their power has been released. . The results of this control experiment are universal among multi-string battery packs with obvious consistency.
6. Solving the consistency problem is equivalent to a revolution in BMS and electric vehicles
The practical problems of rapid attenuation of electric vehicle battery capacity and rapid reduction of cruising range have seriously affected people's enthusiasm for purchasing electric vehicles. For many electric vehicles, the actual cruising range has seriously declined after only two or three years of use, and the overall performance of the battery pack has even deteriorated. Reaching the standard that requires replacement is neither what manufacturers want to see nor what consumers want. The degradation of the battery pack is not caused by the degradation of the entire battery pack, but by the degradation of one or more strings. For this In this case, passive equilibrium can do nothing.
The attenuation of the battery pack is not caused by one or two charge and discharge cycles, but requires the accumulation of multiple charge and discharge cycles. Quantitative changes gradually lead to qualitative changes. If battery balancing intervention can be proactively carried out before signs of differences appear in the battery, it will be consistent. If sexual problems are nipped in the bud, the battery pack will always run in good condition and it will be difficult to live longer.
It makes sense that people pay great attention to the decay of electric vehicle batteries. Electric vehicles are expensive. If the battery pack is durable, it can indeed reduce travel costs. However, if the battery pack has a very short life, it is necessary to replace the battery once. The cost of the battery pack is close to 1/3-1/2 of the price of the entire vehicle. The price of replacing the battery pack for some highly subsidized electric vehicles is even higher than the price of the entire vehicle. The overall cost of use will be much greater than that of a fuel vehicle. Although electric vehicles Manufacturers have given a long warranty period, but during the warranty period, the problem of reduced vehicle mileage is very common, and breakdowns occur from time to time. If it were not for various restrictions, consumers would usually not make it their first choice for car purchase.
7. Outlook
The consistency problem is a common problem in electric vehicle battery packs. Its occurrence will cause many chain problems. Although the problem is very common, it is very difficult to solve. There are both technical reasons and cost factors. It is difficult to balance, high efficiency and transfer. The emergence of real-time battery balancing technology has brought hope and dawn for the consistent management of electric vehicles and power and energy storage battery packs. With the continuous optimization and improvement of this technology, it will become increasingly mature. By then, the mileage of electric vehicles will decline rapidly. , SOC jump and a series of problems will be easily solved.
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