Dr. Nigel Thomas – Contributing Author
Like car batteries, one of an aircraft battery’s functions is to start the engines (or the APU). But the comparison stops there as, on board an aircraft, batteries 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 critical products and/or systems and deserve to be treated and maintained with care.
12V car batteries may be defined in terms of cold cranking capacity (current available for 30s at 6V and -18°C) and reserve capacity (discharge time available at 25A and +23°C). Similarly, 24V aircraft batteries are defined in terms of peak power (measured 12V at 0.3s, 15s or 30s) and rated capacity (discharge capacity available during 1h at +23°C). These ratings reflect respectively the battery’s ability to support an engine start and to support emergency loads.
Battery power has an important impact on the life of a turbine, most of which is used up during the engine light-off sequence due to the high temperatures generated. The higher the battery power, the less time the engine takes to start, the lower the temperatures and the lower is the wear and damage to parts.
Ni-Cd aircraft batteries (with VO/VP cells) retain 85% minimum rated capacity and power as a minimum throughout their life as lead acid aircraft batteries.
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 batteries include capacity loss due to active material degradation, loss of contact 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. 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.
All battery technologies can experience the phenomenon of thermal runaway. It occurs when heat generation exceeds heat dissipation causing a temperature rise that leads to even more heat generation. This can occur in intensive use when insufficient time is available for cooling after battery starts. It can also occur due to separator failure or drying out of cells, but, in the case of Ni-Cd, is avoided if maintenance is correctly performed. As a precaution, Ni-Cd batteries are usually fitted with temperature sensors which detect a potential thermal runaway and allow the battery to be disconnected.
Also degradation of the gas barrier is a normal scenario for the end of life of the battery which is not a failure. It becomes a failure if the scheduled maintenance has not been carried out properly: in this case, it is impossible to detect or anticipate the approaching end of life of the cell or membrane.
Generally speaking, the life of vented Ni-Cd batteries is from 6 to 8 years for helicopters, 7 to 9 years on long-range aircraft, 5 to 7 years for regional and commuter aircraft and 10 to 12 years for business jets.
All battery technologies require scheduled checks to ensure safety and to make certain that the battery will be capable of supporting the emergency loads. Maintenance ensures optimum performance and avoids on-board failure that leads to costly delays for operators. Ensuring the performance of a battery is especially important when the battery is used for internal starts as a poor condition battery can lead to excessive turbine wear.
Maintenance intervals are defined by the aircraft manufacturer based on battery manufacturers’ recommendations in relation to the particular aircraft and its usage.
The basis for determining maintenance intervals depends on the energy required for emergency requirements and the electrolyte reserve of the cell. Important factors influencing the interval include the battery charging system, battery operating temperature, type of starting, number of starts, flight duration, ground operation and the battery technology.
Basic maintenance procedures are similar for both lead acid and Ni-Cd. Apart from the usual tools such as torque wrenches and multimeters, equipment is required to charge and discharge the batteries. Vented batteries also require means to adjust electrolyte levels and check cell vents. Specific to Ni-Cd are the means to deeply discharge individual cells in order to erase any cell imbalance. Various specialist manufacturers offer proprietary systems that can accomplish these tasks with the minimum of labour.
• Avoid flattening batteries on board the aircraft: this is one of the most common causes of unscheduled removal of any battery, and of the premature failure of lead-acid batteries.
• Strictly follow safety rules; they reduce the risk of injury arising from electrical, mechanical or chemical dangers.
• Always review the battery history: Saft batteries are
provided a logbook where it should be recorded.
• Ensure the maintenance documentation is up to date and follow it strictly: using the wrong document leads to errors with important consequences. Saft provides all its current Saft documentation on its website and an email alert service when updates are issued.
• Use the correct tools: Saft provides a toolkit containing all the specific tools necessary for maintaining its batteries.
• Always respect the maintenance intervals specified by the aircraft manufacturer and carry out the maintenance defined for the interval.
• During storage, ensure batteries are stored in a cool place and that lead-acid batteries are periodically recharged: Ni-Cd batteries may be stored charged (ready to use) or discharged.
• Ensure that end of life batteries are recycled: Saft can be contacted for information about the nearest collection point for used Ni-Cd batteries
Future of Aircraft Batteries
The need for more advanced battery technology is being driven by the requirements of weight reduction and more electric aircraft. The ‘electrification’ of functions that were previously powered hydraulically, like actuation, requires high voltage architectures.
As of today, only the higher voltage associated with lithium-ion can provide an appropriate solution to this change. In the light of this, cutting edge Li-ion battery systems which meet these new requirements are being developed.
Li-ion is a sensitive electrochemistry which needs a detailed knowledge of its characteristics to allow its benefits to be exploited fully while ensuring maximum safety. We are likely to see continuing improvements in Li-ion performance as new electrode materials and electrolyte compositions are already under study. Nano-materials now being developed will also have a role to play. Nevertheless, Li-ion batteries are not currently envisaged as retrofit solutions so Ni-Cd and lead-acid batteries still have many years of work ahead of them.
Dr. Nigel Thomas has a PhD in chemistry from the University of London and has worked on the technical side of the battery business for 35 years, more than 25 of which have been spent with Saft. For the last 10 years he has been Saft’s main technical specialist for aircraft batteries, providing support to operators and OEM’s developing new aircraft.