Battery is the power source of electric vehicle and the storage device of energy. The key to the development of electric vehicles is to produce low energy and long service life compared with electric vehicles.
- New energy vehicle battery classification
There are many kinds of power batteries for new energy vehicles, which are widely used and have different shapes. There are many classification methods.
1.1 classification according to the working nature and use characteristics of the battery
According to the working nature and use characteristics of batteries, they are generally divided into four categories.
(1) Primary battery. Also known as “primary battery”, that is, a battery that cannot be restored by charging after discharge. In other words, the battery can only be used once, and the battery can only be abandoned after discharge. The reason why such batteries cannot be recharged is that the battery reaction itself is irreversible, or the conditions limit the reversible reaction, such as zinc manganese dry battery, zinc mercury battery and silver zinc battery.
(2) Secondary battery. Also known as “battery”, that is, after discharge, the active substance can be recovered by charging, which can be discharged again, and can be reused for many times. This kind of battery is actually a chemical energy storage device. The battery is sufficient with DC. At this time, the electrical energy is stored in the battery in the form of chemical energy. When discharged, the chemical energy is converted into electrical energy, such as lead-acid battery, nickel cadmium battery, nickel hydrogen battery, lithium-ion battery and zinc air battery.
(3) Reserve battery. Also known as “activated battery”, it is a kind of battery in which the positive and negative active substances are not in direct contact with the electrolyte, and the electrolyte is temporarily injected or activated by other methods before use. The positive and negative active substances of such batteries are chemically deteriorated or self discharged, which are basically eliminated due to isolation from the electrolyte, so that the batteries can be stored for a long time, such as magnesium silver batteries, calcium thermal batteries and lead perchloric acid batteries.
(4) Fuel cell. Also known as “continuous battery”, that is, as long as the active substance is continuously injected into the battery, it can discharge continuously for a long time. Its characteristic is that the cell itself is only a carrier. The fuel cell can be regarded as a kind of cell that sends reactants from the outside when electric energy is needed, such as hydrogen fuel cell and air fuel cell. It must be pointed out that the above classification method does not mean that a certain battery system can only belong to primary battery, secondary battery, reserve battery or fuel cell. On the contrary, a certain battery system can be designed into different types of batteries according to needs. For example, zinc silver battery can be designed as primary battery, secondary battery or reserve battery.
1.2 classification according to battery reaction principle
According to the principle of battery reaction, it can be divided into three categories: chemical battery, physical battery and biological battery.
(1) Chemical battery. The chemical reaction of substances is used to generate electricity. According to the working nature, chemical cells are divided into primary cells, storage batteries, fuel cells and reserve cells.
Chemical batteries are divided into acid batteries, alkaline batteries, neutral batteries, organic electrolyte batteries, non-aqueous inorganic electrolyte batteries, solid electrolyte batteries, etc.
According to the characteristics of batteries, chemical batteries are divided into high-capacity batteries, sealed batteries, high-power batteries, maintenance free batteries, explosion-proof batteries, etc.
According to the positive and negative electrode materials, chemical batteries are divided into zinc manganese battery series, nickel cadmium nickel hydrogen series, lead-acid series, file battery series, etc.
(2) Physical battery. Physical battery is a battery that generates electricity by using physical energy such as light, heat and physical adsorption, such as solar cell, super capacitor, flywheel battery, etc.
(3) Biological battery. Biological cells are batteries that generate electricity by biochemical reactions, such as microbial cells, enzyme cells, biological solar cells and so on.
- Basic terms and performance indicators of battery
There are many kinds of chemical batteries with different performances. The indicators commonly used to characterize its performance include electrical performance, mechanical performance, storage performance, and sometimes service performance and economic cost.
2.1 voltage
Voltage is divided into electromotive force, rated voltage, open circuit voltage, discharge voltage and termination voltage.
(1) Electromotive force. The electromotive force of the battery, also known as the standard voltage or theoretical voltage of the battery, is the difference between the equilibrium potentials of the two electrodes constituting the battery.
(2) Terminal voltage. The terminal voltage of the battery refers to the potential difference between the positive and negative poles of the battery.
