# Performance indicators of electric car batteries

(1). Voltage (V)

①Electromotive force: the potential difference between the positive electrode and the negative electrode of the battery, usually represented by the symbol E.

②Open circuit voltage: the terminal voltage of the battery when it is open. Generally, the open circuit voltage is approximately equal to the electromotive force of the battery.

③Rated voltage: the voltage that the battery should reach when working under the conditions specified in the standard.

④Working voltage (load voltage, discharge voltage): the voltage displayed during the discharge process after the load is connected to both ends of the battery.

⑤Termination voltage: When the battery is discharged under the discharge conditions specified by a certain standard, the battery voltage will gradually decrease. When the battery is no longer suitable for discharging, the lowest working voltage of the battery is called the termination voltage.

The discharge curve refers to the change curve of the battery’s working voltage with time when the battery is continuously discharged under certain discharge conditions. Figure 1 shows the discharge curve under different current conditions. The change in the battery discharge process can be characterized on the graph, and the discharge time and discharge capacity can also be calculated from the discharge curve. If the discharge time rate (discharge rate expressed in terms of discharge time) is small, the working voltage drops fast, the end voltage is low, and the discharge time is short, which will affect the actual use of the battery. The working voltage drops slowly and can often output more energy. The rate of change of the operating voltage is sometimes referred to as “the smoothness of the discharge curve”.

(2). Battery capacity (A·h)

①Theoretical capacity: According to the characteristics of battery active materials, the highest theoretical value calculated according to Faraday’s law, generally expressed by mass capacity (A·h/kg) or volume capacity (A·h/L).

{Note: The Faraday’s law mentioned here refers to Faraday’s first law, that is, the mass of the substance that undergoes a chemical change at the electrode interface is proportional to the amount of electricity introduced. }

②Actual capacity: The amount of electricity that can be output under certain conditions is equal to the product of the discharge current and the discharge time.

③Nominal capacity (nominal capacity): used to identify the appropriate approximate capacity of the battery. Since the discharge conditions are not specified, only the capacity range of the battery is indicated, and there is no exact value.

④Rated capacity (guaranteed capacity): The minimum capacity that the battery should discharge according to the discharge conditions specified by a certain standard.

⑤ State of Charge (SOC): The state of charge refers to the change of battery capacity, which is the ratio of the remaining capacity of the battery to the rated capacity under the same conditions at a certain discharge rate. SOC=1 means that the battery is fully charged. As the battery discharges, the charge of the battery gradually decreases. At this time, the relative amount of the percentage of SOC can be used to indicate the change state of the charge in the battery. Generally, the high discharge efficiency area of ​​the battery is SOC=50%~80%. Accurate real-time identification of SOC is a key technology of battery management system.

⑥Depth of discharge (DOD): the percentage of discharge capacity to rated capacity, and the following relationship exists with SOC:

DOD=1-SOC

The depth of discharge has a great influence on the service life of the secondary battery. Generally, the higher the commonly used depth of discharge of the secondary battery, the shorter its service life. Therefore, the secondary battery should be avoided as much as possible during the use of the battery. Deep discharge.

(3). Power (W, kW)

Under a certain discharge system (discharge conditions), the energy output by the battery per unit time is called the battery power. The power of the battery determines the acceleration performance of the electric car. The power of a battery is usually expressed in terms of specific power and power density.

① Specific power (W/kg): refers to the electric power that a unit mass battery can emit. ②Power density (WL): refers to the electric power that a unit volume battery can emit.

(4). Energy (W·h, kW·h)

The energy of the battery determines the driving distance of an electric car, and the specific indicators are as follows.

①Nominal energy: the energy that the battery can output under the discharge conditions specified in the standard. The nominal energy of a battery is the product of the battery’s rated capacity and the rated voltage.

② Actual energy: the energy that the battery can output under certain conditions. The actual energy of the battery is the product of the actual capacity of the battery and the average operating voltage.

③Specific energy (W·h/kg): refers to the energy that a unit mass battery can output. The quality of the battery is the sum of the quality of the structural parts of the battery and the quality of the electrolyte.

④ Energy density (W·h/L): refers to the energy output per unit volume of the battery.

In the application process of power batteries in electric cars, the battery pack installation needs to be equipped with components such as battery boxes, connecting wires, current and voltage protection devices, so the actual battery pack’s specific energy is more than 20% lower than the battery’s specific energy.

(5). Internal resistance

When the current flows through the inside of the battery, it receives resistance, which reduces the voltage of the battery. This resistance is called the internal resistance of the battery. Due to the internal resistance of the battery, the terminal voltage of the battery is lower than the electromotive force and open circuit voltage during discharge, and the terminal voltage is higher than the electromotive force and open circuit voltage during charging.

(6). Life

The work of the accumulator is a cyclic process of continuous charging and discharging. It is discharged according to certain standards. When the capacity of the battery drops to a certain specified value, the discharging must be stopped and the battery needs to be charged before it can be used. In each cycle, the chemically active substances in the battery undergo a reversible chemical reaction. With the increase of the number of charging and discharging, the chemical active substances in the battery will be deteriorated, and its chemical function will gradually weaken, so that the efficiency of charging and discharging of the battery will gradually decrease, and finally the battery will lose all functions and be scrapped.

The life of the battery is the time from the beginning of use to the end of life. It is usually expressed by the number of cycles and service life.

①Number of cycles: The number of charge/discharge from the first charge to the end of life of the battery is called the number of cycles, also known as the cycle life. The number of cycles of the battery is related to the form of charging and discharging of the battery, the temperature of the battery and the depth of discharge (when the depth of discharge is low, it is helpful to extend the life of the battery), especially the environment in which the battery is used in electric cars, including each of the battery packs. The balance of the battery, the installation and fixing method, the vibration received and the installation of the circuit, etc., will all affect the number of working cycles of the battery.

②Service life: the number of years from the beginning of use of the battery to the end of life.

(7). Discharge rate (discharge rate)

The rate of battery discharge is called the discharge rate. The discharge rate is expressed in two ways: time rate and rate.

①Hour rate (also called hour rate): The discharge time that elapses when the battery is discharged at a certain intensity until the battery voltage drops to the termination voltage.

② Rate: The ratio (multiple) of the current value of the battery discharged at a certain intensity to the current value of the rated capacity.

When the discharge current is greater than or equal to the value of the rated capacity, the discharge current value is expressed by a multiplier; if the discharge current is less than the value of the rated capacity, the discharge current value is expressed by a time rate. The rated capacity of a battery is usually expressed by “C”, that is, the discharge rate is expressed by adding a coefficient before C. For example, 2 times rate, namely 2C, its discharge current value is twice the rated capacity current value, and the rated capacity is discharged in about 0.5h; 2 hour rate, namely 0.5C, its discharge current value is 1/ of the rated capacity current value 2, and the rated capacity is about 2h to finish.

(8). Self-discharge rate

The self-discharge rate refers to the rate at which the battery discharges itself under no load during the storage time, causing the battery capacity to be lost. The self-discharge rate is expressed by the percentage of battery capacity decrease per unit time (month or year).

(9). Cost

The cost of the battery is related to the technical content, materials, production method and production scale of the battery. Currently, the cost of newly developed batteries with high specific energy is higher, which makes the cost of electric cars higher. The development and development of high-efficiency and low-cost batteries is The key to the development of electric cars. Figure 2 shows the production cost structure of electric cars.