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Nickel-cadmium battery vented cell type
Vented cell (wet cell, flooded cell) is a type of nickel-cadmium battery that is used when large capacities and discharge rates are required. The term vented refers to a difference to common NiCd batteries which are sealed.
They are used in aviation, rail and mass transit, backup power for telecoms, engine starting for backup turbines etc. Using vented cell NiCd batteries results in reduction in size, weight and maintenance requirements over other types of batteries. Vented cell NiCd batteries have long lives (up to 20 years or more, depending on type) and operate at extreme temperatures (from -40°C to +70°C).
Additional recommended knowledge
Sealed vs. Vented
Traditional NiCd batteries are of the sealed type (see nickel-cadmium battery article) which means that charge gas is normally recombined and they do not release any gas unless severely overcharged or a fault develops. Vented cells on the other hand have a vent or low pressure release valve that releases any generated oxygen and hydrogen gasses when overcharged or discharged rapidly. Since the battery is not a pressure vessel it is safer, weighs less and has a simpler and more economical structure. This also means the battery is not normally damaged by excessive rates of overcharge, discharge or even negative charge.
Structure and chemistry
A steel battery box contains the cells connected in series to gain the desired voltage (1.2V per cell nominal). Cells are made of light durable polyamide or nylon with multiple nickel-cadmium plates welded together for each electrode inside. A separator or liner made of silicon rubber acts as an insulator and a gas barrier between the electrodes. Cells are flooded with an electrolyte of 30% aqueous solution of potassium hydroxide (KOH). The specific gravity of the electrolyte does not indicate if the battery is empty or full but changes mainly with venting of water. The top of the cell contains a space for excess electrolyte and a pressure release vent. Large nickel plated copper studs and thick interconnecting links assure minimum electrical resistance for the battery.
The venting of gases means that there must be an excess of water in the cells (hence the name flooded cells). This also means the lost water must be periodically replaced during maintenance. Depending on the charge-discharge cycles and type of battery this can mean a maintenance period of anything from a few months to a year. Because the chemistry affects the volume of the liquid, the level must be topped-up only at the end of the charge cycle when the battery is fully charged.
Vented cell voltage rises rapidly at the end of charge allowing for very simple charger circuitry to be used. Typically a battery is constant current charged at 1CA rate until all the cells have reached 1.55V. Another charge cycle follows at 0.1CA rate, again until all cells have reached 1.55V. The charge is finished with a top-up charge, typically for 4 hours at 0.1CA rate. During the top-up charge the cell voltages will go beyond 1.6V and then slowly start to drop. No cell should rise above 1.71V (dry cell) or drop below 1.55V (gas-barrier broken).
Charging is done with vent caps loosened or open. A stuck vent might increase the pressure in the cell. It will also allow for refilling of water to correct levels before the end of the top-up charge while the charge current is still on. However cells should be closed again as soon as the vents have been cleaned and checked since carbon-dioxe dissolved from outside air carbonates the cells and ages the battery.
In an aircraft installation with a floating battery electrical system the regulator voltage is set to charge the battery at constant potential charge (typically 14 or 28 volts). If this voltage is set too high it will result in rapid water loss. A broken regulator diode will allow the charge voltage to rise well above this, causing a massive overcharge with boiling over of the electrolyte.
Battery maintenance period depends on the number of charge cycles, the type of charge and temperature conditions where the battery has been kept. Capacity testing is done with a constant current discharge of 1CA. For avionics batteries the limit for maintenance is 85% of the rated capacity. The maintenance consists of equalizing the cells by individually short circuiting them. The battery is left overnight in this condition and charged up again with final check of liquid levels. Any cells which do not hold their capacity at the second capacity test will be replaced. With proper maintenance modern cells have a life time well beyond 20 years. The cells will age slowly due to carbon-dioxide carbonating the cells. To prevent this, the vents should be kept closed and pressure tested periodically.
A typical aircraft battery used for engine starting has been rated to deliver over 15 kilowatts of power for 15 seconds. The load operates at around the internal resistance of the battery and the voltage loss at battery is equal to the load. Essentially the battery can be short circuited and it is able to dissipate the heat generated by the internal resistance for this short period of time.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nickel-cadmium_battery_vented_cell_type". A list of authors is available in Wikipedia.|