The internal resistance of a battery consists of three parts: Ohmic polarization (conductor resistance) and electrochemical polarization and concentration polarization resistance. During the charging and discharging process, the resistance changes, and the internal resistance decreases during the charging process. Conversely, the internal resistance increases.
Temperature also has a significant impact on the internal resistance of batteries. In low-temperature conditions such as below 0 ℃, for every 10 ℃ decrease in temperature, the internal resistance increases by about 15%. One of the important reasons is the increase in specific resistance due to the increase in viscosity of sulfuric acid solution. At higher temperatures, such as above 10 ℃, the diffusion rate of sulfate ions increases, and the concentration polarization effect will significantly decrease, resulting in a decrease in polarization resistance. However, the conductor resistance increases with increasing temperature, but the rate of increase is relatively small.
The internal resistance of a battery is related to the size of the discharge current. In the case of instantaneous high current discharge, the sulfuric acid solution inside the electrode gap rapidly dilutes, while more than 90% of the sulfuric acid molecules in the solution outside the electrode hole do not have time to diffuse into the electrode gap. In this way, the specific resistance of the solution in the electrode hole increases, and the terminal voltage significantly decreases. But after the discharge stops, as the high concentration of sulfuric acid molecules diffuse into the pores of the electrode plate, the specific resistance of the solution in the pores of the electrode plate decreases, and the terminal voltage rises.
In addition, the internal resistance of thin plate batteries is significantly smaller than that of thick plate batteries, because there are more thin plates than thick plate batteries of the same capacity. Therefore, when discharging at the same current, the current density of thin plate batteries is smaller, and their polarization at each pole is much smaller.