- Charging and Discharging Definition: Charging is the process of restoring a battery’s energy by reversing the discharge reactions, while discharging is the release of stored energy through chemical reactions.
- Oxidation Reaction: Oxidation happens at the anode, where the material loses electrons.
- Reduction Reaction: Reduction happens at the cathode, where the material gains electrons.
- Electron Flow in Discharge: During discharging, electrons flow from the anode to the cathode through an external circuit.
- Role of External DC Source in Charging: An external DC source is used in charging to reverse the discharging reactions, restoring the battery to its charged state.
The charging and discharging of a battery involve oxidation and reduction reactions. A rechargeable Battery stores energy when an external source drives its discharge reactions in reverse within approved operating limits.
The zinc-chlorine example below illustrates electron transfer.
Zinc loses two electrons and forms a zinc ion with oxidation state +2.
Each chlorine atom accepts one electron and forms a chloride ion.
Zinc ions and chloride ions can then form zinc chloride (ZnCl2).
Zinc is oxidised because it loses electrons. Chlorine is reduced because it gains electrons.
Oxidation raises an element’s oxidation number, while reduction lowers it. In this example zinc changes from 0 to +2. The oxidation number of a chlorine atom changes from 0 to -1.
Discharging of Battery
A cell has positive and negative electrodes separated by electrolyte and usually a separator. When a load closes the external circuit, oxidation at one electrode supplies electrons while reduction at the other consumes them.
During discharge, oxidation occurs at the negative electrode, which is the electrochemical anode. Electrons leave it through the external circuit.
During discharging of battery, reduction occurs at the positive electrode, which is the electrochemical cathode. Electrons flow from the negative electrode through the load and enter the positive electrode.
Ions move through the electrolyte and separator to maintain charge balance and complete the internal circuit. The exact ion species and directions depend on the cell chemistry; reaction products do not all travel from one electrode to the other.
In a nickel-cadmium cell, cadmium is the negative electrode during discharge. Cadmium reacts with OH – and releases two electrons to form cadmium hydroxide.
The positive active material is nickel oxyhydroxide. It accepts electrons and is reduced to nickel hydroxide while hydroxide ions are regenerated according to the shown half-reaction.
Charging of Battery
During charging of battery, a controlled DC source drives current opposite to the discharge direction. Connect a chemistry-compatible charger with its negative terminal to the battery’s negative electrode and its positive terminal to the positive electrode. The charger must enforce the specified current, voltage, temperature and termination limits.
The charger supplies electrons to the negative electrode, where reduction restores charged negative material.
At the positive electrode, the charger removes electrons and oxidation restores charged positive material. Under strict electrochemical naming, the negative electrode is the cathode during charge and the positive electrode is the anode. Battery documents often keep the physical labels negative and positive to avoid this role reversal.
For a rechargeable nickel-cadmium cell, charging of battery reduces cadmium hydroxide at the negative electrode back towards cadmium and releases hydroxide ions (OH–) according to the reverse half-reaction.
At the positive electrode, nickel hydroxide is oxidised towards nickel oxyhydroxide while the corresponding reaction involves hydroxide ions and water.
Controlled charging of battery restores usable chemical potential but does not reverse every ageing or side reaction. Overcharge can generate heat and gas, so charge termination and backup protection are required.
