1) The capacity of the battery
The amount of electricity that the battery can give under certain discharge conditions is called the capacity of the battery, which is commonly expressed as C. However, when a battery is used as a power source, since its terminal voltage is a variable value, it is more accurate to use ampere-hour (Ah) to represent the power supply characteristics of the battery. The battery capacity is defined as:
Theoretically, t can tend to infinity, but in fact, if the battery continues to discharge when the discharge voltage is lower than the termination voltage, it may damage the battery, so there is a limit to the t value. The so-called termination voltage refers to the voltage at which the battery cannot work normally when the battery is lower than this specified voltage. In other words, if the battery continues to discharge and use below the termination voltage, it may cause permanent damage to the battery.
In the battery industry, the continuous discharge time of the battery is expressed in hours or minutes. The common ones are: C24, C20,
C10, C8, C3, C1 and other nominal capacity values. Battery capacity can be divided into theoretical capacity, rated capacity, and actual capacity.
(1) The theoretical capacity is the highest theoretical value obtained by calculating the mass of the active material according to Faraday’s law.
(2) The actual capacity refers to the power that the battery can output under certain conditions, which is equal to the product of the discharge current and the discharge time, and its value is less than the theoretical capacity.
(3) Rated capacity, also known as nominal capacity and guaranteed capacity, is the minimum capacity that the battery should release under certain discharge conditions according to the standards issued by the state or relevant departments. The capacity released by the fixed battery generally adopts the 10h rate as the rated capacity of the battery, and is used to calibrate the model of the battery. The rated capacity or nominal capacity of the battery is indicated by the letter C. For example, a battery with a rated capacity of 6Ah, C=6Ah; a battery with a rated capacity of 24Ah, C=24Ah.
In order to compare different series of batteries, the concept of specific capacity is commonly used, that is, the amount of electricity that can be given by a unit volume or unit mass of batteries, which are called volume specific capacity and weight specific capacity, respectively, and their units are Ah/L (ampere-hour/liter). ) or Ah/kg.
Among the indicators to measure the battery, the rated voltage and rated capacity of the battery are the two most commonly used technical indicators. For example, the rated voltage of the Japanese Yuasa NP6-12 battery is 12V, and the rated capacity is 6Ah/20h; the rated voltage of the German Sunshine A406/165 battery is 6V, and the rated capacity is 165Ah/20h.
In the case of constant current discharge, the battery capacity is:
In the formula, C is the amount of electricity discharged by the battery, the unit is Ah: I is the discharge current, the unit is A: t is the discharge time, and the unit is h.
The essence of the concept of capacity is the representation of the energy conversion of the battery. For example, considering the fact that the terminal voltage E of the battery is equal to 12V and remains almost unchanged in actual use, the output energy expression is W(t)=IxVxt=IxExt, Therefore, from the perspective of energy effect, 6Ah can be understood as the energy released by the NP6-12 battery while keeping the terminal voltage unchanged. If it is discharged with a current of 6A, it can work normally for 1h or discharge with a current of 1A. It can work normally for 6h.
2) The voltage of the battery
(1) Open circuit voltage. The terminal voltage of the battery in the open circuit state is called the open circuit voltage. The open circuit voltage of the battery is equal to the difference between the positive potential and the negative potential of the battery when the external circuit is disconnected (that is, when no current flows through the two poles). The open circuit voltage of the battery is represented by Vk, that is
In the formula, EZ is the positive electrode potential of the battery; Ef is the negative electrode potential of the battery.
(2) Working voltage refers to the voltage displayed during the discharge process after the battery is connected to the load. Also known as load (load) voltage or discharge voltage, discharge voltage is commonly expressed as V:
In the formula, I is the discharge current of the battery; R0 is the ohmic resistance of the battery; Rj is the polarization resistance of the battery.
(3) Initial voltage. The working voltage of the battery at the beginning of discharge is called the initial voltage.
(4) Charging voltage. The charging voltage refers to the voltage applied by the external power source to both ends of the battery when the battery is being charged.
