Self-Discharge

Causal Mechanism

This loss of charge is driven by thermodynamic instability and parasitic chemical reactions within the cell. These reactions include the slow oxidation of the electrolyte on the cathode surface and the reduction of species at the anode. Additionally, microscopic metal contaminants can create high-resistance electrical paths that allow a continuous leakage current to flow.

The rate of these reactions is highly dependent on the storage temperature and the state of charge. High temperatures and high states of charge accelerate the self-discharge rate, leading to faster energy loss and quicker cell degradation.

Testing Protocol

Sourcing specifications define the maximum acceptable rate of energy loss for delivered cells. To measure this parameter, cells are stored under controlled conditions, and their open-circuit voltage is measured at the start and end of the storage period. Cells that show a high rate of voltage decay are rejected as outliers, as this drift often indicates the presence of an internal defect.

During pack assembly, cells are sorted by their self-discharge rate to ensure that they remain balanced over time. This sorting is necessary because a cell with a high self-discharge rate will eventually unbalance the entire pack, reducing its usable capacity.

Sourcing Strategy

Sourcing teams use this metric to evaluate the quality and purity of the raw materials used by the manufacturer. High-quality electrodes with minimal impurities exhibit very low self-discharge rates, ensuring a long shelf life and stable performance. Agreements with suppliers often include guarantees regarding the maximum self-discharge rate under specified storage conditions.

This guarantee protects the buyer from receiving cells that degrade quickly during transit or warehouse storage. By prioritizing cells with low self-discharge, sourcing teams can ensure the reliability of their finished products.