Marine Engine Q & A- Electrical-sealed Nickel-Cadmium Batterries

Sealed Nickel-Cadmium Batteries
Gassing occurs as a conventional battery approaches full charge, and increases during any overcharge due to electrolysis of water in the electrolyte by the current supplied but no longer being used in charging. The gas is released through the vent to prevent pressure build-up and this loss together with loss from evaporation makes topping UP necessary. While on charge, the active material of the plates is being changed, but when the change is complete and no further convertible material remains, the electrical charging energy starts to break down the electrolyte. Oxygen is evolved at the positive plates and hydrogen at the negative
Sealed batteries (Fig. 1.3) are designed to be maintenance-free and although developed from and having a similar chemical reaction to the open type, will not lose water through gassing or evaporation. The seal stops loss by evaporation and gassing is inhabited by modification of the plates.
Sealed cells are made with surplus cadmium hydroxide in the negative plate so that it is only partially charged when the positive plate is fully charged. Oxygen is produced by the charging current at the positive plate (40H ˉ2Hˉ2O+4eˉ+O2) but no hydrogen is generated at the negative because some active material remains available for conversion. Further, the oxygen from the positive side is reduced with water at the negative plate (2Hˉ2O+4eˉ+O2 40H ˉ) so replacing the hydroxyl ions used in the previous action. The process leaves the electrolyte quantity unaffected. The hydroxyl ions acting as current carriers within the cell travel to the positive electrode.
Sealed batteries will accept overcharge at a limited rate indefinitely without pressure rise. Charging equipment is therefore matched for continuous charging at low current or fast charging used with automatic cut-out to prevent excessive rise of pressure and temperature. Rise of pressure temp and voltage all occur as batteries reach the overcharge area but the last two are most used as signals terminate the full charge.

Determination of copper, cadmium and lead in seawater and mineral water by flame atomic absorption spectrometry after coprecipitation with aluminum hydroxide [An article from: Analytica Chimica Acta]