### What is the battery charge-discharge curve?

The curve of the battery voltage changing with the charging time is called the charging curve; the curve of the battery voltage changing with the discharging time is called the discharge curve.

1) Discharge time rate and discharge rate

(1) Rate of discharge. The discharge rate of the battery is the length of the discharge time to express the discharge rate of the battery, that is, the capacity of the battery to discharge with a specified current within a specified discharge time. The discharge time rate can be determined by the following formula:

In the formula, TK (T10, T3, T1) represent the hourly discharge rates of 10, 3, and 1, respectively; CK (C10, C3, and C1) represent the hourly discharge capacities of 10, 3, and 1, respectively (A h); IK (I10, I3, I1) represent the hourly rate discharge current (A) of 10, 3, and 1, respectively.

(2) Discharge rate. The discharge rate (X) is a multiple of the discharge current being the rated capacity of the battery, namely

In the formula, X is the discharge rate; 1 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 (A), 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 6A·h of the above-mentioned NP6-12 battery is measured at a discharge rate of 20h (ie, a discharge rate of 0.05C). For NP6-12 batteries, 0.05C equals 0.3A of current.

2) 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 (W·h). 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 weight, which are called volume specific energy and weight specific energy, respectively, in units of W·h/L and W·h/kg.

3) 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, usually expressed by P, and the unit is watts (W). The power of the battery is divided into theoretical power and actual power. The theoretical power is the product of the discharge current and the electromotive force under a certain discharge condition, and the actual power of the battery is the product of the discharge current and the average working voltage under a certain discharge condi

(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 the volume specific power (W/L) or the mass specific power (W/kg), respectively. 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.

4) Cycle life

The cycle life, also known as the service cycle, refers to the number of charges and discharges experienced by the battery under certain discharge conditions before the battery capacity drops to a specified value.

5) Self-discharge

The self-discharge of the battery refers to the self-discharge phenomenon of the battery when the battery is placed in an open circuit. The self-discharge of the battery will directly reduce the output power of the battery and reduce the battery capacity. 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. Of the two electrodes, the self-discharge of the negative electrode is dominant. The occurrence of self-discharge causes the active material to be consumed and converted into heat energy that cannot be used. The size of self-discharge can be expressed by the self-discharge rate (that is, the percentage of battery capacity reduction in a specified time):

In the formula, Y% is the self-discharge rate; C1 is the capacity of the battery before shelving; C2 is the capacity after the battery is shelved; t is the shelving 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, the surface state, the composition and concentration of the electrolyte, the impurity content, etc., and also depends on the environmental conditions (such as temperature and humidity). And other factors.

6) 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 the polarization internal resistance includes electrochemical polarization and concentration polarization. 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 battery’s state of charge; while the polarization internal resistance increases with the current density, but it is 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.

There is a big difference in internal resistance between a high-quality battery and a poor battery. High-quality batteries can continue to discharge large currents because their internal resistance is small. The poor quality of the battery is not the case. Due to its large internal resistance, when a large current is discharged, the terminal voltage drops rapidly, and it is close to the termination voltage before reaching the required time. On the other hand, due to the large internal resistance , in the process of charging and discharging, the power consumption increases and the battery heats up.

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 identify and judge the performance of VRLA batteries by online detection of the internal resistance (or conductance) of VRLA batteries.

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 consists of three parts: ohmic internal resistance RΩ, concentration polarization internal resistance Rc and activation polarization internal resistance Re.

The ohmic internal resistance RΩ 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 can be considered constant during each inspection of the VRLA battery internal resistance.

Since the concentration polarization internal resistance is caused by the change of the concentration of the reactive ions, as long as there is an electrochemical reaction in progress, the concentration of the reactive ions is always changing, so its value changes, the measurement method is different or the measurement duration is different. Different, its measured results will be different.

The activation polarization internal resistance is determined by the nature of the electrochemical reaction system. The system and structure of the VRLA battery are determined, and the activation polarization internal resistance is also determined; the electrode structure and state occur only in the later life of the VRLA battery or in the later stage of discharge. It only changes when the reaction current density changes due to the change, but the numerical change 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 VRLA batteries will cause the reduction of active substances or the aging of the paste, which will increase the electrochemical resistance of VRLA batteries. When the VRLA battery is charged and discharged, the electrochemical resistance of the VRLA battery will change temporarily 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 reason for the abnormal or increased electrochemical resistance of the VRLA battery.