The state of charge of lead-acid batteries can be determined from the equation: as the acid content in the electrolyte increases after charging, the density also increases; when the electrolyte is discharged, due to the formation of compound PbS0 in the form of yellow acid in the graph, it is stored in the electrode plate, and the sulfuric acid in the electrolyte decreases. This reduction has a strict quantitative relationship in terms of quantity: for every 2 electrons flowing through the external circuit, it must be consumed? A molecule of acid. This provides a possibility that during the discharge process, the decrease in density of the sulfuric acid electrolyte can be used to determine the amount of electricity released by the battery. Conversely, during the charging process, the increase in electrolyte density can also be used to determine the effective amount of electricity charged in the battery. In many cases, the process stipulates the use of measuring electrolyte density to determine the state of charge of the battery, and the theory is based on the above. In large capacity fixed batteries, a density meter is usually installed, which stores plastic balls of different densities in small compartments according to their density sizes. As the electrolyte density changes with the degree of charge and discharge, the corresponding plastic balls will have up and down displacement. The density of the electrolyte can be easily determined based on the values on the surface of the board, which in turn determines the state of charge of the battery. On the side of some batteries, there are colored balls with different densities of red, yellow, and green, which represent three states of charge: "should be charged", "still usable", and "fully charged". A regular suction type float densitometer can also serve the same purpose. Obviously, the above measurement judgments must be correct under the premise that the original electrolyte density value in the battery is known, the total amount of acid in the battery remains unchanged, the electrolyte density is uniform up and down, and the electrode plate is not broken or powdering. These conditions no longer exist after the battery is used, and the deviation between various parameters and the original is increasing, so the accuracy of this detection method is also decreasing.