- 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 high 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 to 70 °C).
Sealed vs. vented
Traditional NiCd batteries are of the sealed type (see nickel–cadmium battery) which means that charge gas is normally recombined and they release no 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 gases 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 silicone 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 discharged or fully charged 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 effective series resistance for the battery.
The venting of gases means that the battery is either being discharged at a high rate or recharged at a higher than nominal rate. This also means the electrolyte lost during venting must be periodically replaced through routine 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.
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 1 CA rate until all the cells have reached at least 1.55 V. Another charge cycle follows at 0.1 CA rate, again until all cells have reached 1.55 V. The charge is finished with an equalizing or top-up charge, typically for not less than 4 hours at 0.1 CA rate. The purpose of the over-charge is to expel as much if not all the gases collected on the electrodes, hydrogen on the anode and oxygen on the cathode, and some of these gases recombine to form water which in turn will raise the electrolyte level to its highest level after which it is safe to adjust the electrolyte levels. During the over-charge or top-up charge, the cell voltages will go beyond 1.6V and then slowly start to drop. No cell should rise above 1.71 V (dry cell) or drop below 1.55 V (gas barrier broken).
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 V). If this voltage is set too high it will result in rapid electrolyte loss. A failed charge regulator may allow the charge voltage to rise well above this value, 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 in service. Capacity testing is done with a constant current discharge of 1 CA. For avionics batteries the limit for maintenance is 100% of rated capacity but in the older batteries (pre 1999) and some less efficient batteries it is considered 85% of the rated capacity. With the intervention of more technologically advanced materials, some manufacturers are considering raising the 'end of life' capacity for High Power cells to 110% 1 CA. The maintenance consists of equalizing the cells by individually short circuiting them through a current limiting resistor. 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 lifetime well beyond 20 years but most manufacturers recommend 5000 hours. 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 thus the voltage loss at the battery is equal to the voltage across load. These are the conditions which provide the maximum possible power to the load (See Maximum power transfer theorem).
- General Electric, "Nickel–Cadmium Battery Application Engineering Handbook", 1971
- Marathon Battery Company, "Care and Maintenance of Nickel–Cadmium Batteries"
- SAFT, "NiCd Aircraft Batteries, Operating and Maintenance Manual (OMM)", 2002
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