What are the charging characteristics of lithium batteries?

The charging characteristics of the battery refer to the changes of terminal voltage u, temporary electromotive force E, static electromotive force Ej and electrolyte density with the charging time during the constant current charging process. The main sign of the battery charging terminal is: the terminal voltage of the single battery increases to about 2.7v, but no longer increases; The specific gravity of electrolyte no longer increases; A large number of bubbles appear in the electrolyte, presenting a so-called boiling state.

When the battery discharge reaches cut-off voltage or termination voltage, charging characteristics vary with charging current, charging time and temperature. Figure 8-9 shows the typical charging characteristic curve of common battery. As can be seen from the figure, charging characteristic curves of different types of batteries are different, which are greatly different from discharge curves. This leads to the personalized requirements for charging mode. According to the requirements of charging characteristics, different types of batteries have different charging methods, the same kind of battery can also be used in a variety of charging methods. Its commonness is in charging process must control voltage and temperature strictly, prevent overcharge and overheat, may cause storage battery permanent damage otherwise, produce an accident even. As for the battery charging method, it is generally based on the control of charging voltage and current, forming the voltage control charging, current control charging, voltage and current combined control charging. After a long period of exploration, it is found that there are some general charging methods suitable for different batteries, such as single constant current charging at a low charging rate, or using pulse current at a high charging rate, the discharge must be stopped and the battery needs to be charged in time. However, it is clear that they do not fully meet the requirements of personalized security and efficiency.

Fully charged

According to the regulations of the IEC/T21, VRLA battery (25 + 2) ℃, the temperature at a constant voltage u charge 16 h, or in constant voltage u charge, charge to 3 h charging current is still stable, these two conditions are VRLA battery is fully sufficient electricity. The constant voltage U value is generally stipulated by the battery manufacturer, and the charging voltage and charging method can be different according to the different USES of the battery. Full charge is also called recovery charge.

As shown in figure 2-6 represents GFM500 in discharge depth is 100% with 0.1 C10A current, pressure limiting 2.35 V (25 ℃) for charging characteristic curve. As can be seen from the figure, after fully discharging the battery, 24h after charging, charging power can reach more than 120%.

As shown in figure 2-7 GFM500 after discharge depth is 100% with 0.1 C10A current, the pressure limiting 2.23 V (25 ℃) for charging characteristic curve. After 24 hours of charging, the charge can reach more than 110%.

2. Qualitative analysis of charging characteristic curve

After 100% discharge of the VRLA battery (see figure 2-6 and figure 2-7), during the charging process, the battery current is constant within 0 ~ 7.5h in the early charging period, that is, the current is maintained at 0.1c10a. The terminal voltage of the battery gradually rises to the average charging voltage of 2.35v/R or floating charging voltage of 2.23v/only, and then the voltage remains constant. During the charging time from 7.5h to 10h, the charging current rapidly decays according to the exponential law, and slows down within 10h to 20h. The current does not change from a few hours before the end of charging. The analysis of each stage is shown in the figure.

(1) before the battery is charged to 70% ~ 80%, the current limiting characteristic of the rectifier is used to maintain the charging current unchanged. In this process, the terminal voltage of the battery rises almost in a straight line.

Where, phi + and phi 1 are the positive or negative polarization potential of the battery with charging current, including the balance electrode potential E+ or E-, as well as the over potential vertical + or vertical - generated by electrochemical polarization and concentration polarization. In the process of constant current charging, the electrolyte concentration in the pore of electrode surface active material increases or becomes more negative. The intensification of electrochemical polarization and ohmic polarization with the increase of vertical + or vertical - current density, the value increases in positive or negative direction. Under the action of concentration polarization, there is a tendency to maintain vertical + lift and continue to become negative. The terminal voltage of the cell is not limited to the set value until the rectifier changes from stable current mode to stable voltage mode.

(2) when the terminal voltage of the current rises to the vicinity of the voltage stabilizing point, since the charging process has reached the middle and late stage, the number of PbSO4 on the positive electrode plate is not large at this time, so the exchange current density increases with the decrease of the reaction area, so the electrochemical polarization has become smaller, and the internal resistance of the battery has also decreased significantly. However, the real surface area of charging has become smaller, which results in the increase of the electrode's real current density. Then, the concentration of electrolyte near the electrode surface increases, resulting in serious influence of concentration polarization and rapid attenuation of current in the battery.

(3) when charging to the later stage, the battery current has significantly decreased, so the concentration polarization effect decreases accordingly. The effect of electrochemical polarization increases, so the current continues to decay, but at a slower rate.

(4) at the end of charging, most of the current charged into the battery is used to maintain the oxygen circulation in the battery, and only a small current is used to maintain the recovery of the active material, so the battery current is stable and unchanged.

When charging the VRLA battery with different charging voltages, the air pressure increases slowly when the voltage is 2.23v/unit, and the air pressure at the later stage of charging is stable. If more than 2.40v is used, the water will decompose more and more hydrogen will be generated in the battery, which will lead to the failure of oxygen circulation after the increase in quantity. Therefore, high voltage long-term charging is not recommended.

3. Balanced charging

VRLA battery in the use of the process, sometimes the capacity, terminal voltage is not consistent, in order to prevent its development into a fault battery, so to regularly perform balanced charging. In addition, in case of the following situations, balanced charging is also required: first, power supply to the communication load alone for more than 15min; second, the capacity of the battery is insufficient after deep discharge.

The balanced charging method depends on the specific situation.

(1) it is hoped that VRLA battery characteristic parameters can be improved through balanced charging. In this case, regular full charging method can be adopted. The charging monitor is used to set the cycle charging time (for example, every three months or half a year) during maintenance. When the VRLA battery floating charge runs to the set time, the rectifier will automatically raise the battery end pressure, and then it will turn to floating charge after a few hours of charging. The backward VRLA battery capacity can be made up by increasing the charging current by increasing the terminal voltage of the battery.

(2) it is hoped to restore the capacity of VRLA battery after discharge through balanced charging. There are two common methods: one is to completely charge the battery, and the other is to float the battery first and then boost the voltage, that is, to adopt the incremental voltage method.

The time required for charging is determined by the depth of battery discharge, the size of current limiting value selection, the temperature during charging and the performance of charging equipment. The set point of flow limiting point of various products is not the same, usually (0.15 ~ 0.25)C10A, but some countries also use 0.1C10A, and some use 0.3C10A. The charging time should not be too long. As mentioned before, the oxygen recombination efficiency in VRLA battery is the highest when the floating charging pressure is taken as the value. However, the balanced charging voltage is already high. When the voltage increases by 100mV, the floating charging current increases by 10 times on average. Therefore, charging time is too long, which not only increases the surplus gas in VRLA battery and affects the oxygen recombination efficiency inside VRLA battery, but also increases the corrosion speed of the grid, thus damaging the battery. 

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