Shortly thereafter, oxygen begins to evolve at the positive plate. Little or no recombination actually takes place. This results in the water of the electrolyte being broken down by hydrolysis. The act of reversing current flow, or charging as it is commonly known, in an electro-chemical couple is endothermic by nature.
Charging cools up to the point of gas generation/recombination. At the onset of gassing the reaction changes from endothermic to exothermic. In other words, the cell/battery begins to generate heat.
Whenever gassing occurs, the temperature increases presenting the potential for thermal run-away regardless of electro-chemistry. The higher the recombination rates, the greater the potential for thermal run-away during charge.
Thermal run-away (Vicious Cycling) Thermal run-away is a potential problem in any electro-chemistry that involves gaseous evolution and/or recombination. This is especially true at higher temperatures. Oxygen recombination, high temperature operating conditions, and a limited amount of free electrolyte (acting as a heat sink) all contribute to the probability of thermal run-away. Any increase in the temperature of an electro-chemical couple will reduce overall resistance.
In a constant potential (DC) system, a decrease in impedance will result in an incremental increase in current draw. This in turn increases gas generation and the cycle accelerates. Eventually the heat generated will become destructive. Some VRLAB manufacturers limit operation above 84 F (30 C), warning that long-term exposure to temperatures above this value can shorten battery life.
Advanced NCAB manufacturing techniques and improved materials have significantly reduced the likelihood of thermal run-away in today's NCAB. In addition, the significantly larger electrolyte reserves in NCAB provide excellent heat dissipation.
The introduction of temperature sensors and 20 cell batteries in the early 1970s also had a significant impact in reducing the incidence of thermal run-away. True incidents of thermal run-away have been virtually eliminated since then. It is important to note that a hot battery is not necessarily an indication that a thermal event has occurred.
All lead-acid batteries are subject to the same physical laws regardless of whether a flooded variety or valve regulated. The chemical process is the same. VRLABs are just as prone to a phenomenon, known as sulfating, as any other lead-acid technology.
To counter-act sulfating, manufacturers of VRLAB stress the importance of boost charging on a regular basis while the battery is in storage. One recommends a boost charge frequency of every 90 days; another stresses the importance of purchasing the "freshest" battery available to ensure the longest possible service life.
Nickel cadmium aircraft batteries do not suffer from extended long-term storage.
VRLABs employ a microporous glass mat to immobilize the electrolyte within the cell. While this provides excellent aerobatic capabilities, it can be detrimental in storage. During storage of VRLABs, the electrolyte stratifies. Stratification increases the probability of sulfating in the area of highest concentration. Performance is, therefore, significantly reduced.
Charging has little or no effect in reconstituting the electrolyte to its original saturation of a VRLAB. NCAB electrolyte will stratify if stored for an extended period of time; however, the electrolyte is not immobilized and can be reconstituted by repeated cycling to restore performance.
Sudden death active material and inter-cell connection corrosion is another issue to be considered when discussing lead-acid technology. Lead-acid batteries work because of the corrosive effects of sulfuric acid on the active materials: lead dioxide and pure lead. The corrosive action of the acid causes the active material to swell on the positive electrode and has been documented to cause shedding of active materials. Corrosion will lead to internal shorts between the electrodes and/or opens at inter-cell connections. A short or an open connector will render the unit inoperable The electrolyte in NCAB has no corrosive effect on the active material or on the plate, tabs, or terminal connections.
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