(5) Float voltage. The float voltage of the battery is the voltage value set when the charger floats the battery. The battery requires the charger to have an accurate and stable float voltage value. The charging voltage value is generally small, and artificially increasing the floating charging voltage value is harmful to the battery and not beneficial.
(6) Termination voltage. The battery discharge termination voltage is the lowest working voltage at which the battery discharge voltage drops to the point where it cannot continue to discharge. It is generally stipulated that when the fixed battery is discharged at a rate of 10h, the termination voltage of the single battery discharge is 1.8V (relative to the single 2V battery).
3) Battery charge and discharge curve
The curve of the battery voltage changing with the charging time is called the charging curve, and the curve of the battery voltage changing with the discharging time is called the discharge curve.
4) Discharge time rate and discharge rate
(1) Rate of discharge. The discharge rate of the battery is expressed by the length of the discharge time to represent the discharge rate of the battery, that is, the capacity discharged by the battery with the specified current during the specified discharge time, and the discharge rate can be calculated as follows:
In the formula, TK (T10, T3, T1) represents the discharge rate of 10, 3, and 1 hour, respectively; CK (C10, C3, C1) represents the discharge capacity of 10, 3, and 1 hour, respectively, and the unit is IK (I10, I3, I1) represent 10, 3, and 1 hour rate discharge currents, respectively, and the unit is A.
(2) Discharge rate. The discharge rate (X) is the multiple of the discharge current to the rated capacity of the battery. which is:
In the formula, X is the discharge rate; I is the discharge current; C is the rated capacity of the battery.
In order to compare 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 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. Therefore, the capacity index 6Ah of the above-mentioned NP6-12 battery is measured at a discharge rate of 20h, that is, at a discharge rate of 0.05C. For NP6-12 batteries, 0.05C equals 0.3A of current.
5) Energy and specific energy
(1) Energy. The energy of the battery refers to the electric energy that the battery can give under a certain discharge system, usually expressed in W, and its unit is watt-hour. The energy of the battery is divided into theoretical energy and actual energy. The theoretical energy can be expressed by the product of the theoretical capacity and the electromotive force, and the actual energy of the 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 battery is the energy given by the battery per unit volume or unit mass, which are called volume specific energy and mass specific energy, respectively, in units of W·h/L and W·h/kg.
6) Power and specific power
(1) Power. The power of the battery refers to the amount of energy given by the 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 battery is divided into theoretical power and actual power. The theoretical power is under certain discharge conditions. The product of the discharge current and the electromotive force, and the actual power of the battery is the product of the discharge current and the average working voltage under certain discharge conditions.
(2) Specific power. The specific power of the battery refers to the power output by the battery per unit volume or unit mass, which is called volume specific power (unit: W/L) or mass specific power (unit: W/kg). The specific power is an important performance technical index of the battery. The specific power of the battery is large, which means that it has a strong ability to withstand high current discharge.
7) Cycle life
The cycle life, also known as the service cycle, refers to the number of charges and discharges that the battery undergoes before the battery capacity drops to a specified value under certain discharge conditions.
The self-discharge of the battery refers to the automatic discharge phenomenon of the battery when the battery is in an open circuit. The self-discharge of the battery will directly reduce the output power of the battery and reduce the capacity of the battery. The generation of self-discharge is mainly due to the fact that the electrodes are in a thermodynamically unstable state in the electrolyte, and the two electrodes of the battery undergo redox reactions. Among the two electrodes, the self-discharge of the negative electrode is dominant, and the occurrence of self-discharge causes the active material to be consumed and converted into unusable thermal energy. The size of self-discharge can be expressed by the self-discharge rate, that is, the percentage of battery capacity reduction within a specified time;
In the formula, Y is the self-discharge rate; C1 is the capacity before the battery is put on hold; C2 is the capacity after the battery is put on hold; T is the lay-up time of the battery, generally expressed in days, weeks, months or years.
The size of the battery self-discharge rate is determined by kinetic factors, mainly depending on the nature of the electrode material, surface state, composition and concentration of the electrolyte, impurity content, etc., and also depends on the environmental conditions of storage, such as temperature and humidity. factor.
