Basic parameters of electric vehicle power battery
1. The voltage of the power battery
(1) Open circuit voltage. The terminal voltage of the power battery in the open circuit state is called the open circuit voltage. The open circuit voltage of the power battery is equal to the difference between the positive electrode potential and the negative electrode potential of the power battery when the power battery is open (that is, when there is no current passing through the two poles). The open circuit voltage of the power battery is expressed by Vk, namely
Vk=Ez-Ef
In the formula, Ez is the positive potential of the power battery; Ef is the negative potential of the power battery.
(2) Working voltage. The voltage displayed during the discharging process after the power battery is connected to the load, also known as the load (load) voltage or the discharge voltage. The discharge voltage is usually represented by V
V=Vk-I(Ro+Rj)
In the formula, I is the discharge current of the power battery; Ro is the ohmic resistance of the power battery; Rj is the polarization resistance of the power battery.
(3) Initial voltage. The working voltage of the power battery at the beginning of discharge is called the initial voltage.
(4) Charging voltage. Charging voltage refers to the voltage applied to both ends of the power battery by an external power source when the power battery is being charged.
(5) Float charging voltage. The floating charge voltage of the power battery is the voltage value set when the charger floats the power battery. The power battery requires the charger to have an accurate and stable float voltage value. The high float voltage value means that the energy storage is large, and the float voltage value of the poor quality power battery is generally small, and artificially increasing the float voltage value is harmful and not beneficial to the power battery.
(6) Termination voltage. The end-of-discharge voltage of the power battery is the lowest working voltage at which the power battery discharges when the voltage drops to the point where it can no longer be discharged. Generally, it is stipulated that when the fixed-type power battery is discharged at a rate of 10 hours, the final discharge voltage of the single power battery is 1.8V (relative to a single power battery). Body 2V power battery, at 25℃).
2. Charging and discharging curve of power battery
The curve of power battery voltage changing with charging time is called charging curve, and the curve of power battery voltage changing with discharging time is called discharging curve.
3. discharge time rate and discharge rate
(1) Discharge time rate. The discharge time rate of the power battery is the length of the discharge time to express the discharge rate of the power battery, that is, the capacity of the power battery at the specified current during the specified discharge time. The discharge time rate can be determined by the following formula
Tk=Ck/Ik
In the formula, Tk (T10, T3, T1) respectively represent the hourly discharge rate of 10, 3, 1, etc.; Ck (C10, C3, C1) respectively represent the hourly rate discharge capacity of 10, 3, and 1, (Amp-hour); Ik (I10, I3, I1) respectively represent 10, 3, 1 hour rate discharge current, (A).
(3) Discharge rate. The discharge rate (X) is a multiple of the discharge current for the rated capacity of the power battery, namely
X=I/C
In the formula, X is the discharge rate; I is the discharge current; C is the rated capacity of the power battery.
In order to compare power batteries with different capacities, the discharge current is not expressed as an absolute value (ampere), but expressed as the ratio of the rated capacity C to the discharge system time, which is called the discharge rate or discharge rate. The discharge rate of the 20h system is C/20=0.05C, and the unit is A. For the NP6-12 type power battery, 0.05C is equal to 0.3A current.
4. Energy and specific energy
(1) Energy. The energy of the power battery refers to the electric energy that the power battery can give under a certain discharge system, usually expressed in W, the unit is Wh, the energy of the power battery is divided into theoretical energy and actual energy, and the theoretical energy can be the product of the theoretical capacity and the electromotive force. , And the actual energy of the power battery is the product of the actual capacity and the average working voltage under certain discharge conditions.
(2) Specific energy. The specific energy of the power battery is the energy given by the power battery per unit volume or unit weight, which is called volume specific energy and weight specific energy, and the units are Wh/L and Wh/kg.
5. Power and specific power
(1) Power. The power of the traction battery refers to the amount of energy given by the traction battery in a unit time under a certain discharge system. It is usually expressed by P and the unit is W. The power of the traction battery is divided into theoretical power and actual power. The theoretical power is in The product of the discharge current and the electromotive force under a certain discharge condition, and the actual power of the power battery is the product of the discharge current and the average working voltage under a certain discharge condition.
(2) Specific power. The specific power of the power battery refers to the power output by the power battery per unit volume or unit mass, respectively called the volume specific power W/L or the mass specific power W/kg. The specific power is an important performance technical index of the power battery. The large specific power of the power battery indicates that it has a strong ability to withstand large current discharge.
6. cycle life
Cycle life, also known as service cycle, refers to the number of charge and discharge times that the power battery experiences before the power battery capacity drops to a certain value under certain discharge conditions.
