Maintenance of helicopters’ onboard batteries is a critical function and it is structured on the basis of their component maintenance manuals. However, the knowledge of industry practitioners on how to accomplish onboard batteries related maintenance tasks sometimes is limited only to what is available in maintenance manuals. To inform professionals involved in helicopter maintenance, we reached out to experts in the field and identified a few factors to consider when performing and managing maintenance on these critical components.
The first thing to consider is that batteries have their maintenance manual and their catalogue of parts, both the intervals and the maintenance procedures are codified. The maintainer has to diligently follow the instructions issued by manufacturers. “The helicopter manufacturer liaises with the battery manufacturer in order to establish the maintenance intervals. From a maintenance standpoint the battery is treated as if it were an aircraft. Some batteries require a yearly check, whereas others require a check twice a year, depending on the model”, says Claudio Girardi, maintenance manager at Adria Air Services.
Different Types of Batteries
There are three main types of batteries that are most commonly installed on helicopters: lead, nickel-cadmium, and gel batteries.
Lead batteries are used in small aircraft and they are the easiest ones to maintain, it could even be said that - under certain circumstances - there is no need for maintenance at all. “Lead batteries, once positioned on board, have their own life cycle at the end of which - instead of performing maintenance by emptying the batteries and checking the cells – they are often replaced by new batteries. The advantage of lead batteries is that they are simple and safe, on the contrary they do not maintain a constant charge level. The charge degrades over time; little by little the curve of the voltage decreases because the cells of the batteries progressively lose their holding ability,” says Fabrizio Segrè, quality manager at helicopter maintenance company Euroavia.
Nickel-cadmium batteries are installed most often on more sophisticated turbine-powered helicopters requiring a significant electric charge upon starting. These batteries also have pros and cons. An advantage is that they maintain a constant charge over time, but a disadvantage is nickel-cadmium batteries do not provide notice as to when their charge level is about to drop or when they are having issues in maintaining the nominal charge level. “Nickel-cadmium batteries maintain their nominal charge level - e.g. 24 volts - and then, all of a sudden, this level drops to zero. Instead of having a descending curve like lead batteries,which allows one to predict their performance over time, nickel-cadmium batteries have a straight line, which at a certain point drops all of a sudden”,” says Segrè. “Also, thermal runaway is a second aspect of nickel-cadmium batteries that needs consideration, because it determines maintenance cycles. Issues in the battery’s cells, internal short-circuits, and degrading insulation can potentially overheat the battery and produce a destructive result. Thermal runaway is due to the particular internal structure of nickel-cadmium batteries, which can continue to overheat even if the electrical load inducing an increase of the battery’s temperature in the first place is discontinued. It is for this reason that helicopters equipped with nickel-cadmium batteries must have installed a sensor connected to a warning light in the cockpit to monitor the battery’s temperature.”
“Lead and nickel-cadmium batteries are not compatible with one another; they need to be managed into two separate environments for health and safety reasons, since the gaseous emissions of these two different types of batteries can create an explosive compound. The two types are also to be disposed of separately,” says Girardi. “There is a recent evolution with regard to lead batteries, they are being increasingly less manufactured with a liquid acid inside but rather with the electrolyte in the form of gel; this makes the handling of batteries safer.” Gel batteries are gaining momentum because they have the same simplicity as lead batteries as well as the same effectiveness as nickel-cadmium batteries.
Even if nickel-cadmium batteries tend to be preferred for installation on turbine-powered helicopters, they are not, per se, an optimal solution. Some business jet aircraft manufacturers, for example, are increasingly making it possible to install lead batteries as an alternative to nickel- cadmium batteries. When the voltage is the same it is theoretically possible to use either type of battery. It is down to the helicopter manufacturer to decide whether lead, nickel-cadmium or both may be installed.
From the perspective of the manufacturer having to make this decision, the discriminating factors are the capability of batteries to maintain their charge level over time and their overheating potential. “The reason why nickel-cadmium batteries continue to be the preferred choice of turbine-powered helicopter manufacturers has to do with the fact that nickel-cadmium batteries have a constant charge level, whereas lead batteries progressively lose their charge,” says Segrè. “With regard to the safety implications of their overheating potential, nickel-cadmium batteries have more overheating potential compared with lead batteries and the extreme consequence of this is battery explosion. It should be pointed out, however, that in case of battery overheating, a helicopter can relatively easily find a suitable landing spot within a handful of minutes from the time the overheating is noticed; it is also for this reason that turbine-powered helicopters tend to be equipped with nickel-cadmium batteries”.
