The potential difference is set up by the electromotive force generated by chemical reactions. The electrolyte used in automotive batteries is a strong solution of sulfuric acid and distilled water. The chemical reaction inside a car battery can be reversed. Therefore, a car battery is known as a secondary cell. A secondary cell battery can be recharged by applying a reverse voltage.
Car batteries have three main jobs.
Starting the car is most demanding of these jobs. In order to start a car the battery must supply 100 to 200 amps of current over a short period of time. This can become very difficult during the winter. The chemicals in the battery can become too cold to stir up the power necessary to start the car motor. The second job of the car battery is to keep itself charged by providing current for the car generator's field.