The Battery’s Role

Sept. 17, 2010
Much of the electronic equipment in use today requires some type of self-contained reserve power

So just what role do batteries play in the aircraft of today? Well, I guess a proper answer would begin with defining exactly what a battery is and what it can do.

To most, the first thing that comes to mind is a storage device for direct current (DC) electrical energy. In certain cases it can be related to a hydraulic accumulator known to store hydraulic fluid under pressure. Batteries are also often related to capacitors. By simplest definition a capacitor consists of two conductors separated by a di-electric material or an insulator. Certain chemical reactions are known to produce electrical energy. Inserting a piece of zinc and a piece of copper in a potato and connecting them to a light-emitting diode or a digital multimeter will provide evidence that electrical energy is in fact being produced.

Just what roles do batteries play in the aircraft of today?

Design philosophies, of the airframe manufacturer, define the role of how and where batteries will interact within their products. Turbine aircraft electrical system design is in part based on the selected engines. Larger engines can be started more efficiently using an air turbine starter while smaller engines most often use some form of electrical starter motor. Lighting up the engines is in most cases the biggest demand on a battery system. Circuits identified by a manufacturer as “Required for Flight” will sometimes have a redundant power source, an ideal job for batteries. One of the more obvious examples is the emergency lighting system required on many aircraft.

There has been some notoriety involving batteries used in Emergency Locator Transmitters (ELT). Technical Standard Order C97 provides a minimum performance standard for certain lithium batteries. Earlier safety problems with the lithium sulfur dioxide chemistry lead to cases of exploding, venting violently, corroding, and burning. As a result, the FAA issued a series of three airworthiness directives (AD). The final AD, issued in February 1980, is current and requires that lithium sulfur dioxide batteries used in U.S. registered civil aircraft meet the requirements of TSO-C97.

Much of the electronic equipment in use today requires some type of self-contained reserve power and understanding the type of energy cell along with the reason for installation may assist in forecasting when replacement or servicing may be needed.

Computerized devices requiring a specific shutdown process utilize internal batteries to ensure the deactivation occurs properly irregardless of available aircraft power. Often when these devices degrade a fault may be noticed during the next starting sequence. This is a common occurrence with certain flight management systems (FMS). Proactive measures such as placing a conservative calendar replacement interval on such devices may reduce malfunctions at inopportune times.

Airworthiness requirements for transport category aircraft dictate that certain backup equipment requires an independent power source in the event of aircraft electrical failure. In addition to providing a standby power source the requirement states a monitoring system must also be incorporated to alert the flight crew when the reserve system may not be able to perform as it should.

In certain cases electrical transients occur during engine start or shutdown. Several manufacturers elect to install backup battery packs to sustain operation of certain relevant equipment during these transition periods. This can include maintaining FMS flight plans for short periods of time even when the FMS may otherwise appear to be dormant.

Aviation batteries come in many forms. Shapes, sizes, and electrical characteristics are frequently customized for certain functions. Even the materials used can be optimized for various operating environments.

Lead-acid battery

Lead-acid batteries were invented in 1859 by French physicist Gaston Planté and are the oldest type of rechargeable battery. Despite having a very low energy- to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. This type battery does tend to degrade with age and should be replaced prior to the capacity dropping below required minimums.

Lithium-ion battery

Lithium-ion batteries are common in much of today’s electronics with one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. Beyond consumer electronics this type battery is growing in popularity for aerospace applications due to their high energy density. Research is yielding a stream of improvements in technology, focusing on energy density, durability, cost, and safety.

Shelf life can be considered a disadvantage in certain lithium batteries. Over time, the cell’s capacity diminishes as the increase in internal resistance reduces the ability to deliver current and is more pronounced in high demand applications. The decrease means that older batteries do not charge as much as new ones and therefore capacity is decreased.

In January 2008, the U.S. Department of Transportation ruled that passengers on board commercial aircraft could carry lithium batteries in their checked baggage if the batteries are installed in a device. Types of batteries affected by this rule are those containing lithium, including Li-ion, lithium polymer, and lithium cobalt oxide chemistries. Lithium-ion batteries containing more than 25 grams (0.88 ounces) equivalent lithium content (ELC) are exempt from the rule and are forbidden in air travel. This restriction greatly reduces the chances of the batteries short-circuiting and causing a fire.

Additionally, a limited number of replacement batteries may be transported in carry-on luggage. Such batteries must be sealed in their original protective packaging or in individual containers or plastic bags.

Some shipping agents restrict air shipping of lithium and lithium-ion batteries, and products containing them.

Nickel-cadmium battery

The nickel-cadmium battery (commonly abbreviated NiCd or NiCad) is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes.

The abbreviation NiCad is a registered trademark of SAFT Corporation, although this brand name is commonly used to describe all nickel-cadmium batteries. The abbreviation NiCd is derived from the chemical symbols of nickel (Ni) and cadmium (Cd).

There are two types of NiCd batteries: sealed and vented.

Sealed NiCd cells may be used individually, or assembled into battery packs containing two or more cells. When NiCds are substituted for other battery types, the lower terminal voltage and smaller ampere-hour capacity may reduce performance and this should be considered prior to any substitution. Miniature cells find their way into devices with computer-memory requirements along with many consumer electronics.

Specialty NiCd batteries are used in cordless and wireless telephones, emergency lighting, and other applications. With a relatively low internal resistance, a NiCd battery can supply high surge currents. This makes them a favorable choice for remote-controlled devices, as well as cordless power tools. Larger wet cells are used for main aircraft batteries, electric vehicles, and standby power.

Nickel-cadmium cells have a nominal cell potential of 1.2 volts. This is lower than the 1.5 volts of alkaline and zinc-carbon primary cells, and consequently they are not appropriate as a replacement in all applications. However, the 1.5 volts of a primary alkaline battery refers to its initial, rather than average, voltage. Unlike alkaline and zinc-carbon primary cells, a NiCd cell’s terminal voltage only changes a little as it discharges. Because many electronic devices are designed to work with primary cells that may discharge to as low as 0.90 to 1.0 volts per cell, the relatively steady 1.2 volts of a NiCd is enough to allow operation.

Recently, nickel-metal hydride (Ni-MH) and lithium-ion batteries (Li-ion) have become commercially available and less costly. Where energy density is important, Ni-Cd batteries are now at a disadvantage compared to Ni-MH and Li-ion batteries. However, the Ni-Cd battery is still very effective in situations requiring very high discharge rates as the Ni-Cd can endure such discharge with no damage or loss of capacity. Recharging prior to a complete drain can have a negative impact on this type battery as it tends to have a memory.

When compared to other forms of rechargeable batteries, the NiCd has a number of distinct advantages including durability and tolerance to deep discharge for long periods. In fact, NiCd batteries in long-term storage are typically stored fully discharged. This is in contrast to lithium-ion batteries, which are less stable and will be permanently damaged if discharged below a minimum voltage. NiCd batteries typically last longer, in terms of number of charge/discharge cycles and when compared to lead-acid batteries, NiCd batteries have a much higher energy density. A NiCd battery is smaller and lighter than a comparable lead-acid type.

Batteries contain materials considered hazardous and should always be handled and disposed of with care.

I guess it’s a shame potato batteries never caught on in the aerospace environment. I can see an alternative to paying the inflated price for airline snacks and spent battery disposal could be handled by the digestive system.

What a concept! AMT

Jim Sparks has been in aviation for 30 years and is a licensed A&P. He is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. He can be reached at [email protected].