Length Of Service Life Research Articles

Heat Effects during the Operation of Lead-Acid Batteries

Thermal events in lead-acid batteries during their operation play an important role; they affect not only the reaction rate of ongoing electrochemical reactions, but also the rate of discharge and self-discharge, length of service life and, in critical cases, can even cause a fatal failure of the battery, known as “thermal runaway.” This contribution discusses the parameters affecting the thermal state of the lead-acid battery. It was found by calculations and measurements that there is a cooling component in the lead-acid battery system which is caused by the endothermic discharge reactions and electrolysis of water during charging, related to entropy change contribution. Thus, under certain circumstances, it is possible to lower the temperature of the lead-acid battery during its discharging. The Joule heat generated on the internal resistance of the cell due to current flow, the exothermic charging reaction, and above all, the gradual increase in polarization as the cell voltage increases during charging all contribute to the heating of the cell, overtaking the cooling effect. Of these three sources of thermal energy, Joule heating in polarization resistance contributes the most to the temperature rise in the lead-acid battery. Thus, the maximum voltage reached determines the slope of the temperature rise in the lead-acid battery cell, and by a suitably chosen limiting voltage, it is possible to limit the danger of the “thermal runaway” effect. The overall thermal conditions of the experimental cell are significantly affected by the ambient temperature of the external environment and the rate of heat transfer through the walls of the calorimeter. A series of experiments with direct temperature measurement of individual locations within a lead-acid battery uses a calorimeter made of expanded polystyrene to minimize external influences. A hitherto unpublished phenomenon is discussed whereby the temperature of the positive electrode was lower than that of the negative electrode throughout the discharge, while during charging, the order was reversed and the temperature of the positive electrode was higher than that of the negative electrode throughout the charge. The authors relate this phenomenon to the higher reaction entropy change of the active mass of the positive electrode than that of the negative electrode.

The use of ultrasonics in the optimisation of welding processes

Summary A non‐destructive method for measurement of residual welding stress is described. The results are in good agreement with another proven semi‐destructive method. Excellent practical perspectives for a vast range of parameters, important to welders, with a direct influence on length of service life are given. The welding process creates complex residual stress fields which may now be evaluated experimentally by various semi‐destructive or destructive methods. In the case of welded compounds, these stresses are often high and may reach the elastic limit of the base metal, thus influencing the mechanical properties of welded assemblies. A new non‐destructive, ultrasonic technique of evaluating stresses is presented here. The potential uses of this technique are to measure near surface stress and apply it to the optimisation of welding processes.

気象因子を考慮したコンクリートの凍害劣化予測

This study is aimed to estimate the frost damage of concrete under natural weatering conditions. An empirical formula based on the meteorological factors was obtained by means of multiple regression analysis of the results of freezing-and-thawing tests which were carried out under several experimental conditions to evaluate the effects of the freezing-and-thawing conditions (lowest temperature, keeping time of lowest temperature, cooling rate) and the curing conditions of concrete (water content). The formula describes the number of equivalent freezing-and-thawing cycles to ASTM C666 A. Applying meteorological data in some cities of Japan to this formula, the number of equivalent cycles in one year and the length of service life were calculated. The correspondence of estimated value to the results of exposure test for 15 years was investigated.