(3) Open circuit voltage. The open circuit voltage of the battery is the battery voltage without load. The open circuit voltage is not equal to the electromotive force of the battery. It must be pointed out that the electromotive force of the battery is calculated from the thermodynamic function, and the open circuit voltage of the battery is actually measured.
(4) Operating voltage. The actual discharge voltage of a battery under a load usually refers to a voltage range. For example, the working voltage of lead-acid battery is 1.8V ~ 2V; The working voltage of Ni MH battery is 1.1V ~ 1.5V; The working voltage of lithium ion battery is 2.75V ~ 3.6V.
(5) Rated voltage. It refers to the recognized standard voltage when the battery of the electrochemical system works. For example, zinc manganese dry battery is 1.5V, nickel cadmium battery is 1.2V and lead-acid battery is 2V.
(6) Termination voltage refers to the voltage value at the end of discharge, which varies according to the load and use requirements. Take lead-acid battery as an example: the electromotive force is 2.1V, the rated voltage is 2V, the open circuit voltage is close to 2.1V, the working voltage is 1.8V ~ 2V, and the discharge termination voltage is 1.5V ~ 1.8V (the discharge termination voltage is also different according to the discharge rate).
(7) Charging voltage. It refers to the voltage charged by the DC voltage of the external circuit to the battery. Generally, the charging voltage is greater than the open circuit voltage of the battery, usually within a certain range. For example, the charging voltage of nickel cadmium battery is 1.45v ~ 1.5V; The charging voltage of lithium ion battery is 4.1V ~ 4.2V; The charging voltage of lead-acid battery is 2.25V ~ 2.7V.
(8) Voltage efficiency. It refers to the ratio of the working voltage of the battery to the electromotive force of the battery. When the battery is discharged, the working voltage of the battery is less than the electromotive force due to the existence of electrochemical polarization, concentration polarization and ohmic voltage drop. Improving electrode structure (including real surface area, porosity, pore size distribution, size of active material particles, etc.) and adding additives (including conductive materials, expanders, catalysts, hydrophobics, doping, etc.) are two important ways to improve battery voltage efficiency.
2.2 internal resistance
The steady-state model of the battery in a short time can be regarded as a voltage source, and its internal impedance is equivalent to the internal resistance of the voltage source, which determines the service efficiency of the battery. The internal resistance of the battery includes the resistance of the positive and negative plates, the resistance of the electrolyte, the resistance of the diaphragm and the resistance of the connector.
(1) Positive and negative plate resistance. At present, the positive and negative plates of lead-acid batteries commonly used are paste coated, which are composed of lead cake, antimony alloy or lead calcium alloy grid frame 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 charge and discharge, so its resistance is the inherent resistance of the grid. The resistance of the active material changes with the different charging and discharging states of the battery.
When the battery is discharged, the active material of the electrode plate changes into lead sulfate. The greater the content of lead sulfate, the greater the resistance. When the battery is charged, lead sulfate is reduced to lead. The smaller the content of lead sulfate is, the smaller the resistance is.
(2) Electrolyte resistance. The resistance varies with the concentration of electrolyte. Within the specified concentration range, once a certain concentration is selected, the electrolyte resistance will change with the degree of charge and discharge. When the battery is charged, the electrolyte concentration increases and the resistance decreases while the active substance of the electrode plate is reduced; When the battery is discharged, the electrolyte concentration decreases and the resistance increases while the active material of the electrode plate is sulfated.
(3) 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 running time. Because some lead slag and other deposits are on the separator during the operation of the battery, the porosity of the separator decreases and the resistance increases.
(4) 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 composed of positive and negative plates. 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 internal resistance of the battery will gradually increase during discharge and decrease during charging. For the same batch of batteries, those with too large or too small internal resistance are abnormal. Too small internal resistance may mean micro short circuit, and too large internal resistance may be the aging of electrode plate, loss of active material and capacity attenuation. The change of internal resistance can be used as one of the sufficient reference basis for battery cracking.
