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Electric vehicle batteries are divided into two categories: battery and fuel cell. The battery is applicable to pure electric vehicles, including lead-acid battery, nickel-metal hydride battery, sodium-sulfur battery, secondary lithium battery and air battery.

Fuel cells are dedicated to fuel cell electric vehicles, including alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), proton exchange membrane fuel cells (PEMFC), and direct methanol fuel cells (DMFC).

It varies slightly with the types of electric vehicles. In pure electric vehicles equipped only with batteries, the role of batteries is the only power source of the vehicle drive system. In the hybrid electric vehicle equipped with traditional engine (or fuel cell) and battery, the battery can play the role of both the main power source of the vehicle drive system and the auxiliary power source. It can be seen that at low speed and starting, the battery plays the role of the main power source of the vehicle drive system; At full load acceleration, it acts as an auxiliary power source; It plays the role of storing energy during normal driving, deceleration and braking.

Fuel cells are anodized by fuel and reduced by oxidant at cathode. If gaseous fuel (hydrogen) is continuously supplied on the anode (i.e., the negative pole of the external circuit, also known as the fuel pole), and oxygen (or air) is continuously supplied on the cathode (i.e., the positive pole of the external circuit, also known as the air pole), the electrochemical reaction can occur continuously on the electrode and generate current. It can be seen that fuel cells are different from conventional cells. Its fuel and oxidant are not stored in the battery, but in the storage tank outside the battery. When it works (output current and do work), it needs to continuously feed fuel and oxidant into the battery and simultaneously discharge the reaction products. Therefore, in terms of working mode, it is similar to conventional gasoline or diesel generator. Since fuel and oxidant are continuously fed into the fuel cell during operation, the fuel and oxidant used in the fuel cell are both fluid (gas or liquid). The most commonly used fuels are pure hydrogen, various hydrogen-rich gases (such as reforming gas) and some liquids (such as methanol aqueous solution). The commonly used oxidants are pure oxygen, purified air and other gases and some liquids (such as hydrogen peroxide and nitric acid aqueous solution).

The function of fuel cell anode is to provide a common interface for fuel and electrolyte, and to catalyze the oxidation of fuel. At the same time, it transmits the electrons generated in the reaction to the external circuit or to the collector plate first and then to the external circuit. The function of cathode (oxygen electrode) is to provide a common interface for oxygen and electrolyte, to catalyze the reduction of oxygen, and to transmit electrons from the external circuit to the reaction site of the oxygen electrode. Because most of the reactions on the electrode are multiphase interfacial reactions, in order to improve the reaction rate, the electrode is generally made of porous materials and coated with electrocatalysts.

The function of electrolyte is to transport the ions produced by the fuel electrode and oxygen electrode in the electrode reaction, and to prevent the straightness between the electrodes

Then transfer electrons.

The role of the diaphragm is to conduct ions, prevent the direct transfer of electrons between electrodes and separate oxidants and reducing agents. Therefore, the diaphragm must be electrolyte resistant and insulating material with good wettability.

Battery pack

The battery pack of electric vehicle is composed of multiple batteries stacked in series. A typical battery pack has about 96 batteries. When charged to 4.2V lithium-ion batteries, such battery packs can generate a total voltage of more than 400V. Although the vehicle power supply system regards the battery pack as a single high-voltage battery and charges and discharges the entire battery pack every time, the battery control system must consider the situation of each battery independently. If the capacity of one battery in the battery pack is slightly lower than that of other batteries, its charging state will gradually deviate from other batteries after several charging/discharging cycles. If the charging state of this battery is not periodically balanced with other batteries, then it will eventually enter a deep discharge state, resulting in damage and eventually forming a battery failure. To prevent this, the voltage of each battery must be monitored to determine the charging state. In addition, there must be a device to charge or discharge the batteries separately to balance the charging state of these batteries.

Communication interface is an important consideration of battery pack monitoring system. For the communication in PC board, the common options include serial peripheral interface (SPI) bus and I2C bus. The communication cost of each bus is very low, which is suitable for low interference environment. Another option is the Controller Area Network (CAN) bus, which is widely used in automotive applications. CAN bus is very good, with error detection and fault tolerance characteristics, but its communication cost is very high, and the material cost is also very high. Although the connection from the battery system to the main CAN bus of the car is worthwhile, it is advantageous to use SPI or I2C communication in the battery pack.

