Quantifying secondary reactions in VRLA batteries. What purity standards and other factors are necessary to prevent negative plate discharge?
In a VRLA cell with a fully developed oxygen cycle, the negative plates will suffer discharge during float service if their self-discharge rate is too high to be balanced by the corrosion rate of the positive grids. Positive grids have been developed with very low corrosion rates. However, there is little quantitative data available on negative plate self-discharge rates that are required to balance such low corrosion rates. This paper presents values measured from several different battery brands, American and European. The data shows clearly that some production batteries have high negative self-discharge rates that account for the observed tendency for self-discharge in service. Surprisingly, the data also shows that other brands have achieved extremely low self-discharge rates-low enough to be compatible with the best positive grids that the authors have tested. These results are very encouraging for VRLA technology and confirm that the principle can be realized. The focus now shifts to the practical issues: finding the source of the impurities that increase hydrogen evolution and weighing the cost and practicality of reducing them.
Hydrogen sulfide in VRLA cells
This investigation of hydrogen sulfide (H 2 S) in VRLA cells produced three major results. First, VRLA cells produce significant amounts of H 2 S even at normal float voltages. Second, the lead dioxide in the positive plates absorbs much of this H 2 S. Third, this H 2 S cycle in VRLA cells results in an equilibrium concentration level of no more than 1 ppm of H 2 S in the gas space of a typical cell. The data further suggests that even this low amount may decline over time.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.