2.3 capacity and specific capacity
1) Capacity
It refers to the amount of electricity that can be released under certain discharge conditions after the battery is fully charged. It is represented by symbol C, and its unit is ampere hour (a · h) or milliampere hour (MA · h). The capacity is related to the discharge current and the cut-off voltage of charge and discharge. The capacity of battery can be divided into theoretical capacity, rated capacity, actual capacity and nominal capacity.
(1) Theoretical capacity. Assuming that all electrode active substances participate in the electrochemical reaction of the battery, the electric quantity that can be provided is the highest theoretical value calculated according to Faraday’s law.
(2) Rated capacity. Rated capacity, also known as guaranteed capacity, refers to the minimum amount of electricity that can be released by the battery under certain discharge conditions in accordance with the standards issued by the state or relevant departments when designing and manufacturing the battery.
(3) Actual capacity. Actual capacity refers to the amount of electricity actually discharged by the battery under certain discharge conditions. It is equal to the product of discharge current and discharge time. For practical chemical power supply, its actual capacity is always lower than the theoretical capacity, and usually 10% ~ 20% higher than the rated capacity. The capacity of the battery is related to the quantity and activity of active substances on the positive and negative electrodes, as well as the structure and manufacturing process of the battery and the discharge conditions (current and temperature) of the battery. The comprehensive index of factors affecting battery capacity is the utilization rate of active substances. In other words, the more fully utilized the active material, the higher the capacity given by the battery. Using thin and porous electrodes and reducing the internal resistance of the battery can improve the utilization of active substances and improve the actual output capacity of the battery.
(4) Nominal capacity. Nominal capacity (or nominal capacity) is the approximate value used to identify the appropriate battery. When specifying discharge conditions, it generally refers to the discharge capacity at 0.2C discharge.
2) Specific capacity
In order to compare different series of batteries, the concept of specific capacity is often used. Specific capacity refers to the amount of electricity that can be given by the battery per unit mass or unit volume, which is correspondingly called mass specific capacity or volume specific capacity.
When the battery is working, the amount of electricity passing through the positive and negative electrodes is always equal. However, in the actual design and manufacture of batteries, the capacities of positive and negative electrodes are generally not equal, and the capacity of batteries is limited by electrodes with small capacity. In the actual battery, the positive capacity limits the capacity of the whole battery, while the negative capacity is excess.
2.4 energy
The energy of the battery refers to the electric energy that the battery can output under a certain discharge system, which is usually expressed in watt hours (w · h). The energy of the battery reflects the power capacity of the battery. It is also a measure of energy conversion in the process of battery discharge. It affects the driving distance of electric vehicles.
(1) Theoretical energy. If the dummy battery is always in equilibrium in the discharge process, its discharge voltage maintains the value of electromotive force, and the utilization rate of active material is 100%, that is, the discharge capacity is equal to the theoretical capacity, then the energy output by the battery under this condition is the theoretical energy, that is, the maximum work done by the reversible battery under constant temperature and voltage.
(2) Actual energy. The actual energy is the energy actually output when the battery is discharged. It is numerically equal to the product of the actual capacity of the battery and the average working voltage of the battery.
(3) Specific energy. Specific energy is divided into mass specific energy and volume specific energy.
Mass specific energy refers to the energy that can be output by the battery per unit mass. The unit is usually w · H / kg.
Volume specific energy refers to the energy that can be output by a unit volume of battery, also known as energy density. The unit is usually w · H / L.
Specific energy is often used to compare different battery series.
Specific energy is also divided into theoretical specific energy and actual specific energy.
Theoretical specific energy refers to the energy that can be output theoretically when the reaction material of 1kg battery is completely discharged. According to the theoretical mass specific capacity of positive and negative active substances and the electromotive force of the battery, the theoretical mass specific energy of the battery can be calculated directly. If the electrolyte participates in the reaction of the battery, the theoretical amount of electrolyte needs to be added. The theoretical specific energy only considers the specific energy under the completely reversible battery reaction according to the battery reaction formula, so it is an idealized model. For practical batteries, the actual specific capacity is more meaningful. Because the battery reaction cannot reach a completely reversible charge discharge and energy state, and many necessary auxiliary materials occupy the mass and volume of the battery in the actual battery. Actual specific energy refers to the actual output energy of a 1kg battery during discharge, which is expressed as the ratio of the actual output energy of the battery to the mass (or volume) of the whole battery. Due to the influence of various factors, the actual specific energy of the battery is far less than the theoretical specific energy.
