Cause of Self Discharging of Batteries [PDF]

Self-discharge in VRLA (Valve Regulated Lead-Acid) batteries is a phenomenon where the battery gradually loses its capacity over time while in storage, especially in open-circuit conditions. The causes of self-discharge are complex, but they primarily relate to chemical reactions occurring within the battery components. Let’s delve deeper into the factors contributing to self-discharge:

1. Cathode (Negative Electrode): Self-discharge mainly occurs at the cathode or negative electrode of the VRLA battery. The active material in the negative electrode is typically spongy lead. These materials have an electrical potential in the battery’s electrolyte that is less negative than the potential required for the hydrogen evolution reaction. As a result, a substitution reaction can take place. Additionally, if there are metallic impurities present in the negative electrode, they can react with the active material, forming micro-cells that lead to the dissolution of negative electrode metal and the precipitation of hydrogen. These reactions ultimately reduce the battery’s capacity.

Self-discharge rate:

It is the percentage of reduced capacity per unit of time is known as Self-discharge rate.

1. Electrolyte and Impurities: Impurities present in the electrolyte can also contribute to self-discharge. These impurities can react with the electrode materials, leading to capacity loss. Ensuring the purity of the electrolyte is crucial in minimizing self-discharge.
2. Concentration Cells: Self-discharge can be influenced by concentration cells that form within the battery. Due to differences in sulfuric acid concentration between the upper and lower ends of the electrodes and within the pores of the electrodes, concentration cells are created. In regions with relatively dilute sulfuric acid, lead dioxide in the positive electrode can act as the negative electrode and consume oxygen. In areas with higher sulfuric acid density, lead dioxide acts as the anode and reverses its process, turning into lead sulfate. These concentration cells can form during charging and discharging, resulting in the precipitation of oxygen. However, as the concentration of sulfuric acid tends to equalize over time, the concentration cell dissipates, and self-discharge stops.

The rate of self-discharge at the positive electrode is influenced by various factors, including the composition of the grid alloy and the concentration of the electrolyte. Different values of self-discharge correspond to different concentrations of sulfuric acid.

To minimize self-discharge in VRLA batteries, it is essential to maintain high purity levels in the battery components, carefully control the composition of the grid alloys, and ensure uniform concentration of the electrolyte throughout the battery. These measures help reduce the occurrence of self-discharge and extend the battery’s storage life and overall performance.