For the host plant engineers, the knowledge of power battery must be known,which is one of the three major new energy (battery, motor, electronic control) core technology. However, at present, many engineers know little about the power battery. The main reason is that the power battery is in the electrical field, and the Automobile Institute is not. Here are some brief introductions for your reference.
1. What is a battery? What is its function?
Let's talk about the pool first. Cups, buckets, water tanks, pools, cups, barrels, pools, and ponds here have a common feature,whose function is to hold water. However, water is liquid and it flows downhill. Here is the question, when the pool with a hole drilled,and the water is poured in. A pool of water will eventually dry. What is the scientific truth in this process?
a) empty pool, empty volume can hold water;
b) The water itself cannot enter the pool, it is the person who pours it in;
c) With the existence of water pressure, water will flow from height to ground.
In the same way, the battery is the volume of “electricity”. At first the inside of the battery is “empty” and there is no electricity. The electricity is charged by people and because of the voltage difference, the battery can be discharged.
The pool is about the principle of physics and it contains water,which is molecular structure. However, the battery is about the principle of electrochemistry and is equipped with “electrical”which is smaller than the molecules.
2. The basic knowledge of dry batteries
Commonly used radios, tape recorders, cameras, electronic clocks, toys, and other batteries are classified as dry batteries. The electrolyte in a dry cell is a non-flowable paste called a dry cell (see Figure 1), which is what is said with respect to a cell with a mobile electrolyte.
Figure 1 Dry cell shape and internal structure
The outer shell is a cylindrical container made of zinc. A carbon rod is set in the center of the zinc cylinder, and a copper cap is fixed on the top of the carbon rod. Carbon rods and zinc cans are called electrodes for dry cells. The carbon rod that collects the positive charge is called the positive electrode (the symbol +, indicates the positive electrode of the battery), and the zinc tube that collects the negative charge is called the negative electrode (the symbol - indicates the negative electrode of the battery).
The basic principle of the discharge: Carbon dioxide is filled with magnesium dioxide around, the zinc electrode forms the shell of the dry cell, and the carbon electrode is placed in the center. The electrons are given with electronized zinc metal (oxidation), and the electric current flowing into the outside reaches the carbon electrode. Magnesium dioxide near the carbon electrode gets electrons (reducing action) to generate hydroxide ions, and forms a new compound called magnesium oxide. The oxidation reaction pushes out the electrons of the negative electrode of the battery and the reduction reaction absorbs them at the positive electrode. Dry batteries do not have recharging characteristics for the user and are disposable batteries.
3. Power battery must be rechargeable
(1) The biggest difference between dry batteries and current power batteries is:
a) The dry battery electrolyte is a non-flowable paste and it does not have recharging characteristics;
b) The power battery is an electrolyte in which electrolyte lithium ions can flow, and can be charged thousands of times;
(2) Basic similarities between dry batteries and current power batteries:
a) Positive electrode, negative electrode, shell, electrochemical reaction;
b) The dry battery and the power battery have the same physical characteristics on the monomer, and they are cylindrical and square.
Figure 2 Power Battery Round and Square
4.what is the meaning of power battery pack? Why do you need to make a battery pack after connecting one section in series (parallel)?
For the engine manufacturer's engineers, the fuel car is a fuel tank, which is a liquid volume. But seeing the electric car is a power battery pack after another, open the battery pack, there are many countless sections of a battery. Can you make a battery pack directly from a power battery?
The flashlight is generally equipped with two dry batteries, some with three sections and five sections. Dry cell specifications are No. 1, No. 2, and No. 5. The power battery called a battery the monomer, and called a group in parallel a module. The modules connected in series called a battery pack, and the battery pack is called a battery system in series.
Monomer (or single block) is the smallest unit of battery product. Can you make a big battery? For example, put 3 batteries into a flashlight. In practice, this is a deficiency of the principle of electrochemical cells. One section of dry battery can only do 1.5V, people need 3V power, must be 2 series dry batteries in series. The capacity of a single cell can't be done very well. People have to have a very large power supply and they must connect individual cells in parallel.
Figure 3 Internal structure of a power battery pack
Vehicle power supply is generally 300V, 200A or more, the current basic method is to unite a single battery into a module, the module battery into a battery pack in series, the battery pack into a power system in series.
5.What is the battery management system (BMS) and what is its function?
The 12-meter electric bus generally has 6-8 battery packs, and each battery pack is connected in parallel (in series) with thousands of single battery. So how do many battery systems with a certain number of individual battery cells work together in orderly manner? Simply, organization generates management. The battery management system comes into being.
The single battery is an object, similarly, the management of the battery must also be an “object”, rather than a person. This object is the computer management system. To sum up, the battery management system (BMS) is an intelligent management system that applied by computer technology.
Lithium-ion power battery working process:During charging and discharging process, lithium ions move back and forth between positive and negative poles through the electrolyte. The number of lithium ions that move back home is not enough. It must be well controlled. If so,it can be recharged repeatedly without reducing the capacity. Otherwise, the capacity of the battery will be permanently reduced or even explode.
Figure 4 Battery Management System (BMS) Management Objects and Contents
The battery management system is closely integrated with the power battery of the electric vehicle. The battery management system usually has the function of measuring the battery voltage to prevent abnormal conditions such as battery over-discharge, over-charge, and over-temperature. Its basic function is :Real-time detection of voltage, current, and temperature , as well as leakage detection, thermal management, battery equalization management, alarm reminding, calculation of the remaining capacity (SOC), discharge power, reporting of battery deterioration degree (SOH) and remaining capacity (SOC) state help it works,it also uses the algorithm to control the maximum output power , and to use the algorithm to control the charger for the optimal current charging,.Besides,by using the CAN bus interface, the on-board controller and motor control the system monitor the real-time communications.
6.The basic functions of the three important subsystems of the battery management system
(1) SOC estimation function
It is very important to accurately estimate the SOC value, and its algorithm is one of the core competitiveness of related companies. The SOC has high estimation accuracy and can have higher cruising range for the same amount of batteries. Therefore, high-precision SOC estimation can effectively reduce the required battery cost. The SOC is the transmission information calculated based on the monitored external characteristic information. The SOC informs the owner of the current power, it also allows the car to understand its own power, preventing overcharge and over discharge, to help improve the consistency of the balance,and to increase the output power. The interior of the system is calculated through complex algorithms to ensure the continuous and stable operation of the vehicle safety.
(2) Thermal management function
Thermal management mainly includes determining the optimal operating temperature range of the battery, calculation of battery thermal field and temperature prediction, selection of heat transfer medium. Make sure the battery operates within the proper temperature range and reduce the temperature difference between individual battery modules.
(3) Balance control function
Balanced control is divided into active equilibrium and passive equilibrium.Active equalization is to balance the capacity or voltage difference generated between the battery cells during charging, discharging or placement of the battery pack to eliminate various inconsistencies generated within the battery.
The balanced approach is mainly based on passive equalization, and the single battery is used to shunt the power resistance resistors in parallel, and can only perform equalization work during the charging process.The working principle is to find the difference between the series cells through the collection of the voltage, and to set the "upper threshold voltage" of the charging voltage as a benchmark. When the upper threshold voltage is detected and the difference with the battery in the adjacent group is detected, that is, the battery with the highest single cell voltage in the battery group is discharged in parallel by the power consumption resistance of the single cell, and so on. The single cell with the lowest voltage reaches the "upper threshold voltage" as a balancing cycle.