The specific energy of battery is a comprehensive index, which reflects the quality level of battery. The specific energy of battery affects the whole vehicle quality and driving range of electric vehicle, which is an important index to evaluate whether the power battery of electric vehicle meets the predetermined driving range.
2.5 efficiency
As an energy storage, the power battery converts electric energy into chemical energy when charging and releases electric energy when discharging. In this reversible electrochemical conversion process, there is a certain energy loss. It is usually expressed by the capacity efficiency and energy efficiency of the battery. For electric vehicles, the driving range is one of the most important indicators. On the premise of a certain amount of battery power and output impedance, according to the law of energy conservation, the output energy of the battery pack is transformed into two parts: one part is lost on the resistance as heat consumption; The other part is provided to the motor controller and transformed into effective power. The ratio of the two parts of energy depends on the ratio of the output impedance of the battery pack to the equivalent input impedance of the motor controller. The smaller the impedance of the battery pack, the smaller the useless heat consumption and the greater the output efficiency.
(1) Capacity efficiency. Capacity efficiency refers to the ratio of the output capacity when the battery is discharged to the input capacity when charging. The main factor affecting the capacity and efficiency of the battery is the side reaction. When the battery is charged, part of the power is consumed in the decomposition of water. In addition, self discharge, falling off of electrode active material, caking and shrinkage of porosity also reduce the capacity output.
(2) Energy efficiency. Energy efficiency, also known as electric energy efficiency, refers to the ratio of the energy output during battery discharge to the energy input during charging. The reason affecting energy efficiency is that the battery has internal resistance, which increases the battery charging voltage and decreases the discharge voltage. The energy loss of internal resistance is lost in the form of battery heating.
2.6 power and specific power
Battery power refers to the energy output per unit time of the battery under a certain discharge system, and the unit is watt (W) or kilowatt (kw).
The power output per unit mass or unit volume of battery is called specific power, and the unit is w / kg or w / L. If the specific power of a battery is large, it indicates that more energy is given in unit weight or unit volume per unit time, that is, the battery can discharge with a larger current. Therefore, the specific power of the battery is also one of the important indexes to evaluate the performance of the battery.
2.7 discharge current and discharge depth
When it comes to battery capacity or energy, it is necessary to point out the discharge current or discharge conditions, which are usually expressed by discharge rate.
(1) Discharge rate refers to the rate of discharge, which is commonly expressed by “hour rate” and “magnification”. Hour rate refers to the discharge rate expressed in discharge time (H), that is, the time required to discharge the rated capacity with a certain discharge current. Magnification refers to the current value output by the battery to release the rated capacity within the specified time, which is numerically equal to the multiple of the rated capacity. For example, 2 “rate” discharge means that the discharge current value is twice the rated capacity. If the battery capacity is 3a · h, the discharge current should be 2 × 3 = 6a, that is, 2 “rate” discharge.
(2) Discharge depth, a measure of discharge degree, is the percentage of discharge capacity and total discharge capacity, referred to as DOD (depth of discharge). The discharge depth is closely related to the charging life of the secondary battery:
The deeper the discharge depth of the secondary battery, the shorter its charging life. Therefore, deep discharge should be avoided as far as possible.
2.8 charging
How much power the battery has, also known as the remaining power, is often taken as its ratio to the rated capacity or actual capacity, which is called the degree of charge. Charge is the parameter data that people are most concerned about in use, and it is also the most difficult to obtain. People have tried to calculate the charge by measuring the change of internal resistance, voltage and current, and have done a lot of research work. However, until now, any formula and algorithm can not be effectively supported by statistical data, and the indicated charge degree always changes nonlinearly.
2.9 storage performance and self discharge
For all chemical power supplies, even if they are placed in an open circuit without contact with the external circuit, the capacity will decay naturally. This phenomenon is called self discharge, also known as charge holding capacity.
