In a fully charged state, there are 2 sulfuric acid molecules in the electrolyte; when the electricity is discharged, the 2 sulfuric acid molecules become 2 water molecules. Sulfuric acid is a good conductor of electricity, while water is non-conductive. The conductivity of everyday water is due to impurities in the water. The resistivity of highly purified water used in the semiconductor industry can reach up to 17m². In batteries, sulfuric acid added All are excessive, so after discharge, the electrolyte in the battery is still an aqueous solution of sulfuric acid, but the content of sulfuric acid is reduced, and the resistance of the electrolyte increases at this time. This process of changing from small to large is continuous and gradual, and in the process of charging, the above process is reversed.
On the positive and negative plates, when fully charged, whether it is Pb or PbO2, the resistivity is very small, both 10-4~10-3Ω/cm, after the discharge of the plates, the active material becomes PbSO4, Its resistivity is very large, about 1010Ω/cm. This factor is combined with the change in the resistance of the electrolyte, and the resistance of components such as separators constitutes the static internal resistance of the battery. The so-called static internal resistance refers to the internal resistance of the battery measured under the condition of no charge and discharge.
The battery also has a dynamic internal resistance, which changes with the magnitude of the discharge current. This is because in the discharge reaction, the electrolyte is not uniform. The density of the electrolyte close to the surface of the electrode plate is reduced due to the reaction with the active material on the electrode plate. It will take some time for the sulfuric acid in the electrolyte far from the electrode plate to diffuse to the surface of the electrode plate, which results in a concentration gradient of the electrolyte solution. The greater the discharge current, the greater the concentration gradient, the lower the sulfuric acid concentration on the surface of the plate, and the more the terminal voltage of the battery drops. When the discharge stops, the concentration polarization disappears rapidly, and the terminal voltage rapidly rises to a stable value. Therefore, the greater the discharge current, the greater the dynamic internal resistance of the battery.
From the above analysis it can be seen that:
Internal resistance of battery = dynamic internal resistance + static internal resistance (electrolyte resistance + plate resistance + component resistance)
Therefore, when describing the internal resistance of a battery, be sure to explain its discharge conditions.
The measurement method of internal resistance is shown in Figure 1. The formula for measuring the internal resistance of the battery is as follows.
In the formula, U1——the no-load voltage of the battery;
U2——When discharging with I, the battery terminal voltage;
r – the internal resistance of the battery.
The physical meaning of internal resistance is: when discharging with current I, the internal resistance r of the battery consumes the voltage value of U1-U2. When using the tape recorder and TV in the car, the current consumption is very small, only about 1A. The internal resistance of the battery is also small, which does not affect the use of such electrical appliances. When the engine is started, the peak current reaches about 200A, and the internal resistance of the battery increases several hundred times in an instant. Sometimes the car cannot be started, commonly known as “insufficient power”. After the charging operation, the charge of the battery is very high, which reduces the internal resistance, and the car can be started.
On some large-scale power equipment, the schematic diagram of the starting circuit is shown in Figure 2. The battery is used to supply power to the high-power starting contactor to pull in the coil. When K is closed, the coil is energized and closed, the starting contact is connected, and the battery is powered by a large current. Discharge the starter motor. At this time, if the internal resistance of the battery is too large, the terminal voltage of the battery will drop rapidly, so that the pull-in force of the contactor is not enough, causing the contactor to be disconnected and the high-current discharge to be interrupted. With the interruption of the high current discharge, the coil is pulled in again because the terminal voltage rises again. This kind of action keeps going back and forth, commonly known as “playing the board”. This situation is caused by the excessive dynamic internal resistance of the battery.
The internal resistance of the battery is a function of the discharge current, and its specific value is directly related to the discharge current. It is meaningless if it is not measured under discharge conditions. Now there is a conductance battery internal resistance meter on the market, which can only measure the static internal resistance of the battery, but not the dynamic internal resistance of the battery. Therefore, the measurement data cannot express the power supply capability of the battery, and the user cannot take effective maintenance of the battery according to the measurement data.