Determining the optimal discharge strategy for a lithium-ion battery using a physics-based model

• Low discharge current related degradation mechanisms are simulated in a physics-based model.
• Impact of the state-of-charge and cycle number on these mechanisms is investigated.
• A generic lithium ion cell's optimal discharge current and its variation are determined.
• Linearity of penalty-based model validated and modified to include a 2-stage strategy.
• The results impact the design and implementation of battery controllers for multi-cell battery packs.

Multi-cell, penalty minimizing discharging schemes were developed based on experimental data and high-level models in attempt to optimize discharge capacity and cycle life based on an unspecified optimal load current. Degradation due to high discharge current is well known, but the degradation behavior of the cell at low discharge current has not been thoroughly studied. To facilitate the implementation of the penalty method in multi-cell controllers, a physics-based pseudo-2D lithium ion model is used to investigate the degradation effects at low discharge current. The results of the model show that the optimal current, depending on the application, is not necessarily constant, but instead shows a 2-stage decrease within each discharge cycle. The optimal current also decreases as the cycle number increases.