[[TitleIndustry]]

Lead-acid battery charging and discharging working principle

Date:Aug 31, 2019

The lead-acid battery is composed of an electrolyte in which the positive and negative plates are infiltrated between them. To be more detailed, the positive and negative plates and the electrolyte form their respective “half cell". In the respective half-cell configuration, the positive plate has a positive potential and the negative plate has a negative potential. The basic unit can be regarded as the above two. The half-cell battery is made up of a positive electrode plate-electrolyte-electrolyte-negative electrode plate. The relative potentials of positive and negative pole are 2V. The 6 single cells are connected in series to be the 12V battery which is commonly used in electric vehicles. 

When the lead-acid battery is fully charged, the material on the positive electrode plate is lead dioxide (PbO2), the material on the negative plate is fluffy lead (Pb), and the density of electrolyte sulfuric acid (H2SO4) is about 1.33 g/cm3 (refers to Lead-acid batteries for electric vehicles, the density of lead-acid batteries for other uses is slightly lower). During the discharge process, electrons are obtained by the lead dioxide on the positive electrode of the discharge circuit, and the lead on the negative electrode plate loses electrons, respectively producing divalent lead (Pb2+) and reacting with sulfuric acid in the electrolyte to precipitate on the respective plates and form Lead sulfate (PbSO4), the xygen ions and hydrogen ionization react and form water. As the discharge progresses, the electrolyte concentration decreases, and lead sulfate on the positive and negative plates gradually accumulates. When this process develops to a certain extent, the discharge polarization phenomenon becomes more and more serious, the potential of the positive electrode plate tends to be more and more negative, the potential of the negative electrode plate tends to be more positive, and the density of sulfuric acid in the electrolyte becomes lower and lower. The battery voltage is low to the termination voltage and the discharge will be terminated.

During the charging process, the divalent lead ions in the solution transfer electrons to the external circuit for oxidation to positive tetravalent lead (Pb4+), while the oxygen ions in the electrolyte water (HO2) and the positive tetravalent lead enter the positive electrode plate. grid. Since the divalent lead in the solution is consumed, the lead sulfate on the positive electrode plate is continuously dissolved, and the lead dioxide is continuously formed; the lead sulfate on the negative electrode plate is first dissolved into divalent lead and sulfate (SO4), and the divalent lead is charged. The electrons from the loop are reduced to lead on the negative plate. At the same time, hydrogen and sulfate remaining in the electrolyte synthesize sulfuric acid. As the charging progresses, the lead sulfate on the electrode plate is gradually dissolved, and the electrolyte concentration is continuously increased. When this process is carried out to a certain extent, the charging polarization phenomenon becomes more and more serious, and the positive and negative plates successively separate oxygen and hydrogen, respectively, and the charging current is more and more hydrolyzed, and the density of sulfuric acid in the electrolyte is getting higher and higher, the positive electrode plate The potential tends to be the most positive, the potential of the negative plate tends to be the most negative, the battery voltage continues to rise, and eventually returns to the above fully charged state.

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