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Lead-Acid Batteries |
R. W. Stuart Before you got to the RC field and used a battery for fuel pumping and for engine cranking, you used a battery to start your car. Since we depend on lead-acid batteries, both "wet" cell batteries and "gel" cell batteries, it might be useful to review the lead-acid system. A battery is made of cells connected to provide the voltage and current to do a job. Most modern lead-acid batteries are nominally 12 VDC, but some older 6 VDC batteries are still around and most full size aircraft batteries are 24 VDC. All electrochemical cells are composed of two different "metals" in chemical contact with any electrolyte. Selection of the electrolyte depends on the chemistry of the materials used in the battery and the discharge-charge chemical processes involved. In the lead-acid system the two metals are lead for the negative plate and lead dioxide, acting as a "metal", for the positive plate. The electrolyte is sulfuric acid, either liquid or gelled. The plates are constructed of lead alloy coated with lead dioxide (positive plate) and spongy pure lead (negative plate), all sandwiched into a grid with insulating spacers between plates. In a fully charged cell the emf is 2.1 volts and the specific gravity of the sulfuric acid is 1.31 at 80 degrees F. The letters emf stand for electromotive force and is proper nomenclature-- TV Channel 8 improperly used "emf" for "electromagnetic field" when they produced their series on magnetic fields around power lines. A discharged cell has lead sulfate, which is poorly soluble, adhering to both plates and an acid gravity of 1.16 with a no-load emf of 1.75 volts. As the battery is charged the positive plate becomes lead dioxide, the negative plate becomes spongy pure lead, the emf becomes 2.1 volts, and the sulfate ions produced "boot" the acid gravity to 1.31. The most imperative thing to do to live with lead-acid batteries is to recharge the battery at once when totally discharged and to recharge partially discharged batteries at regular intervals. As the lead sulfate lays on the plate (discharged condition) it "hardens" with time and temperature to a form that cannot be chemically converted by the charging voltage. Once the lead sulfate "hardens" the battery is ruined- done- kaput!!. Recharge lead-acid batteries the same day they show serious discharge and recharge several times per year even when on the shelf. Charging the batteries requires a charger designed for lead- acid: 14.7 volts for a 12 volter and 7.3 volts for a 6 volter. Fluid batteries are charged until the built-in indicator reads charged or until the gravity is 1.3. Gel cells should be charged until the surface of the battery being charged feels slightly warmer than a similar surface at ambient temperature. Also the warm surface test may be used for charge end indication on fluid filled batteries. Charge at a mild rate as quick charge is tough on lead-acid batteries. Gel cells have the obvious advantage of not being sensitive to gravity orientation and sedimentation is not a gel cell problem. I have had gel cell batteries in service for more than 15 years-- just keep them charged. Gel cell batteries are easy to repair (replace a cell) and easy to rebuild into 6 or 12 volt assemblies. Keep the lead-acid batteries, both gel cell and fluid, charged and they will give good service for many years. Dead shorts may explode a battery or warp the plates into a shorted condition, so be careful. Also, a metal fuel can dropped across the terminals of a battery could cause instant vaporization of the metal of the can, boiling of the fuel, and ignition-- resulting in an "eargeshsplitten loudenboomer". Again, be careful. |
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