A voltage range.

At present, the positive and negative plates of lead-acid batteries in common use are of paste type, which are composed of lead-antimony alloy or lead-calcium alloy grids and active substances. Therefore, the plate resistance is also composed of grid resistance and active material resistance. The grid is in the inner layer of the active material and will not change chemically during charging and discharging, so its resistance is the inherent resistance of the grid. The resistance of the active material changes with the charging and discharging state of the battery.

When the battery is discharged, the active material of the electrode plate changes to lead sulfate (PbSO4). The greater the content of lead sulfate, the greater the resistance. When the battery is charged, the lead sulfate is reduced to lead (Pb). The smaller the lead sulfate content is, the smaller the resistance is.

Electrolyte resistance

The resistance of electrolyte varies with its concentration. Once a certain concentration is selected within the specified concentration range, the electrolyte resistance will change with the degree of charging and discharging. When the battery is charged, the electrolyte concentration increases and its resistance decreases while the active material on the electrode plate is reduced; When the battery is discharged, the electrolyte concentration decreases and its resistance increases while the active material of the electrode plate is sulfated.

Diaphragm resistance

The resistance of the separator varies according to its porosity. The resistance of the separator of the new battery tends to a fixed value, but its resistance increases with the extension of the battery operation time. Because some lead slag and other deposits are deposited on the separator during the operation of the battery, the porosity of the separator decreases and the resistance increases.

Connector resistance

The connector includes the inherent resistance of the metal such as the connecting strip when the single battery is connected in series, the connection resistance between the battery plates, and the metal resistance of the connector of the positive and negative plates forming the pole group. If the welding and connection contact are good, the connector resistance can be regarded as a fixed resistance.

The internal resistance of each battery is the sum of the resistance of the above objects. The relationship between the internal resistance R of the battery and the electromotive force, terminal voltage and discharge current: Rs=(E-Uf) ÷ If

The internal resistance of the battery will gradually increase during the discharge process, and gradually decrease during the charging process. Therefore, in the process of battery charging and discharging, the terminal voltage will also change due to the change of its internal resistance. Therefore, the terminal voltage is lower than the electromotive force of the battery when discharging, and higher than the electromotive force of the battery when charging.

life

The battery life has two concepts: "dry storage life" and "wet storage life". It must be pointed out that these two concepts are only for the size of battery self-discharge, not the actual service life of the battery. The real life of the battery refers to the actual use time of the battery.

For the primary battery, the battery life is the working time (related to the discharge rate) that characterizes the rated capacity.

For the secondary battery, the battery life is divided into charge and discharge cycle life and wet shelving service life.

The charge-discharge cycle life is an important parameter to measure the performance of secondary battery. It is called a cycle (or a cycle) to undergo a charge and discharge. Under a certain charge-discharge system, the number of charge-discharge times that the battery can withstand before the battery capacity drops to a specified value is called the charge-discharge cycle life of the secondary battery. The longer the charge-discharge cycle life, the better the performance of the battery. Among the commonly used secondary batteries at present, the cycle life of cadmium nickel battery is 500~800 times, that of lead acid battery is 200~500 times, that of lithium ion battery is 600~1000 times, and that of zinc silver battery is very short, about 100 times.

The charge-discharge cycle life of secondary battery is related to the discharge depth, temperature, charge-discharge system and other conditions. The so-called "discharge depth" refers to the percentage of the discharged capacity of the battery in the rated capacity. By reducing the depth of discharge (i.e. "shallow discharge"), the charging and discharging cycle life of the secondary battery can be greatly extended.

The service life of wet storage is also one of the important parameters to measure the performance of secondary batteries. It refers to the time from the beginning of the charge-discharge cycle to the end of the charge-discharge cycle life of the battery after adding the electrolyte (including the time when the battery is in the discharge state during the charge-discharge cycle). The longer the service life of wet storage, the better the battery performance. Among the commonly used batteries at present, the service life of cadmium nickel battery in wet storage is 2-3 years, lead-acid battery is 3-5 years, lithium-ion battery is 5-8 years, and zinc-silver battery is the shortest, only about 1 year.

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