Power batteries for electric cars are one of the key factors affecting the development of electric cars. At present, the power batteries of new energy cars mostly use lithium-ion batteries, which are small in size, light in weight and high in working voltage (about nickel-hydrogen batteries, nickel-cadmium batteries). 3 times), long life, many cycles, no memory effect, low self-discharge rate, no pollution, and good safety performance. Lithium-ion batteries mainly include manganese lithium-ion batteries and iron phosphate lithium-ion batteries. The latter has a longer life and higher safety performance.
Different types of power batteries have different charging characteristics, and the best charging rate varies between 0.2-2.0C. In the case of the same rated voltage of the battery system, the maximum charging voltage also reflects a certain difference due to the difference in the type and structure of the battery. For different types of batteries, charging methods and charging control strategies are also different, and different charging methods should be used according to the different characteristics of the batteries.
Electric cars with different operating modes have different requirements for charging time, and different charging times require different charging methods to meet. In the case that electric cars do not require high charging time, regular charging can be used during the outage time to extend the driving range of electric cars; when the charging time is more urgent, fast charging or battery packs are required Quick replacement and timely replenishment of electric energy for electric cars.
The charging and discharging efficiency of the power battery is affected by the environmental conditions of the charging station, especially by the environmental temperature. At normal temperature, the battery charge acceptance ability is strong, with the decrease of the ambient temperature, its charge acceptance ability gradually decreases. Therefore, as the ambient temperature decreases, the power demand of the charging station will increase. Therefore, when constructing a charging station, it is necessary to ensure that its environment is not affected by man-made temperature conditions as much as possible.
Different types of power batteries have different charging characteristics, which are mainly manifested in parameters such as maximum acceptable charging current, maximum charging voltage, charge/discharge rate, charge/discharge termination voltage, cycle life, and charge retention capability. The higher the charging current and the higher the charging voltage, the greater the power demand for the charger. The charging characteristics of lithium-ion batteries are mainly affected by the charging current, the state of health (SOH), the state of charge (SOC) of the battery, and the number of cycles.
1. Lead-acid battery
Lead-acid batteries are the most widely used batteries, as shown in Figure 1. Lead-acid batteries use lead oxide as the positive plate, sponge lead as the negative plate, and aqueous sulfuric acid as the electrolyte. The charging and discharging process relies on the chemical reaction between the active material on the electrode plate and the electrolyte. The main advantages of lead-acid batteries are stable voltage and low price. At the same time, they also have problems such as low specific energy, short service life and frequent daily maintenance. Lead-acid batteries are widely used in domestic low-speed electric cars. In addition to the advantages mentioned above, lead-acid batteries are also cheaper than other batteries. Lead-acid batteries have 2V, 4V, 6V, 8V, 12V and 24V series. The discharge time of a lead-acid battery can be roughly calculated using the following formula:
Discharge duration=rated capacity×discharge capacity rate×[1+temperature coefficient×(ambient temperature-25)]/discharge current——(1)
Lead-acid batteries are inexpensive and have low endurance. Therefore, it is not very suitable for electric cars to be completely powered by lead-acid batteries.
2. Lithium iron phosphate battery
Lithium iron phosphate battery is shown in Figure 2. It is a lithium ion secondary battery and is mainly used as a power battery. The discharge efficiency is high. In the case of rate discharge, the charging and discharging efficiency can reach more than 90% (the lead-acid battery is about 80%). %). Among all kinds of batteries, the safety of lithium iron phosphate batteries is also higher than other batteries. The theoretical life span can reach 7 to 8 years, and the actual life span is 3 to 5 years. The performance-price ratio is theoretically 4 times that of lead-acid batteries. Times more.
The disadvantage of lithium iron phosphate batteries is that the price is higher than other types of batteries, and the battery capacity is small, the mileage is short, and basically cannot be recycled after being scrapped, and has no recyclable value. The application of lithium iron phosphate batteries in electric cars will increase the overall cost, and the batteries cannot be recycled, resulting in waste and consumption of resources.
3. Lithium iron manganese phosphate battery
BYD’s latest research on iron manganese phosphate lithium ion battery is shown in Figure 3. It is an improved type under the lithium iron phosphate route. It adds manganese to the lithium iron phosphate material, which is called lithium iron manganese phosphate. The energy density of this battery has reached the density of ternary materials, breaking through the energy density limit of traditional lithium iron phosphate batteries, and is better than ordinary lithium iron phosphate in cost control, and has been applied to BYD electric cars. In terms of endurance, it is more durable than current lithium iron phosphate batteries.
4. Lithium Cobalt Acid Battery
The battery of the TESLA electric car adopts the NCA series (nickel-cobalt-aluminum system) 18650 cobalt-acid lithium ion battery provided by Panasonic, and the single-ion battery capacity is 3100mAh. 18650 cobalt oxide lithium ion batteries, these 18650 cobalt oxide lithium ion batteries are equally distributed by bricks and pieces one by one, and finally form an 85kwh 18650 cobalt oxide lithium ion battery pack, which is located on the underbody of the car body, as shown in Figure 4.
The lithium cobalt oxide battery has stable structure, high capacity ratio, outstanding comprehensive performance, poor safety, and very high cost. It is mainly used for small and medium-sized batteries with a nominal voltage of 3.7V. TESIA combines lithium cobalt oxide batteries together, which is safe Sex has become a very important issue. TESLA distributes the safety device in the lithium cobalt oxide battery pack to each 18650 cobalt oxide lithium ion battery. Each 18650 cobalt oxide lithium ion battery has a fuse at both ends. When the lithium cobalt oxide battery is overheated or current When it is too large, the fuse will be cut off, so as to avoid an abnormal condition (overheating or excessive current) of a lithium cobalt oxide battery from affecting the entire lithium cobalt oxide battery pack.
From this point of view, although the lithium cobalt oxide battery has its own defects, its safety can be basically guaranteed through the packaging of TESLA. Obviously, such a solution is very suitable for application in pure electric cars. The cruising range and total capacity of lithium cobalt oxide battery packs are higher than those of other batteries. If the safety of lithium ion batteries is further improved, it can be popularized and applied in electric cars.