When a battery discharges, it converts stored chemical energy into electrical energy through controlled electrochemical reactions. These processes involve coordinated electron flow through external circuits and ion movement within internal electrolytes, maintaining charge balance while delivering power to connected devices. The discharge characteristics vary significantly between galvanic (spontaneous) and electrolytic (forced) modes, with performance further influenced by discharge rates quantified through C-rate metrics.

During galvanic discharge in a typical lithium-ion cell, oxidation occurs at the anode where lithium atoms release electrons to become lithium ions. The general half-reaction can be represented as Li → Li⁺ + e⁻. These electrons travel through the external circuit to power devices, while the lithium ions migrate through the electrolyte toward the cathode. Simultaneously, reduction takes place at the cathode where lithium ions recombine with electrons and integrate into the host material, such as lithium cobalt oxide (LiCoO₂), following the reaction Li⁺ + e⁻ + CoO₂ → LiCoO₂. This electron-ion coupling maintains electrical neutrality while sustaining current flow.

Ion transport mechanisms differ by battery chemistry. In liquid electrolytes, lithium ions solvated by organic carbonate solvents diffuse toward the cathode, whereas in solid-state batteries, ions hop through crystalline lattice structures. The electrolyte must exhibit high ionic conductivity but negligible electronic conductivity to prevent self-discharge. Separators with microporous structures further ensure physical isolation of electrodes while permitting ionic flow.

Electrolytic discharge occurs when external power sources drive reverse reactions, as seen in rechargeable batteries during charging. Here, electrons are forced into the anode, attracting positive ions back through the electrolyte. This non-spontaneous process requires energy input exceeding the cell's potential, with overpotentials arising from activation barriers and ohmic losses. The same electrodes and electrolytes facilitate both galvanic and electrolytic processes in secondary cells, distinguishing them from primary batteries designed for single-use discharge.

Discharge rates profoundly impact capacity utilization and voltage stability. The C-rate specifies current relative to nominal capacity; a 1C rate discharges total capacity in one hour, while 2C accomplishes this in 30 minutes. Higher C-rates induce several effects:

Capacity Reduction:
0.5C may deliver 100% rated capacity
1C typically provides 95-98%
2C often yields 90-92%
5C can drop to 80-85%

This stems from kinetic limitations - at high currents, concentration gradients prevent complete active material utilization before reaching cutoff voltage. Lithium diffusion in electrodes becomes rate-limiting, causing premature voltage drop.

Voltage profiles exhibit distinct C-rate dependencies:
Low C-rates (below 0.5C) show flat discharge plateaus with gradual decay
Medium C-rates (0.5-2C) display slight sloping voltage decrease
High C-rates (above 3C) demonstrate pronounced voltage sag and early termination

The voltage drop derives from overpotentials:
Ohmic overpotential scales linearly with current (IR drop)
Activation overpotential follows logarithmic current dependence
Concentration overpotential grows exponentially at high rates

Internal resistance plays a key role, composed of:
Electrolyte ionic resistance (40-50% of total)
Electrode electronic resistance (20-30%)
Interface charge-transfer resistance (20-30%)
This resistance causes heat generation proportional to I²R losses, raising cell temperature during high-rate discharge.

Different battery chemistries exhibit unique discharge signatures. Lithium iron phosphate (LFP) cells maintain exceptionally flat voltage profiles until sudden drop-off, while nickel-manganese-cobalt (NMC) chemistries show continuous sloping curves. These variations originate from differing thermodynamic properties and phase transition behaviors during lithiation/delithiation.

Temperature interacts with discharge rate effects. Low temperatures exacerbate capacity loss at high C-rates due to increased electrolyte viscosity and slowed ion mobility. Below -20°C, standard lithium-ion batteries may retain less than 50% capacity even at moderate 0.2C rates. High temperatures improve rate capability but accelerate degradation mechanisms.

State of charge (SOC) influences discharge dynamics. Most batteries deliver higher voltages when fully charged, with nonlinear voltage-SOC relationships. The open-circuit voltage (OCV) represents equilibrium potential at zero current, while operating voltage under load reflects OCV minus overpotentials. Accurate SOC estimation requires accounting for these C-rate and temperature dependencies.

Polarization phenomena during discharge create transient voltage responses. When current is applied, voltage immediately drops due to ohmic resistance, then gradually declines from activation and concentration polarization. Upon current interruption, voltage recovers through relaxation processes as concentration gradients dissipate. These effects become more pronounced at higher C-rates.

Material properties dictate rate capability. Anodes with small particle sizes and high surface areas improve high-rate performance by shortening lithium diffusion paths. Cathodes with open crystal structures facilitate faster ion insertion. Electrolyte formulations with high conductivity and low viscosity minimize ohmic losses. These design parameters enable tradeoffs between energy density and power density.

Discharge termination criteria prevent damage from over-discharge. Most systems employ voltage cutoffs (typically 2.5-3.0V for lithium-ion) or coulomb counting methods. Deep discharge below these limits can cause copper current collector dissolution or cathode structural degradation, compromising cycle life.

The discharge process represents a complex interplay of electrochemical thermodynamics and kinetics. While the basic electron-ion coupling remains consistent across battery types, specific characteristics emerge from materials selection, cell design, and operating conditions. Understanding these fundamental processes enables optimization of battery systems for diverse performance requirements, balancing energy delivery against longevity considerations across various discharge scenarios.