A battery works by storing electrical energy from the alternator as chemical energy. This chemical energy is reconverted on-demand from the electrical system.
It develops voltage from the chemical reaction produced when two dissimilar metals such as the positive and negative plates, are immersed in the electrolyte (a solution of sulphuric acid and water). In a typical starter battery, the voltage is approximately 2 volts per cell, for a total of 12 volts. Electricity flows from the battery as soon as there is a circuit between the positive and negative terminals. This happens when any load that needs electricity such as a radio, immobiliser or alarm is connected to a battery.
Most people don’t realise that a lead acid battery operates in a constant process of charge and discharge. When a battery is connected to a load that needs electricity, such as the starter in your car, current flows from the battery and it begins to be discharged.
A battery becomes charged when current flows back into it, restoring the chemical difference between the plates. This happens when you are driving without any accessories and the alternator puts current back into the battery.
As a battery discharges, the lead plates become more chemically alike, the acid becomes weaker and the voltage drops. Eventually the battery is so discharged that it can no longer deliver electricity at a useful voltage. You can recharge a discharged battery by feeding electrical current back into the battery. A full charge restores the chemical difference between the plates and leaves the battery ready to deliver its full power.