9) Internal resistance
The internal resistance of the battery refers to the resistance received by the current passing through the interior of the battery, which includes ohmic internal resistance and polarization internal resistance, and polarization internal resistance includes electrochemical polarization internal resistance and concentration polarization internal resistance. Due to the existence of internal resistance, the working voltage of the battery is always less than the open circuit voltage or electromotive force of the battery.
The ohmic internal resistance is generated by the battery grid, active material, diaphragm and electrolyte. Although it follows Ohm’s law, it also changes with the change of the battery’s state of charge, while the polarization internal resistance increases with the increase of the current density. But not a linear relationship. Therefore, the internal resistance of the battery is not constant, it changes continuously with time during the charging and discharging process, that is, it changes with the continuous change of the composition state of the active material, the concentration of the electrolyte and the temperature.
Good batteries and poor batteries differ greatly in internal resistance. The reason why a good quality battery can continue to discharge a large current is because its internal resistance is small, while the poor quality battery is not. Because of its large internal resistance, On the one hand, when the high current is discharged, the terminal voltage drops rapidly, and it is close to the termination voltage if the required time is not reached.
Since the VRLA battery is sealed, it is not as transparent and intuitive as the ordinary lead-acid battery, and it cannot directly measure the electrolyte density, which brings certain difficulties to the use and maintenance work. Therefore, people hope to identify and predict the performance of VRLA batteries by detecting the internal resistance of VRLA batteries. At present, the imported and domestic conductivity testers used to measure the internal resistance of VRLA batteries have been applied in some departments. However, it can be found in practice that it is not satisfactory to use online measurement of VRLA battery internal resistance (or conductance) to identify and judge the performance of VRLA battery.
From a macro perspective, if the open-circuit voltage of the VRLA battery is V0, and its terminal potential is V when the current I is discharged, then r=(V0-V)/I is the internal resistance of the VRLA battery. However, the internal resistance of the VRLA battery obtained in this way is not a constant, it not only varies with the working state and environmental conditions of the VRLA battery, but also varies with the test method and test duration. The essence is that the internal resistance r of the VRLA battery includes complex and changing components.
Macroscopically measured internal resistance r (ie steady-state internal resistance) of VRLA battery is composed of three parts: ohmic internal resistance RΩ, concentration polarization internal resistance Rc and activation polarization internal resistance Re.
The ohmic internal resistance includes the resistance of all parts such as electrodes, diaphragms, electrolytes, connecting bars and poles inside the battery. Although it will change due to grid corrosion and electrode deformation during the entire life of the VRLA battery, it will be In the process of VRLA battery internal resistance, it can be considered as constant.
Since the concentration polarization internal resistance is caused by the change of the concentration of reactive ions, as long as there is an electrochemical reaction in progress, the concentration of reactive ions is always changing, so its value is in a state of change, and the measurement method is different or Depending on the duration of the measurement, the measured results will also vary.
The activation polarization internal resistance is determined by the nature of the electrochemical reaction system. The chemical system and structure of the VRLA battery are determined, and the activation polarization internal resistance is also determined: only in the later life or discharge stage of the VRLA battery, the electrode structure and The change occurs when the state changes and the reaction current density changes, but its value is still small.
The conduction path of metal resistance inside VRLA batteries has always plagued VRLA battery testing, because VRLA battery performance degradation occurs particularly quickly, possibly in the interval between annual capacity tests. The abnormal internal resistance of the failed VRLA battery indicates that the pole, internal busbar and grid of the VRLA battery have been chemically corroded. At this time, it will be seen that the contact surface of the copper pad immersed in the electrolyte has been corroded or the lead pole has fallen off The phenomenon.
The plate paste, electrolyte and separator of the battery constitute the electrochemical resistance part of the internal resistance of the VRLA battery. The long-term use of the VRLA battery will cause the reduction of active substances or the aging of the paste, which will increase the electrochemical resistance of the VRLA battery. When the VRLA battery is charged and discharged, the electrochemical resistance of the VRLA battery will also temporarily change due to the change of the specific gravity of the electrolyte and the change of the composition of the isolation net or the chemical composition of its surface. Creep, blockage, short circuit or vulcanization of the isolation net is the cause of abnormal or increased electrochemical resistance of VRLA batteries.