The power battery system is the energy source of the whole vehicle and provides driving electric energy for the whole vehicle. The volume, shape and technical parameters of battery system affect the driving performance of electric vehicle. It is one of the most important subsystems of electric vehicle. The parameter matching of electric vehicle power battery system mainly includes the selection of battery type, battery pack voltage and energy.
Electric vehicles require the power battery system to have high specific energy and specific power to meet the requirements of vehicle driving range and power performance. At the same time, it also hopes that the power battery system has the charge discharge cycle life equivalent to the service life of the vehicle, with high efficiency, good performance price ratio and maintenance free characteristics. Specific requirements are as follows:
(1) High energy density to improve operation efficiency and mileage.
(2) High output power density to meet driving performance requirements.
(3) The operating temperature range in summer and winter is wide.
(4) Long cycle life to ensure the service life and total mileage of the battery.
(5) No memory effect, so as to meet the charging needs when the vehicle is often in the incomplete discharge state.
(6) The self discharge rate is small, which can meet the needs of vehicle shelving for a long time.
In addition, the battery is required to be safe, reliable and recyclable. At present, lithium-ion battery has attracted more and more attention because of its superior performance such as high energy and fast charge and discharge speed. It is a product with the best market prospect at present.
The voltage level of the power battery system shall be consistent with the voltage level of the motor and meet the requirements of motor voltage change. Due to the power consumption of accessories such as electric air conditioner, electric vacuum pump and electric power steering pump, the total voltage of battery pack shall be greater than the rated voltage of motor.
There are generally two types of power batteries: energy type and power type. In order to meet the driving of pure electric vehicles, energy type batteries are required. When matching, the energy requirements are mainly checked, that is, the batteries should have large capacity to increase the driving range of vehicles. The battery capacity is directly proportional to its power. The larger the capacity, the greater the output power, so its output power can meet the requirements of the whole vehicle power system. Therefore, the battery capacity is mainly determined according to its driving range, and the determined battery capacity must meet the standards of existing products in the market, and the design is carried out through repeated verification of existing products.
- Advantages and disadvantages of major power batteries
At present, the available power batteries for electric vehicles mainly include lead-acid batteries, nickel cadmium batteries, nickel hydrogen batteries and lithium-ion batteries. Among them, lead-acid battery has the advantages of good high current discharge performance, good safety, low price and rich material resources, but its mass specific energy and volume specific energy are small, which seriously restricts its application in the field of electric vehicles. Nickel cadmium battery has good charge discharge rate characteristics, but it has some problems, such as memory effect, containing heavy metals and environmental pollution. Although Ni MH battery has the advantages of large charge discharge ratio, no hidden danger of environmental pollution and no memory effect, Ni MH battery should not be used in parallel and the working voltage is low. Lithium battery has the advantages of high working voltage, large specific energy and specific volume, low self discharge rate, no memory effect, high charge discharge efficiency, long cycle life and no pollution. Lithium battery has high energy density; The average voltage of the single battery is 3.6V, which reduces the probability of battery failure and greatly prolongs the service life of the battery pack. Since its launch, lithium battery has been widely recognized by people for its excellent performance and great development in technology. It is considered to be an ideal power source for the new generation of electric vehicles.
- Main performance parameters of power battery
The main performance index of power battery is the key parameter to evaluate the performance of power battery, and it is also the primary basis for the selection of power battery of electric vehicle. The main performance indexes of general power battery include:
(1) Mass specific power (w / kg): the mass specific power of the battery represents the power that can be provided by the battery per kilogram of mass. Its size determines the maximum power that the battery can output, marks the acceleration performance and maximum speed of the vehicle, and has a direct impact on the power performance of electric vehicles. In pure electric vehicles, the specific power of batteries is the most critical factor.
(2) Mass specific energy (w · H / kg): represents how much energy can be provided by the battery per kilogram of mass, which can indicate the driving ability of pure electric vehicles.
(3) Cycle life: the work of the power battery is a cycle process of continuous power off, discharge, charging and discharge. Every time it is charged and discharged, the chemical substances in the power battery will have a reversible chemical reaction. With the increase of charging and discharging times, the chemically active substances in the power battery will age, gradually weaken its chemical function, reduce the charging and discharging efficiency of the power battery, and finally lose its charging and discharging function partially or completely. The number of working cycles of power battery is an important index to measure the service life of power battery. It marks the economy and convenience of use.
(4) Price cost: the cost of the battery is related to the new technical content, materials, manufacturing method and production scale of the battery, which represents the economy of use. At present, the newly developed power battery with high specific power has high cost.
- Parameter matching of power battery
The parameter matching of electric vehicle power battery system mainly includes the selection of battery type, battery pack voltage and energy.
(1) Selection of power battery type. According to the previous analysis of the characteristics of each type of battery and the matching principle of power battery, select the appropriate power battery.
(2) Selection of battery pack voltage. When matching the voltage of the power battery pack, the voltage level of the power battery pack is required to be consistent with the voltage level of the driving motor and meet the requirements of the voltage change of the driving motor. Moreover, due to the power consumption of accessories such as electric air conditioner, electric vacuum pump and electric power steering pump on the electric vehicle, the total voltage of the battery pack should be greater than the rated voltage of the motor.
(3) Selection of battery pack energy. Battery energy index is the most important parameter to reflect the value of battery. For pure electric vehicles, the energy of the battery pack is determined by the driving range of the electric vehicle. The driving range of electric vehicle can be measured by working condition method or constant speed method. The former method refers to putting the electric vehicle in the working condition test environment and passing the driving test at the working condition driving speed. The total driving distance during the test is the driving range; The latter method refers to driving the electric vehicle on the road at a constant speed. Due to the different driving range of the electric vehicle at different speeds, according to the relationship between the driving range and the vehicle speed, there is a large driving range when the vehicle speed is within the range of 30km / H ~ 63km / h. generally, when calibrating the driving range of the electric vehicle, the speed is taken as (40 + 2) km / h or (60 ± 2) km / h, and the total driving distance during the test is the driving range. Constant speed method is often used to measure the driving range of electric vehicles.