1、Basic Electrochemical Principles 
The charging and discharging process of maintenance-free lead-acid batteries is based on electrochemical redox reactions. When discharging, metal lead is the negative pole, the oxidation reaction occurs, the electrode reaction formula is Pb +SO42-===PbSO4+ 2e-; lead dioxide is the positive pole, the reduction reaction occurs, the electrode reaction formula is PbO2+2e-+4H++SO42-=PbSO4+2H2O, and the chemical energy is converted into electrical energy. When charging, under the action of external power supply, the reaction proceeds in the reverse direction, electrical energy is converted into chemical energy, and lead sulphate is oxidised to lead dioxide and reduced to lead at the positive and negative electrodes respectively, restoring the battery to its initial state.  

2、Oxygen cycle principle 

Maintenance-free lead-acid batteries usually adopt cathode absorption sealing technology. At the late charging stage, oxygen will be generated at the positive electrode. Due to the design of the internal structure of the battery, the oxygen can diffuse quickly to the negative electrode and react with the lead at the negative electrode to generate lead sulphate, which inhibits the generation of hydrogen and reduces the decomposition of water, thus reducing the consumption of electrolyte. 

3、Material and Structure Principle 
Adopting lead-calcium alloy grid frame, compared with the traditional lead-antimony alloy grid frame, it can change the reverse electromotive force of the battery after fully charging, reduce the overcharge current, and lower the liquid gasification rate, thus reducing the loss of electrolyte. At the same time, the battery adopts sealed structure, such as valve-regulated sealed lead-acid battery is equipped with one-way valve, when the internal pressure exceeds a certain value, the valve opens to release the gas, and then closes automatically, preventing the entry of external air, which also avoids the leakage of the electrolyte and the large amount of sulphuric acid gas discharged. 

4、Depleted Liquid Design Principle 
Some of the maintenance-free lead-acid batteries adopt the lean design, where the acid is adsorbed on the special partitions (such as asbestos or glass fibre partitions) between the plates, which avoids the generation of acid mist and a large amount of gases due to the flow of electrolyte during charging and discharging of the traditional batteries, and reduces the depletion of the electrolyte and the pollution of the surrounding environment.