Research On Liquid Density Measurement Of Lead-acid Batteries By Optical Critical Angle Method

Since the invention of the lead-acid battery,the lead-acid battery has been improved for156years,and has become the most widely used chemical batteries. Compared to other chemical batteries, the lead-acid battery has a large capactiy,high stable performance and efficiency and long life.Commonly the lead-acid battery has been used in the back source of a car, the telecommunication industry and the computer UPS field. Large lead-acid batteries become the key part of many special fileds, such as the electric emergency power, the ship electric propulsion sytem, the internet, the financial data backup system, the telecommunication system and the locomotive power system.The preformance of the lead-acid battery varies in the production process and long-term utilization.If serval lead-acid batteries in the battery pack are damaged, the electrical load will be unbalanced,the productivity and life of the battery pack will reduce,and above special fields will be influenced. In order to keep the lead-acid battery in normal work condition,timely detect bad batteries and extand the life the battery pack,it is imporotant to online monitor the SOC (state of charge) of each lead-acid battery with high precision.This thesis studies the physical and chemical characteristics of the lead-acid battery and the battery electrolyte, proposes the online density measurement method of the battery electrolyte by optical critical angle method,and lays the foundation for the high accruate SOC measurement of the lead-acid battery.The main research are summarized as following:Firstly, this thesis researches on the changing composition characterstic of the battery electrolyte in the charging and dis charging process of the lead-acid battery.The SOC of the lead-acid battery is directly related to the density of the battery electrolyte,and the density correspondes to the refractive index.Several refractive index measurement methods are compared in the actual battery electrolyte. This thesis demonstrates the density measurement of the battery electrolyte by optical critical angle method,and establised the relationship between the bright-dark bourdary line of the reflection.Secondly, this thesis studies the high accurate determination of the bright-dark boudary line on the critical angle refractometer. This thesis proposes the improved differential algorithm to determinat the boundary line at sub-pixel accuracy on the basis of the differential algorithm.The improved differential algorithm makes up for non-response of the differential algorithm in small refractive index changing.The theoretical calculation and experimental results verifiy the effectiveness of the algorithm. And this thesis proposes a by optical heterodyne method to enhance the SNR.Also this thesis researches on the salinity of the sodium chloride solution by the improved differential algorithm and the refractive index mesurement system.Thirdly, this thesis analysises the reflective ratio curve of the critial angle device at different refractive index values.The incident angle and the refractive index of prism restricts the upper limition.When the refracitve index of a liquid is more than nprismsin(θmax)> the reflective ration curve is gradual changing. This thesis demonstrates the fourier power spectrum and the centroid algorithm to extand the mesurement range.The experimental results show these algorithms can significantly extand the measurement range at10-4RIU accuracy.Fourthly, this thesis designes a density measurement sensor based on the critical angle method for the battery electrolyte in detail.And this thesis proposes a divergent point source to solve the maldistribution of the light intensity. Experimental results the maxium measurement error of the sensor is0.002g/cm3,meets the technical requirement.Fifthly, several prototype density measurement sensors are assessed by comprehensive tests and field experiments.The sensor has been tested by repeated heating and cooling for one month in the lab and three months in the real environment to verify the acid proof and mechanical seal. This thesis analysises the influence of the bubble,the oil pollution and the impurity in the battery electrolyte,and demonstrates a method to reduce the bad influence.6sensors are tested by online measuremetn for three month in the real environment without calibration and maintenance,the experimental results show that sensors gain remarkable performances and the maximal measurment error of sensors is less than0.005g/cm3.