The Shape of Stored Energy
A battery's discharge curve is its fingerprint. It reveals how voltage declines as charge is extracted, and its shape depends on the underlying electrochemistry. Lithium cobalt oxide cells (in phones) show a smooth downward slope, while lithium iron phosphate cells (in EVs) maintain a flat plateau until suddenly dropping. Understanding these curves is essential for predicting runtime, designing battery management systems, and comparing cell chemistries.
Internal Resistance and IR Drop
The moment current flows, the terminal voltage drops below the open-circuit voltage by I × Rᵢ. This IR drop is instantaneous and proportional to current. In high-power applications like power tools or electric vehicles, minimizing internal resistance is critical. Cell aging, low temperature, and manufacturing defects all increase Rᵢ, degrading performance.
State of Charge and Voltage
As the battery discharges, the active materials at both electrodes change composition. This shifts the thermodynamic equilibrium voltage. In the middle of discharge, voltage changes gradually (the plateau region), but near the endpoints — fully charged and fully depleted — voltage changes rapidly. Battery management systems use this voltage-SOC relationship to estimate remaining charge.
Energy vs. Power
A battery optimized for energy (high capacity, thin electrodes) differs fundamentally from one optimized for power (low resistance, thick current collectors). This simulation shows the tradeoff: increasing load current increases instantaneous power but reduces total delivered energy through greater resistive losses and the Peukert effect.