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.