The self discharge of the battery is measured by the self discharge rate, which is generally expressed by the percentage of capacity reduction per unit time:
Self discharge rate = (battery capacity before storage – battery capacity after storage) / battery capacity before storage × 100%
The self discharge of battery is mainly determined by many factors, such as electrode material, manufacturing process, storage conditions and so on. From the perspective of thermodynamics, the discharge process of the battery is the process of reducing the free energy of the system, so the occurrence of self discharge is inevitable, but the rate is different. The main factors affecting the self discharge rate are the storage temperature and humidity of the battery. The increase of temperature will improve the reaction activity of positive and negative materials in the battery, accelerate the ion conduction velocity of electrolyte, reduce the strength of auxiliary materials such as cadmium, and greatly improve the self discharge reaction rate. If the temperature is too high, the chemical balance in the battery will be seriously damaged, irreversible reaction will occur, and finally the overall performance of the battery will be seriously damaged. The influence of humidity is similar to that of temperature. Too high ambient humidity will also accelerate the self discharge reaction. Generally speaking, under the environmental conditions of low temperature and low humidity, the self discharge rate of the battery is low, which is conducive to the storage of the battery. However, too low temperature may also cause irreversible changes in electrode materials and greatly reduce the overall performance of the battery.
The storage performance of battery refers to the change of main performance parameters after the battery is stored for a certain time under certain conditions, including the decline of capacity, appearance and whether there is deformation or seepage. National standards are limited by battery capacity decline, appearance change and liquid leakage ratio.
2.10 service life
Battery life is divided into storage life and service life.
(1) Storage life includes two concepts: dry storage life and wet storage life. The storage life of batteries with electrolyte added during use is also known as dry storage life. Dry storage life can be very long. The storage life of batteries with electrolyte added before leaving the factory is conventionally called wet storage life (or wet charge life). During wet storage, the self discharge is serious and the service life is short.
(2) Service life refers to the actual use time of the battery. For the primary battery, the service life of the battery is the working time given the rated capacity (related to the discharge rate). For secondary batteries, the battery life is divided into charge discharge cycle life and wet storage service life.
Charge discharge cycle life is an important parameter to measure the performance of secondary battery. Under a certain charge and discharge system, the number of charge and discharge times that the battery can withstand before the battery capacity is reduced to a specified value is called the charge and 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 charge discharge cycle life of nickel cadmium battery is 500800 times, that of lead-acid battery is 200500 times, that of lithium-ion battery is 6001000 times, and that of zinc silver battery is very short, about 100 times.
The charge discharge cycle life of secondary battery is related to discharge depth, temperature, charge discharge mode and other conditions. By reducing the discharge depth (i.e. “shallow discharge”), the charge discharge cycle life of the secondary battery can be greatly prolonged.
- Basic requirements of new energy vehicles for power batteries
The requirements of new energy vehicles for power batteries mainly include:
(1) The specific energy is high. In order to improve the driving range of electric vehicles, the power battery on electric vehicles is required to store as much energy as possible, but electric vehicles can not be too heavy, and the space for installing batteries is also limited, which requires batteries to have high specific energy.
(2) High specific power. In order to make electric vehicles compete with fuel vehicles in acceleration, climbing ability and load driving, batteries are required to have high specific power.
(3) The charging technology is mature and the time is short. The charging technology should be universal and can realize wireless charging. Fast charging can be realized in terms of charging time.
(4) With high continuous discharge rate and low self discharge rate, the battery can meet the requirements of rapid discharge. The self discharge rate should be low and the battery can be stored for a long time.
(5) Adapt to the vehicle operating environment, the battery can work normally and stably under normal temperature conditions, is not affected by the ambient temperature, does not need special heating and insulation system, and can adapt to the vibration of electric vehicles.
(6) Safe and reliable. The terminal post shall be clean and the battery shell shall not be corroded. It shall not cause white burning or burning, and will not cause injury to passengers in case of collision and other accidents. The waste battery can be recycled and reused, and the harmful heavy metals in the battery can be recycled and treated in a centralized manner. The battery pack can be replaced quickly by mechanical device, and the line connection is convenient.
(7) Long life and maintenance free. The cycle life of the battery shall not be less than 1000 times, and maintenance and repair are not required within the limited service life.