Problem
Explain in five lines why, even without external use, a chemical battery discharges over time, and why a “flat” battery sometimes briefly recovers if left to rest. Discuss the role of self-discharge currents and ionic diffusion in the electrolyte.
Solution
Why it discharges on its own (self-discharge) — even at open circuit, slow parasitic chemical reactions occur inside the battery: small internal self-discharge currents make the active electrode materials react without delivering any useful energy externally. The separation between the reactants is never perfect, there are impurities and microscopic conductive paths in the electrolyte: the stored charge is consumed internally. This is why a battery left in a drawer for months is found partially discharged.
Why it “recovers” after resting — during use, the reaction consumes the reactants near the electrode surfaces faster than ionic diffusion can resupply them from the rest of the electrolyte. A concentration polarisation builds up: ions are locally depleted, the terminal voltage collapses and the battery appears flat. Leaving it to rest, diffusion has time to re-equilibrate the concentrations, bringing fresh reactants back to the surfaces: the voltage rises again and the battery delivers current for a while longer. This is only an apparent recovery, due to ion transport, not a real recharge: the total available charge is still decreasing.
Collegamenti
Argomenti: Circuiti elettrici Concetti: Forza elettromotrice