The main energy source of pure electric vehicles is the power battery system, and its performance directly affects the economy, power and reliability of the whole vehicle. The biggest difference between electric vehicles and traditional fuel vehicles is the use of power batteries as power drives, and as an important link connecting battery packs, vehicle systems and motors, the importance of battery management systems (BMS) is self-evident. Perfect BMS can effectively improve the utilization rate of the battery, prevent the battery from overcharging and overdischarge, and extend the service life of the battery, monitor the operating status of the battery pack and each battery cell, effectively prevent the spontaneous combustion of the battery pack, realize the early warning of emergencies, and win time for safety.
The author sorts out the key technologies in the development process of the battery management system, and provides a theoretical basis for the design and test production of the power battery management system. The plan is divided into 5 chapters (as follows) to sort out the key technologies in the development of battery management systems, and today we will first talk about the first chapter: power battery test research.
First of all, it needs to be clear that the object of the battery management system is the battery, and the smallest unit of the battery is the battery cell. A battery cell is made up of complex chemicals that undergo an electrochemical reaction during the charge and discharge of the cell. The bottom layer of the management battery is to establish an accurate lithium-ion power battery model and accurately estimate its state, to obtain the above information, it is necessary to carry out relevant battery test test research to obtain a large number of useful test data.
(1) The working principle of the power battery
Lithium-ion battery is a recyclable rechargeable battery that relies primarily on lithium ions moving between the positive and negative electrodes to operate. Lithium-ion batteries use an embedded lithium compound as an electrode material. At present, the cathode materials used as lithium-ion batteries are mainly common: lithium cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium nickel oxide (LiNiO2) and lithium iron phosphate (LiFePO4)
▲Figure 1 - Six major elements of lithium batteries
(2) Analysis of key indicators of power batteries
The core indicators of the power battery include battery electromotive force, battery terminal voltage, battery internal resistance, battery capacity, and battery state estimation.
1) Battery electromotive force refers to the theoretical calculated value, which is often greater than the battery terminal voltage.
2) The battery internal resistance has a very large impact on the battery, and the battery internal resistance includes ohmic internal resistance and polarization internal resistance. The polarized internal resistance consists of electrochemical polarization and concentration polarization. Electrochemical polarization is caused by the sluggishness of electrochemical reactions on the electrode surface and decreases significantly as the current decreases. Concentration polarization: In the process of battery charge and discharge, due to the slowness of ion diffusion in the electrolyte, the concentration difference between the electrode surface and the electrolyte body is caused. As the current decreases, so does the concentration polarization resistance, but lasts from a few seconds to tens of seconds.
3) Battery capacity refers to the amount of power obtained from the battery under certain discharge rate, ambient temperature, termination voltage and other conditions, that is, the capacity of the battery,
4) Battery state, state of charge (SOC) The most widely used DEFINITION OF SOC is proposed by the American Advanced Battery Federation (USABC), that is, the ratio of the remaining power of the battery to the rated capacity under the same conditions under a certain discharge rate.
(3) Build a power battery test system
In order to obtain the data of the underlying cells, it is necessary to build a relevant test system, which usually includes the following
Figure 2 - Battery test platform equipment layout diagram
To obtain the basic data of lithium batteries, it is necessary to carry out the constant current, constant voltage, constant power charge and discharge, pulse charge and discharge and various urban road cycle conditions of lithium batteries, in order to ensure the safety of the test and the safety of the battery, it is necessary to extract and set important parameters included
Maximum current, minimum current, charge and discharge cut-off voltage, maximum capacity, temperature range, etc. In particular, the battery temperature has a greater impact on battery performance, so the high and low temperature test chamber is the most important equipment for the battery test system.
(4) Carry out power battery performance testing and research
In order to generate a sufficiently rich incentive ground for lithium-ion power batteries, it is usually necessary to design a comprehensive set
The test procedure. Figure 3 The entire test consists of 7 experimental subroutines.
Figure 3 - 7 sub-tests of the battery performance test
For the new battery test, the first step is to complete a battery activation process, generally setting the temperature of 25 ° C, and completing 2-3 standard charge and discharge processes.
(5) Factors affecting the actual capacity of the power battery
Complex, variable and nonlinear are the characteristics of the actual capacity of lithium-ion batteries, which are affected by factors such as discharge rate, temperature, termination voltage, and number of cycles during use. For batteries after multiple cycles of charge and discharge, the actual capacity is less than the rated capacity.
▲Figure 4 - Influencing factors of the actual capacity of the power battery
summary:
According to the current industry development, lithium-ion batteries have obvious advantages over other types of batteries.
The test platform of battery cells and battery packs is constructed, and the characteristics of batteries are tested based on different tests, and the influence of charge-discharge rate on battery polarization characteristics is analyzed, which provides important support for the improved battery composite pulse experiments and dynamic working conditions acquisition methods, which provides important support for the identification and verification of battery model parameters in the later stage. At the same time, in the actual capacity assessment, the relationship between battery capacity and charge and discharge rate, temperature, cycle times and aging needs to be analyzed, and the above basic research is crucial for the design and development of battery management system.
About the Author: Nuts
Senior engineers in luxury car companies switched from engine testing to battery testing.
At present, he focuses on the research of battery testing technology.
I hope to work with you to "electrify, to the future"!