= Electric battery
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= Battery
{synonym}
An <electric battery> converts the available chemical <Gibbs free energy> of one or more <galvanic cells> into electrical <work> delivered to an <electrical load>. The reacting materials are a finite reservoir: a <battery> can sustain a terminal <electric potential difference> while discharging, but cannot supply <energy> indefinitely. Unlike a <capacitor>, whose <capacitor energy> resides in an <electric field>, a <battery> chiefly stores its available <energy> in the chemical state of its materials. The detailed <half-reactions> and apparatus of a <Daniell cell> explain this conversion at the <electrode> level.
During discharge, <electrons> move through metallic wires from the negative <anode>, through the <electrical load>, to the positive <cathode>; external <conventional current> points the other way. Inside the <battery>, <ions> carry <electric current> through the <electrolyte>, and <redox> <chemical reactions> connect ionic transport to electronic transport at the <electrodes>. The same <electron> does not make a lap through both wire and <electrolyte>. <Electric charge conservation> closes the circuit; changing chemical composition supplies the <energy>. See <galvanic cell> for the two transport paths and <electrolyte> for how oppositely moving <ions> contribute to one net <electric current>.
\Image[/battery-charge-paths-and-circuit-energy.png]
{title=<Battery> discharge: charge paths, equivalent circuit and work per coulomb}
{description=Original schematic and stipulated calculation, not a particular commercial <battery> or measured cell. Left: external <electron> motion and internal ionic <conventional current> connect the two <electrodes>. Right: a short-interval model with <electromotive force> $\mathcal E=3\,\mathrm V$, <battery internal resistance> $r=1\,\Omega$ and <electrical load> $R=4\,\Omega$. Bottom: each <coulomb> transported corresponds to $3\,\mathrm J$ of available source <work>, split into $2.4\,\mathrm J$ delivered to the <electrical load> and $0.6\,\mathrm J$ of internal dissipation. Arrows describe transport and an energy budget, not the route of a particular <electron> or the propagation of an <electric field>.}
The source's <electromotive force> $\mathcal E$ measures its driving <work> per unit <electric charge>, in <volts>; the loaded terminal <electric potential difference> $V_L$ measures the electrical <work> per unit <electric charge> delivered outside the <battery>. They agree in an ideal lossless source, or approximately near open circuit, but generally differ during discharge.<University Physics Volume 2 by Samuel J. Ling and others>[§10.1, Introduction to Electromotive Force, work per charge and terminal voltage]{ref}
In the ideal fixed-voltage model used in <fixed-voltage capacitor energy balance>, a <battery> maintains a prescribed terminal <electric potential difference> while <electric charge> flows to or from the <capacitor>. This is a constraint for that calculation, not an inexhaustible reservoir. The <fixed-voltage capacitor energy balance> also accounts for electrical <energy> returned to the source; an actual cell can accept sustained charging only if its chemistry and operating conditions permit it.
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