What happens when a battery discharges?

Questions I am interested in:

• What determines the amount of current a battery can supply?

• What changes when a battery discharges - voltage, max current, or both?

• What is the best predictor of the remaining capacity of e.g. AA battery?

When a battery is in operation an electrochemical reaction known as a Redox reaction takes place. In it, an oxidiser $$\text{O}$$ is oxidised (loses electrons) and a reducing agent $$\text{R}$$ is reduced (gains electrons).

Simplified we can write:

$$\text{O}\to \text{O}^{n+}+n\text{ e}^-$$ $$\text{R} +n\text{ e}^-\to \text{R}^{n-}$$

Note that neutrality is maintained at all times.

By running the reaction with the oxidiser and reducing agent in separate 'compartments' the flow of electrons can be 'harvested' as current.

The voltage a battery (or better put, a cell) can deliver depends exclusively on the type of Redox reaction the cell employs. For example, a typical manganese oxide/zinc cell uses:

$$\text{Zn}+2\text{OH}^{-}\to \text{ZnO}+\text{H}_2\text{O}+2\text{ e}^-$$ $$2\text{MnO}_2 +\text{H}_2\text{O}+2\text{ e}^-\to \text{Mn}_2\text{O}_3+2\text{OH}^{-}$$

and delivers about $$\text{1.43 }\mathrm{V}$$.

Cells 'run out' when either the oxidiser or reducing agent has been depleted.

• The max current is the ratio of its voltage V to internal resistance R, as in I = V/R
• Its voltage remains approximately constant but follows a decay curve characteristic of the battery technology. Max current likewise.
• The remaining capacity is the total capacity (specified in Ampere hours) less the charge supplied to date, calculated as Amps x hours.