Working principle: The positive electrode material of a nickel cadmium battery is a mixture of nickel hydroxide and graphite powder, the negative electrode material is a sponge mesh sieve like cadmium powder and cadmium oxide powder, and the electrolyte is usually a solution of sodium hydroxide or potassium hydroxide. When the ambient temperature is high, use a sodium hydroxide solution with a density of 1.17-1.19 (at 15 ℃). When the ambient temperature is low, use a potassium hydroxide solution with a density of 1.19-1.21 (at 15 ℃). At temperatures below -15 ℃, use a potassium hydroxide solution with a density of 1.25-1.27 (at 15 ℃). To balance low-temperature performance and charge retention capacity, sealed nickel cadmium batteries use a potassium hydroxide solution with a density of 1.40 (at 15 ℃). In order to increase the capacity and cycle life of the battery, a small amount of lithium hydroxide is usually added to the electrolyte (approximately 15-20g per liter of electrolyte). After charging a nickel cadmium battery, the active substance on the positive electrode plate becomes nickel hydroxide [NiOOH], and the active substance on the negative electrode plate becomes metal cadmium; After the discharge of a nickel cadmium battery, the active substance on the positive electrode plate becomes nickel hydroxide, and the active substance on the negative electrode plate becomes cadmium hydroxide. The chemical reaction during the discharge process of the chemical reaction (1) The negative electrode loses two electrons of cadmium on the negative electrode and becomes a divalent cadmium ion Cd2+. Then, it immediately combines with the two hydroxide ions OH - in the solution to generate cadmium hydroxide Cd (OH) 2, which is deposited on the negative electrode plate. (2) The active substance on the positive electrode plate of the positive electrode reaction is nickel hydroxide (NiOOH) crystal. Nickel is a positive trivalent ion (Ni3+), and every two nickel ions in the lattice can obtain two electrons transferred from the negative electrode from the external circuit, generating two divalent ions 2Ni2+. At the same time, two hydrogen ions ionized by every two water molecules in the solution enter the positive electrode plate and combine with two oxygen negative ions on the lattice to generate two hydroxide ions. Then, together with the two existing hydroxide ions on the lattice, they form two nickel hydroxide crystals with two divalent nickel ions. During the charging process, chemical reactions occur. When charging, the positive and negative electrodes of the battery are connected to the positive and negative electrodes of the charger, respectively. The battery undergoes an electrochemical reaction that is completely opposite to the discharge process, that is, the negative electrode undergoes a reduction reaction and the positive electrode undergoes an oxidation reaction. (1) During negative electrode reaction charging, cadmium hydroxide on the negative electrode plate is first ionized into cadmium ions and hydroxide ions. Then, cadmium ions obtain electrons from the external circuit, generating cadmium atoms that attach to the electrode plate, and hydroxide ions enter the solution to participate in the positive electrode reaction. (2) Under the action of an external power source, the positive electrode reacts with two divalent nickel ions in the nickel hydroxide lattice on the positive electrode plate, each losing one electron to form a trivalent nickel ion. At the same time, each of the two hydroxide ions in the lattice releases one hydrogen ion, leaving the oxygen negative ion on the lattice. The released hydrogen ion combines with the hydroxide ions in the solution to form water molecules. Then, two trivalent nickel ions combine with two oxygen negative ions and the remaining two hydroxide ions to form two nickel hydroxide crystals. At the end of battery charging, the charging current will cause a reaction of decomposing water in the battery, and a large amount of oxygen and hydrogen will be precipitated on the positive and negative plates, respectively. From the above electrode reactions, it can be seen that sodium hydroxide or potassium hydroxide do not directly participate in the reaction, but only have a conductive effect. From the perspective of battery reaction, water molecules are generated during the charging process and consumed during the discharge process. Therefore, the electrolyte concentration changes very little during the charging and discharging processes, and the degree of charging and discharging cannot be detected using a density meter