Marine Engine Q & A- Electrical-Nickel-Cadmium Battery Gassing, Topping up,

Gassing
The gases evolved during charging are oxygen (at the positive plates; and hydrogen (at the negative plates). Rate of production of gas increases in periods of overcharge. When hydrogen reaches a proportion of about 4% and up to 74% it constitutes an explosive mixture. Good ventilation of battery compartments is therefore necessary to remove gas. Equipment likely to cause sparking or arcing must not be located or introduced into battery spaces. Vent caps are non-return valves, as shown diagrammatically (Fig. 1.2) so that gas is released but contact by the electrolyte with the atmosphere is prevented. The electrolyte readily absorbs carbon dioxide from the atmosphere and deterioration results because of the formation of potassium carbonate. For this reason, cell vent caps must be kept closed
Topping Up
Gassing is a consequence of the breakdown of water in the electrolyte. This together with a certain amount of evaporation means that topping up with distilled water will be necessary from time to time. High consumption of distilled water would suggest overcharging.
Electrolyte
Potassium hydroxide solution is strongly alkaline and the physical and chemical properties of potassium hydroxide closely resemble those of caustic soda (sodium hydroxide). It is corrosive, so care is essential when topping up batteries or handling the electrolyte. In the event at skin or eye contact the remedy is to wash with plenty of clean water (for 15 minutes) to dilute and remove the solution quickly. Speed is vital to prevent burn damage; and water, which is the best flushing agent, must be readily available. Neutralizing compounds (usually weak acids) cannot always be located easily, although they should be available in battery compartments.
Specific of electrolyte in a Ni-Cd cell is about 1.210 and this does not change with charge and discharge as in lead-acid cells. However, over a period of time the strength f. the solution will gradually drop and renewal is necessary at about a specific gravity of 1.170


Containers
The electrolyte slowly attacks glass and various other materials. Containers are therefore of welded sheet t which is then nickel plated or moulded in high-impact polystyrene casings are preferred when batteries are subject to shock and vibration. Hardwood crates are used to keep the cells separate from each 2t and from the support beneath. Separation is necessary cause the positive plate assembly is connected to the steel casing.