Like car batteries, aircraft batteries serve to start the engines or the APU. But the comparison stops there as aircraft are required to do much more. In-flight electrical generation failure is an emergency that calls on the batteries to power the essential loads until landing and evacuation. They have even been used to restart the engines after the rare cases of engine flame-out. They also act as a buffer regulating the DC network voltage ensuring acceptable power quality for the equipment connected to it. As these various functions attest, aircraft batteries are crucial components and deserve to be treated and maintained with care.
Battery technology and construction
Two chemistries are generally used for today’s aircraft batteries — nickel cadmium (Ni-Cd) and lead-acid. Lead-acid batteries are either vented or valve regulated (VRLA), and are typically used in light and general aviation aircraft. At the other end of the spectrum, vented Ni-Cd batteries dominate larger aircraft and helicopter applications while both VRLA and Ni-Cd types are found in smaller aircraft such as business jets.
Nickel cadmium (Ni-Cd)
Ni-Cd cells contain interleaved electrodes connected via internal current collectors to terminals that pass through the cell cover. Between the electrodes, a non-woven polyamide felt separator keeps the alkaline electrolyte in contact with the active surface. It prevents short circuits while allowing current flow through the electrolyte. The separator system includes an oxygen barrier made of organic or micro-porous synthetic material. During overcharge, this barrier minimizes oxygen recombination to ensure low and stable overcharge current.
The electrode assembly is housed in a rigid plastic container that allows cells to be fitted side-by-side in a battery case. Each cell is equipped with a low pressure vent valve that can be removed to allow water addition. The vent allows gas produced in normal operation to be released while preventing electrolyte from escaping and contaminants from entering. Ni-Cd aircraft batteries usually consist of a metal box containing 20 individual series-cells connected using rigid, highly conductive, nickel-plated copper cell links that are secured by nickel-plated copper nuts on the cell terminals.
Lead-acid batteries use one 12-cell or two 6-cell plastic containers to house individual cells that are series-connected through the cell wall to give a nominal voltage of 24 volts. This provides a ‘mono-bloc’ structure where the cells cannot be individually removed or checked.
Inter-electrode separation is assured by the use of polymeric or glass-fiber mats that maintain the sulphuric acid electrolyte in contact with the active surface. The main difference between vented and VRLA batteries is the use of higher pressure valves and different separators that, in VRLA, are designed to promote oxygen transfer to maximize oxygen recombination.
Mechanism of failure
Over time, all batteries eventually lose their ability to perform and become eligible for scrap and recycling.
In Ni-Cd batteries the three main mechanisms of failure are all progressive and can therefore be predicted in advance, with high reliability, through proper maintenance. They include oxygen barrier failure, separator failure, and irreversible capacity loss due to degradation of active materials.
The principle mechanisms of failure in lead-acid include capacity loss due to active material degradation, loss of the active material from the current collecting structure leading to high internal resistance, and corrosion of the current collecting structure leading to sudden death. In contrast to Ni-Cd batteries, high internal resistance and corrosion can appear rapidly and without warning especially if the battery has been subject to deep discharge, for example during ground operation. Unfortunately these mechanisms of failure are not detectable during maintenance checks.
Much of the electronic equipment in use today requires some type of self-contained reserve power
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