7. self-discharge
The self-discharge of the power battery refers to the automatic discharge of the power battery when it is left open. The self-discharge of the power battery will directly reduce the power output of the power battery and reduce the capacity of the power battery. The main reason for self-discharge is that the electrodes are in a thermodynamically unstable state in the electrolyte, which is the result of the oxidation-reduction reactions of the two electrodes of the power battery. In the two electrodes, the self-discharge of the negative electrode is the main one. The occurrence of self-discharge causes the active material to be consumed and transformed into unusable heat energy. The magnitude of self-discharge can be expressed by the self-discharge rate, that is, the percentage of the reduction in the capacity of the power battery within a specified time.
Y%=[(C1-C2)/C1×T]×100%
In the formula, Y% is the self-discharge rate; C1 is the capacity of the power battery before shelving; C2 is the capacity of the power battery after shelving; T is the shelving time of the power battery, generally expressed in days, weeks, months or years.
The self-discharge rate of power battery is determined by dynamic factors, mainly depends on the nature of the electrode material, surface state, electrolyte composition and concentration, impurity content, etc., and also depends on the environmental conditions such as temperature and humidity And other factors.
8. internal resistance
The internal resistance of the power battery refers to the resistance that the current passes through the power battery, including ohmic internal resistance and polarization internal resistance. Polarization internal resistance also includes electrochemical polarization internal resistance and concentration polarization internal resistance. Due to the internal resistance, the working voltage of the power battery is always less than the open circuit voltage or electromotive force of the power battery.
The ohmic internal resistance is produced by the grid, active material, diaphragm and electrolyte of the power battery. Although it follows Ohm’s law, it also changes with the state of charge of the power battery, while the polarization internal resistance increases with the increase of current density. But it is not a linear relationship. Therefore, the internal resistance of the power battery is not constant, and it changes with time during the charging and discharging process, that is, with the continuous change of the composition state of the active material, the electrolyte concentration and the temperature.
A high-quality power battery and a poor power battery differ greatly in internal resistance. High-quality power batteries can continue to discharge with large currents because their internal resistance is very small, while poor quality power batteries are not. Because of their large internal resistance, the terminal voltage drops quickly during high-current discharge. By the required time, it is close to the end voltage. On the other hand, due to the large internal resistance, the power consumption increases during the charging and discharging process and the power battery heats up. Macroscopically, if the open circuit voltage of the power battery is U, when the current is used 1 When discharging, its terminal potential is U, then r=(U0-U)/I is the internal resistance of the power battery. The internal resistance of the power battery obtained in this way is not a constant, not only varies with the working state and environmental conditions of the power battery, but also varies with the test method and test duration. The essence is that the internal resistance r of the power battery contains complex And it’s changing.
The internal resistance r of the power battery measured macroscopically (that is, the steady-state internal resistance) is composed of three parts: the ohmic internal resistance RΩ, the concentration polarization internal resistance Rc and the activation polarization internal resistance Re.
(1) The ohmic resistance RΩ includes the resistance of all parts such as the electrodes, diaphragms, electrolyte, connecting bars and poles inside the power battery. Although it will change due to grid corrosion and electrode deformation during the entire life of the power battery, The internal resistance of the power battery can be considered unchanged every time.
(2) The internal resistance of concentration polarization is caused by the change of ion concentration during the electrochemical reaction. As long as the electrochemical reaction is proceeding, the concentration of reactive ions is always changing, so its value is in a state of change. Different measurement methods or different measurement durations will result in different results.
(3) The activation polarization internal resistance is determined by the nature of the electrochemical reaction system. Once the power battery system and structure are determined, the activation polarization internal resistance is also determined; only at the end of the life of the power battery or the electrode structure at the end of discharge The change occurs only when the state has changed and caused the reaction current density to change, but its value is still very small.
The conduction path of the internal metal resistance of the power battery has always plagued the power battery test. This is because the performance degradation of the power battery occurs particularly quickly, which may occur in the interval of the annual capacity test. The abnormal internal resistance of the failed power battery indicates that the pole, internal busbar and grid of the power battery have been chemically corroded. At this time, you will see that the contact surface of the copper pad immersed in the electrolyte has been corroded or the lead pole has fallen off Phenomenon.
The electrode paste, electrolyte and separator of the power battery constitute the electrochemical resistance part of the internal resistance of the power battery. The prolonged use of the power battery will cause the reduction of active materials or the aging of the paste, which will increase the electrochemical resistance of the power battery. . When the power battery is charged and discharged, changes in the specific gravity of the electrolyte and changes in the composition of the isolation net or the chemical composition of the surface will cause temporary changes in the electrochemical resistance of the power battery. The creep, blockage, short circuit or vulcanization phenomenon of the isolation mesh is the cause of the abnormal or increased electrochemical resistance of the power battery.