- Influence on battery internal resistance
In lead-acid batteries, the resistivity of the electrolyte is thousands of times greater than the resistivity of various substances on the positive and negative plates. Therefore, under normal circumstances, the internal resistance of a fully charged battery is largely determined by the electrolyte. The internal resistance of a discharged battery is mainly determined by lead sulfate. In order to reduce the internal resistance of the battery and improve the ability of the battery to discharge large currents, it is very important to shorten the distance between the positive and negative plates. Click here to open if you would like to learn more about the effects of battery discharge performance.
Pure water is almost non-conductive, and water purified by ion exchange can obtain a large resistivity, up to 17MΩ/cm. When sulfuric acid is added, the solution can conduct electricity due to the ionization of sulfuric acid. In a very dilute sulfuric acid solution, the resistivity is large due to the small number of ions. As the concentration increases, the number of ions per unit volume increases, and the conductivity of the solution also increases. But this increase is not infinite. Ionization is only one aspect of the problem. On the other hand, the ionized ions with different charges attract each other. When the number of ions in a unit volume increases to a certain extent, the number of ions attracting and merging into sulfuric acid molecules is equal to the number of sulfuric acid molecules ionized into ions. At this time, the resistivity reaches the minimum value. When the content of H2SO4 is increased, the resistivity increases due to the obstruction of ion movement by molecular groups.
Through the analysis of the resistivity of sulfuric acid electrolyte, the following conclusions can be drawn.
①At each ambient temperature, the density corresponding to the lowest resistivity is different. Therefore, after adjusting the density of the electrolyte to a constant value, different internal resistance values will be obtained at different temperatures.
② The resistivity at 0°C is almost 1 times higher than that at 30°C, and the resistivity at -25°C is almost 4 times higher than that at 30°C. Therefore, the internal resistance of the same type of battery will be quite different in north and south, winter and summer.
③ With the change of temperature, the viscosity of H2SO4 solution will change greatly. The viscosity of the sulfuric acid solution determines its diffusion rate in the capillary pores of the plate, which has a great influence on the capacity.
- Impact on battery capacity
In lead-acid batteries, the most basic condition for selecting the concentration of sulfuric acid electrolyte is to meet the requirement that the output of the battery reaches the specified discharge capacity.
The capacity of the battery generally varies with the concentration of the sulfuric acid electrolyte. The concentration of sulfuric acid in the pores of the electrode plate determines the working voltage and output capacity of the battery. If a sufficient amount of sulfuric acid cannot be maintained in the pores of the electrode plate to supply the electrochemical reaction during operation, the terminal voltage of the battery will drop rapidly and the capacity output will be exhausted. This is because the concentration of the sulfuric acid electrolyte determines the change of the plate potential, affects the diffusion rate of the electrolyte, and affects the internal resistance of the lead-acid battery. Especially when the concentration of sulfuric acid electrolyte in the pores of the electrode plate is unbalanced with the concentration outside the electrode plate, causing concentration polarization, the terminal voltage of the lead-acid battery will drop quickly, and the energy output will decrease quickly.
The relationship between sulfuric acid content and battery capacity is shown in Figure 1.
Usually, the choice of sulfuric acid electrolyte concentration varies with the type of battery. The lead-acid battery used for start-up uses a higher concentration of sulfuric acid electrolyte than that of the stationary lead-acid battery. Because the installation of lead-acid batteries for starting on a motor vehicle is limited by volume and weight, the battery tank cannot hold a large amount of sulfuric acid electrolyte. Therefore, it is necessary to choose a sulfuric acid electrolyte with a higher concentration, so that the limited space inside the lead-acid battery can contain the necessary amount of sulfuric acid.
- Impact on battery life
The service life of lead-acid batteries decreases with the increase of the concentration of sulfuric acid electrolyte, and the density should not be higher than 1.30g/cm³. For the current start-up lead-acid batteries, the electrolyte with a density of 1.28g/cm³ is selected based on the principle of the lowest freezing point, not necessarily 1.28g/cm³. In the southern region where the ambient temperature is high, appropriately reducing the concentration of the electrolyte can significantly prolong the service life of the lead-acid battery.
Read more: Nominal and open circuit voltage of lead-acid batteries