How does a battery work? (VW Golf 1)

The Volkswagen, like other cars, has a lead-acid battery. When the battery is discharged, chemical energy is converted into electrical energy, and when it is charged, the reverse process occurs.

The lead plate serves as an electrode and comes into contact with dilute sulfuric acid (electrolyte). The plate, under the influence of the solution, releases positive ions, i.e. electrically charged particles, into the electrolyte. Due to this, an electrical voltage appears between the lead plate and the electrolyte.

In practice, however, they do not rely on this "voluntary" transition of charged particles, but supply the battery with voltage and charge it. This produces the following effect: the lead sulfate of the plates of the battery being charged turns into lead dioxide at the positive electrode, and into porous lead at the negative electrode. At the same time, sulfuric acid is reduced in the electrolyte. The external sign of a fully completed charging process is the rise of gas bubbles.

During discharge, the reverse process occurs. The lead dioxide of the positive plate and the porous lead of the negative plate are converted back into lead sulfate, and the sulfuric acid is weakened and water is formed. Therefore, during discharge, the density of the electrolyte decreases.

The original can be found at VWManual