So the overall reaction of the discharging lead acid battery is the reaction of and lead dioxide electrodes with acid to form and water. This is a spontaneous redox reaction which converts chemical into electric energy.
• It is because the reaction will occur on its own: you don’t need to heat it or provide an external potential difference to force it to occur.
• It is a redox reaction: it involves the transfer of from one chemical to another.
• It converts into energy as the chemical reaction creates a across the electrodes.