This battery contains a series of internal cells that work together to produce power. Each individual cell contains two different types of metal plates sitting in a liquid. The negative anode plate is made from a spongy form of solid pure lead.
The positive cathode plate is coated in a dark brown chemical called lead dioxide. Both plates sit entirely submerged in a strong liquid bath of sulfuric acid. When you turn your car key, the battery releases a massive surge of electrons.
The lead on both plates reacts with the acid to form solid lead(II) sulfate. This chemical reaction also creates water that dilutes the strong acid pool. When your car engine is running, it pushes electricity back into the battery.
This fresh electrical current forces the chemical reaction to run in complete reverse. The solid lead sulfate turns back into pure lead and solid lead dioxide. The water also converts back into strong sulfuric acid for the next engine start.
Students often think the battery somehow creates power out of thin air. In reality, it simply stores chemical energy and releases it as electrical energy later. These batteries are very heavy but remain popular because they are cheap and reliable.