In the charging process, the charging method with constant charging current from beginning to end is called constant current charging.
Constant current charging can make the battery charge a certain amount of electricity quickly, which belongs to compulsory charging. This method is often used in battery supplementary charging, especially in small batteries with good heat dissipation conditions.
Figure 1 shows a set of curves measured under constant current charging conditions. The following analyzes the changes in the battery at each stage of charging.
In the initial stage of charging, the terminal voltage rises rapidly in the 0-A stage, which is due to the PbSO4 → PBO on the negative electrode and PbSO4 → PbO2 on the positive electrode during charging. These two transformations increase the acid concentration between the pores of the electrode plate and cause concentration polarization; At the same time, both resistance polarization and electrochemical polarization increase, which makes the electromotive force of the battery continue to increase, and the dynamic internal resistance of the battery also increases. Because in the formula, u is the charging voltage; Is the electromotive force of the battery; I is the charging current; R is the internal resistance of the battery.
So when charging, u rises. This is the internal condition of the battery corresponding to the curve of section 0-a.
In the A-B segment, the terminal voltage of the battery changes gently. During this period, since the diffusion rate of acid and its ions has been balanced with the rate of acid concentration, only one term has been added to the above voltage expression.
In section B-C, at 6 o’clock, most of PbSO4 on the electrode plate has been transformed into Pb and PbO2. At this time, the terminal voltage of the battery has risen to about 2.3V, and the decomposition of water begins to become obvious, and many bubbles escape from the two poles. Due to the different proportion of active substances in the positive and negative plates, the sequence of gases produced by the positive and negative plates is different. In ordinary batteries, the positive active material is less than the negative, so gas is first produced on the positive. In sealed batteries, in order to produce as little H2 as possible, the negative active material is larger than the positive active material. When an ordinary battery is vented, O2 is generated on the positive plate and H2 twice the volume is generated on the negative plate.
The positive plate is surrounded by new ecosystem 02, and the negative plate is surrounded by H2. Both gases are non-conductive substances. The wrapping of the gas reduces the conductive area of the electrode plate, which increases the internal resistance of the battery. Therefore, under the condition of constant current charging, the terminal voltage of the battery rises sharply again, up to about 2.6V.
In the C-D section, when the voltage rises to 2.6V, if you continue to charge, because the electrochemical conversion of active substances has been completed, the charged power is basically consumed in the decomposition of water, so the terminal voltage will no longer rise. At this time, the battery temperature rises faster. After stopping charging, the terminal voltage of the battery immediately drops below 2.2V within 10s, and the terminal voltage is equivalent to the electromotive force of the battery.
With the diffusion of acid in the pores, the high potential caused by concentration polarization gradually decreases and finally stabilizes at about 2.1V. In the 0-a-b stage of constant current charging, the conversion rate of charging current is very high, up to more than 90%. At this time, almost all the charged electricity is converted into chemical energy and stored in the battery, and the battery temperature rise is low. But after point B, the charging efficiency drops sharply. In some charging processes, the constant current method is adopted before the battery terminal voltage reaches 2.3V during charging. This is the first stage of the three-level charging procedure.
The design working points of starting batteries on motor vehicles are all around point B. Working under the working conditions of more than 6 points, the battery life will be significantly shortened.