Maintaining Nickel-Cadmium Batteries
Compared with lead batteries, nickel-cadmium batteries require more complex maintenance because they have discharging and recharging cycles requiring that the batteries be attended by an operative during this lengthy process; in addition they need to be handled in a shop equipped with an air suction device. “It is in order to ensure that nickel-cadmium batteries maintain their charge level constant over time and exploit their full potential that it is necessary to go through a series of steps in the maintenance process, whereby the batteries are brought to the maintenance shop to be fully discharged and then fully recharged. The charge of the battery is brought to zero in order to make sure that it has no power at all”, says Segrè. “The battery is then fully charged to the nominal value, an energy charge is consequently applied, and a measurement is then taken as to how much voltage is lost in time due to energy absorption. This measurement provides an assessment of the battery’s ‘health’ condition. Once it is ascertained that the battery is fine, a second discharge cycle is performed, the battery is then fully recharged and reinstalled on board. This type of maintenance is more demanding – even in terms of manpower – and requires specific tooling”.
Every battery model has its own maintenance manual and kit for accomplishing charge and discharge tasks. The health of nickel-cadmium batteries is ensured by how accurately the charge and discharge tasks are performed. “When maintenance is being initiated it is of fundamental importance to properly predict the discharge time of the cells, which have to be fully discharged, and proceed to recharge them. In fact, it is possible to experience the so called ‘memory effect’ when a battery is not fully discharged and has a residual charge. When it is recharged, the available charge is not the full charge but rather the full charge net of the residual charge. If a battery is not fully discharged the residual charge actually has a negative effect in the recharge process. This consideration is important because it affects the usable life of a battery, often times you have to change the batteries - or replace their cells - because prior discharge and recharge maintenance cycles have not been properly performed,” says Segrè.
To conclude, lead, nickel-cadmium, and gel batteries are the types that can be most commonly found on helicopters. Turbine-powered helicopters tend to be equipped with nickel-cadmium batteries and these require relatively complex maintenance because of the discharging and recharging cycles that have to be performed and are key to ensuring their health and functionality.
In addition to the more established lead-acid and nickel-cadmium batteries, lithium-ion power solutions have become increasingly popular in recent times. Battery manufacturer True Blue Power has been installing lithium-ion batteries in rotorcraft since 2010, beginning with the True Blue Power TS835, a 4.5 amp-hour emergency backup battery for avionics and instruments. The company has also added lithium-ion main ship batteries, primarily for engine start applications, to its repertoire of certified battery options for rotorcraft. Lithium-ion batteries are available both in the U.S. and Europe and for installation on several helicopter types, including the Leonardo AW169, the Bell 505, the Robinson R44 and R66, and the Airbus H145 and H135. In addition the Sikorsky S-76D and S-92 and the Airbus AS350 currently have pending supplemental type certificates for the installation of lithium-ion batteries.
According to Rick Slater, a director at True Blue Power, lithium-ion batteries provide significant benefits compared with lead-acid and nickel-cadmium batteries. “Lithium-ion batteries are roughly half the weight of lead-acid and nickel-cadmium chemistries. They deliver a significant increase in power for high-power applications like engine or APU start and provide quicker and cooler engine starts on turbine aircraft. They offer good energy characteristics for applications requiring longer availability of battery power and provide a very quick recharging profile compared to lead-acid and nickel-cadmium equivalents”, says Slater.
“Lithium-ion batteries are virtually maintenance free; for True Blue Power batteries only a capacity check every two years is suggested. In addition, they have no memory effect like nickel-cadmium and no reblocking or desulfating like lead-acid. A lithium-ion battery has a much longer product life, which creates overall a significantly reduced cost of ownership covering maintenance, replacement, aircraft down time and dispatch, reliability, and availability.”
Charging of lithium-ion batteries is accomplished through use of a standard ramp charging unit. “Lithium-ion batteries can be charged near lead-acid or ni-cad batteries and do not require special conditions or equipment. Lithium-ion batteries should not be discharged to zero as the battery could be damaged. Top charging to near capacity periodically is recommended if they are going to stay on a shelf for an extended period”, says Slater.
Like other types of batteries on the market today, not all lithium-ion batteries have the same kind of technology and safety features. “The key factor in lithium-ion safety is a combination of cell chemistry, product architecture – relative to safety design – and certification testing. Some lithium chemistries are much more volatile than iron phosphate, particularly those with oxygen as a key element within their basic chemistry. Lithium iron phosphate is more manageable as it relates to heat generation. No oxygen content is present to exacerbate any potential situation involving fire,” says Slater. “Product architecture also affects safety. Having a large number of smaller lithium-ion cells versus a small number of large cells makes managing an exception condition like a thermal
runaway as a result of a single cell short circuit, much more manageable.”
Testing to FAA-specified requirements and receiving certification is the final step to ensure the battery can deliver what it promises, i.e. the ability to withstand complete thermal runaway with no damaging or permanent effects on the surrounding parts or components of the aircraft. “The FAA’s thorough testing and stringent specifications for rechargeable lithium-ion products provide a level of assurance to the pilot and passengers. True Blue Power is the only lithium-ion battery manufacturer to successfully achieve a technical standard order (TSO), type certificate (TC) and supplemental type certificate (STC), further validating the industry’s confidence in this technology and our advanced battery system designs,” concludes Slater.