Measurement of the specific gravity of the electrolyte is a good indicator for state of charge estimation in lead-acid batteries. Unfortunately, the specific gravity of the electrolyte is not measurable in VRLABs. VRLAB design does not allow access to the electrolyte, therefore, the phrase "maintenance- free." Electrolyte replenishment is not possible in VRLABs.
Electrochemistry — Valve Regulated Lead Acid Batteries
Some within the industry identify valve regulated lead-acid batteries as sealed. VRLABs vent when overcharged. During discharge, the pure lead, lead dioxide, and sulfuric acid in the electrolyte interact to form lead sulfate and water.
During the charge sequence, the lead sulfate component is converted to the original state and sulfuric acid is reintroduced into the electrolyte.
Under ideal conditions, both negative and positive electrodes would have 100 percent charge efficiencies. This is not the case. In the real world, each electrode has different efficiencies.
When the conversion of lead sulfate is complete, on the more efficient (negative) electrode, hydrogen gas evolves as the water in the electrolyte begins to break down. Oxygen evolves from the positive electrode shortly thereafter. If the pressure created by gassing reaches a high enough value, the gases are vented overboard, hence the term "valve regulated." Prior to venting, the hydrogen gas is collected within the container. Venting reduces the overall volume of electrolyte and will eventually result in cell dry out.
A 10 percent reduction in electrolyte volume will translate into a 20 percent reduction in performance. The valve in VRLAB technology is intended to assist recombination of oxygen, thereby reducing the amount of water hydrolyzed during the charge process. It is inevitable that some of the generated gasses will escape, however, because the reaction is not 100 percent efficient.
Electrochemistry — Nickel Cadmium Aircraft Batteries
During charge, the active material (cadmium hydroxide) of the negative plate is converted to pure cadmium. Nickel hydroxide in the positive plate is converted to nickelic oxyhydroxides. The electrolyte is not directly involved in the charge/discharge process but serves as a medium for ionic transfer. Specific gravity of the potassium hydroxide/water solution remains virtually unchanged during these processes.
Operations - Valve Regulated Lead-Acid Batteries
During constant current discharge, the VRLAB exhibits a continuous reduction in voltage. This is dependent on the conversion rate of the active materials in the plates and electrolyte as the discharge continues. Here again, temperature has significant influence on discharge characteristics. At the end of discharge there is a sharp reduction in voltage. Basically, the unit runs out of active materials to convert. Both positive and negative electrodes are reduced to lead sulfate and the electrolyte is reduced to water.
During recharge, an initial sharp voltage rise occurs. This is primarily due to the change in internal resistance as the lead sulfate begins to reconvert. As the charge continues, the voltage rate-of-change slows to a gradual upward slope — as the specific gravity of the electrolyte increases.
At the end of charge, another sharp voltage rise is observed as most of the lead sulfate is converted to its original state and gas begins to form on the surface of the electrodes. Water begins to break down at this point. Substantial gassing occurs as the overcharge continues. The sharp voltage rise and gas production is due to the inequitable charge characteristics of the negative and positive plate.
Operations - Nickel Cadmium Aircraft Batteries
During constant current discharge, a NCAB will provide an almost flat curve, with a slight downward slope. When most, if not all, of the active materials convert, as discussed in the electro-chemistry section, the voltage then begins to drop off rapidly. The electrolyte remains a solution of potassium hydroxide and water.
During recharge, a significant voltage rise is observed, followed by a slow continuous rise to a transition voltage. At this point the negative plate is fully charged and begins to generate hydrogen gas. As the over-charge continues, a rapid voltage increase is observed due to the increased impedance of the cell.
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