A primary battery is designed to deliver stored chemical energy through discharge rather than to recover that energy repeatedly by electrical charging. The distinction concerns the practical cell design and reaction reversibility, not whether a balanced chemical equation can be written in both directions.
During discharge, coupled oxidation and reduction at the electrodes supply an external electric current. Reaction products, morphology changes and parasitic processes determine how much active material remains accessible. A rechargeable battery instead manages those changes sufficiently well to recover useful capacity over repeated cycles; nominal chemical reversibility alone does not establish that capability.
A primary battery using metallic lithium and iron disulfide. The overall discharge conversion consumes four lithium atoms and releases four electrons per formula unit. This is distinct from the rechargeable host-to-host transport in a conventional lithium-ion battery.
Battery capacity is the amount of electric charge delivered between specified charged and discharged states under stated conditions. It is , commonly reported in ampere-hours; .
Capacity is distinct from delivered energy, which is . Two batteries with equal ampere-hour ratings can deliver different energies because their voltages differ. Discharge rate, temperature, endpoint voltage and aging affect the measured capacity. Comparing ratings therefore requires the same test conditions and a stated choice of active-material or full-device mass when using specific capacity.
Battery self-discharge is loss of recoverable stored charge through internal processes while a battery supplies no useful external electric current. Parasitic reactions and internal leakage can change the charged state even when the external terminals are open.
A storage experiment compares recoverable capacity after controlled waiting intervals with an otherwise identical freshly charged or prepared cell. Holding temperature and subsequent discharge conditions fixed separates storage loss from a changed capacity test. Terminal voltage alone need not measure the lost capacity, because voltage also depends on chemistry and relaxation. Self-discharge is distinct from the intentional current drawn by a connected sleeping device.
A battery separator keeps the two electrodes from direct electronic contact while allowing the ionic transport needed for the cell reaction. It is electronically insulating but permeable to relevant ions, commonly through an electrolyte-filled porous structure.
The distinction between ionic and electronic paths makes the circuit work: useful electrons travel through the external load, while ions complete the internal charge balance. Separator thickness and pore structure affect internal resistance and transport. A very low ionic resistance alone is not a complete design goal, because the separator must also maintain separation and compatibility with the cell's chemical environment.
A primary battery coupling lithium oxidation to reduction of fluorinated carbon, producing lithium fluoride and carbon.
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