Nowadays, battery manufacturers are vigorously promoting electric vehicles, and how to choose a power battery that is suitable for demand and has sufficient endurance, and has good price and performance safety aspects is particularly important. Relatively new nickel-metal hydride batteries are cheaper and safer than lithium batteries, and are more suitable for electric power systems.
There are no ideal competitors in electric power systems, so lithium batteries are still a good choice. Figure 1 shows five alternative batteries in which a lithium nickel manganese cobalt (NMC) battery, a lithium phosphate battery, and a lithium manganese battery have certain advantages. The most popular lithium-cobalt battery (not listed) in consumer electronics is the highest energy density "computer battery" that is used by Tesla and Smart Fortwo ED as a power source for electric vehicles, but because of its high Disadvantages such as price are not suitable for a large number of applications in general electric vehicles. People need to make compromises in terms of safety, specific energy, cost and high temperature performance.
Figure 1 Electric vehicle battery challenges, opportunities and prospects for 2020
Note: The more the shape extends outward on the axis, the better the battery.
Figure 1 compares the safety, specific energy (also known as capacity), specific power (capacity of large-rate charge and discharge of batteries), high and low temperature performance of batteries, life (including service life and storage life), and cost of five types of batteries. Performance of 6 indicators. However, there is no mention of charging time. If a suitable power outlet is available, all batteries in the electric vehicle power system can be quickly charged. Ultra-fast charging is an exception, but most users can still accept charging for several hours. . Figure 1 also does not list self-discharge performance - this is another battery characteristic that needs attention. In general, lithium batteries have a low self-discharge rate. If the battery is new, self-discharge is almost negligible. However, aging of the battery due to heat accelerates self-discharge of the battery and causes problems in battery management. Among the battery types that are available for electric vehicles, lithium phosphate batteries have higher self-discharge rates than other battery systems.
The performance maps of the five batteries in Figure 1 are not exactly the same, but they are similar, and no battery can stand out. For example, although NCA has a high capacity, it is not safe to the lithium phosphate battery, but the lithium phosphate battery has a very low capacity. Without a perfect battery, automakers can compensate for some of the performance shortcomings by designing battery combinations. Therefore, battery manufacturers generally design batteries according to the order requirements to enhance the important features required for the application. Below are a few important indicators of a battery in an electric powertrain.
figure 2
Safety: It is one of the most important indicators for selecting an electric vehicle battery. An accident in the media and public opinion amplification may make the public deny this electric car. Similar problems have occurred 100 years ago, such as steam engines and gasoline tank explosions. The main concern of people is the thermal runaway of the battery. The use of rugged accessories and well-designed safety circuits should basically solve this problem, but there may still be risks. The battery must be safe after being abused and used for a long time.
Lifetime: Reacts the number of cycles and storage life of the battery. The battery life of most electric vehicles guarantees that the car can travel 160 kilometers (about 100 miles) in one to 10 years. Attenuation of battery capacity caused by battery aging is a serious problem (especially in hot climates). And automakers lack the aging data of batteries under different conditions and climates. Therefore, manufacturers usually increase the size of the battery pack to compensate for the loss of battery capacity to ensure that the car can be used normally within a specified period of time.
High and low temperature performance of the battery: reflects the performance of the battery in the electric car in extremely hot summers and very low temperatures. Unlike an internal combustion engine that can operate normally over a wide temperature range, electric vehicle batteries have higher temperature requirements. When the vehicle's power source is only the battery, the energy required to regulate the temperature of the battery, the heating inside the car, and the temperature drop will also come from the battery.
Specific energy: The energy contained in the unit weight of the battery reflects the mileage of the car. It is clearly recognized that the energy output per unit weight of the battery is only 1% of the fossil fuel. Gasoline 1kg (1.4L 0.37 gallon) capacity is about 12kWh, while 1kg battery is only about 120Wh. But we must know that the motor efficiency is higher than 90%, and the internal combustion engine is only about 30%. Although the efficiency is so large, it is impossible to compare the energy storage capacity of the battery by 2-4 times compared with the internal combustion engine.
Specific power: related to the car's acceleration capability, most electric car batteries are not lower than the power. The electric motor has a better torque ratio than an internal combustion engine of the same horsepower.
Cost: This is the main disadvantage of electric vehicles. BCG predicts that it is difficult to limit the target cost of the battery to $250-400/KWh. And there are other factors that drive cost increases, such as specifying safety protection circuits, battery management status, ambient temperature control retention, and an 8-10 year warranty. These factors lead to the price of a battery equivalent to the price of a non-electric car, basically the price of an electric car has to be doubled.
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