Performance and Safety Requirements for Batteries Installed in Aircraft
Like everything else in our modern world, electrification is extending to aviation. Although much of this transformation involves the aircraft’s onboard power generation capabilities such as generators, alternators, magnetos, and auxiliary turbines, battery energy storage systems are becoming increasingly more important. This ranges from small format batteries that provide keep-alive power for memory circuits in avionics to larger battery devices that provide the main source of power to propel the aircraft.
Given the nature of air travel, such batteries and their component cells must perform as designed and operate safely in their applications. In the United States, the Federal Aviation Administration (FAA) is the primary regulatory authority for aviation and is responsible for developing, implementing, and enforcing regulations to protect the public. This authority extends to the regulation of portable energy products that are considered a part of the aircraft itself.
The FAA produces a multitude of regulations and supporting guidance documents. As a point of fact, there are over fifty types of documents that are used for both internal and external purposes. General guidance on these document types can be found at https://www.faa.gov/guidance. Of interest to aerospace battery compliance, we will focus on two of these document types used to promulgate regulatory information to both FAA personnel and the public, as follows:
- Advisory Circulars (AC’s) are used to uniformly “…deliver advisory material to FAA customers, industry, the aviation community, and the public.” All such ACs are maintained in a common database.
- Technical Standard Orders (TSO’s) are intended to provide guidance of a technical nature to FAA personnel. However, the aviation industry as well as the general public make use of these documents to aid in compliance efforts and to foster a general understanding of the agency’s efforts. Like the ACs, TSOs are maintained in a common database by the FAA.
Like many other regulatory agencies, the FAA will sometimes rely on the industry being regulated as a partner in establishing specific testing requirements. Although this may seem to some as a classic case of “the fox guarding the hen house,” the truth is that the industry is incentivized to help develop a reasonable set of tests sufficient to support the stated intent of showing an acceptable level of both safety and performance. The industry knows that any safety failure has negative consequences for the entire industry, not just the company impacted, both in terms of governmental response as well as damage to the public’s view of the industry itself. They also fully understand that if they fail to develop an acceptable test standard, the regulatory agency could take steps to develop one unilaterally without direct industry participation. Such an outcome would be considered less than ideal by most industry participants.
In the case of aviation, such standards development is commonly coordinated through the Radio Technical Commission for Aeronautics, now referred to simply as RTCA (https://www.rtca.org). RTCA is a non-profit organization founded in 1935 and is self-described on its website as “…the premier Public-Private Partnership venue for developing consensus among diverse, competing interests on critical aviation modernization issues in an increasingly global enterprise.” (The RTCA test standards referenced here are copyrighted materials and can be purchased through RTCA.)
In the case of aviation battery regulations, several standards have been developed over time to address different chemistries. A summary of the regulatory references and their associated standards is given in Table 1.
|Battery Chemistry||Advisory Circular||Technical Std Order||Referenced Test Standard|
|Rechargeable Lithium||AC 20-184||TSO-C179b||See Table 2|
|†Lithium Sulfur Dioxide||—–||TSO-C97||14 CFR § 37.209|
|Nickel Cadmium, Nickel Metal Hydride, Sealed Lead Acid||—–||TSO-C173A||RTCA/DO-293A|
†Lithium Sulfur Dioxide is a specific type of non-rechargeable lithium batteries that have unique regulatory requirements.
Table 1: Linkage of battery chemistry to test standards
The requirements for rechargeable lithium (typically lithium-ion) reflect some further nuanced specifications based upon their configuration and sample size. These requirements are detailed in Table 2.
|Battery Size||Configuration||Energy (Watt-Hours)||Referenced Test Standard|
|Coin and Button Cells||Single or Multi-Cell||Wh < 2||UL 1642, UL 2054, IEC 62133|
|Small/Medium*||Single Cell||2 ≤ Wh < 60||RTCA/DO-347|
|Multi-Cell||2 ≤ Wh < 300|
|Large||Single Cell||Wh ≥ 60||RTCA/DO-311A plus selected tests from RTCA/DO-347|
|Multi-Cell||Wh ≥ 300|
*The terms ”small” & “medium” are not differentiated in TSO-C179b but appear to generally reference the Energy Categories given in RTCA/DO-347. As noted above, they are treated the same for test purposes.
Table 2: Rechargeable lithium test requirements
In addition to the test requirements previously cited, the TSOs noted in Table 1 also refer to other RTCA standards for various design aspects (see Table 3).
|Design Aspect||Advisory Circular||Referenced Test Standard|
Table 3: Additional standards to consider
Note also that certain types of battery-supported equipment have their own separate TSOs that may have battery requirements in addition to those noted so far. An example of this is TSO-C200a, titled “Airframe Low Frequency Underwater Locating Device (Acoustic) (Self-Powered).” These devices use non-rechargeable lithium batteries, but the TSO requires that the requirements given in RTCA/DO-227A be supplemented with selected tests from RTCA/DO-347 which is intended for rechargeable lithium batteries.
It should be clear that compliance with the stated requirements can be complex. The discussion in the preceding paragraphs does not cover every situation but rather attempts to depict those cases considered most typical to illustrate concepts common to the various regulatory requirements. Users of this information are cautioned to fully research their product’s regulatory situation to ensure that the appropriate guidelines are being utilized.
As a general rule, the regulatory requirements should be confirmed early in the process with one’s customer as well as the FAA or their Designated Engineering Representative (DER). From some perspectives, these discussions may be considered a negotiation as it is possible in some cases to modify requirements or have them waived altogether if the specific situation warrants. Any such changes will be recorded in a document known as a Quality Test Plan (QTP).
A QTP is a detailed document that describes the product but, more importantly, defines in detail how the tests are to be run. Development of this document is accomplished by the client with input from their test provider that might include equipment types and additional product-specific detail. The intent is to provide enough detail to reconstruct the test, but not so much detail that the document becomes encumbered with information that does not significantly impact the conduct of the testing. It is not uncommon for such documents to be anywhere from 50-150 pages in length. The QTP will also form the basis for the final report.
It is important to realize that the scope of the testing includes the entire tier structure of the device. This may include component cells, battery packs, or the supported device (the equipment under test, or EUT).
The testing itself may include:
- Electrical performance tests like capacity at temperature or high current discharge;
- Mechanical or environmental tests like vibration, drop, or thermal cycling. These are commonly specified as tests from the current revision of RTCA/DO-160 which covers environmental requirements for aviation electronics;
- Safety tests such as short-circuit or overcharge; and
- EUT-level tests such as thermal runaway containment.
Like the negotiation around the test requirements, there will need to be an agreement with the party responsible for conducting the testing. In some cases, the equipment vendor may have the expertise and equipment necessary to do the work in-house. For others without such internal resources, an external lab that has been accredited to the test standards involved may be selected. There also exists the possibility that a hybrid testing model will be used where both internal and external resources are being used to accomplish the needed testing.
Because of sensitivity around lithium battery safety due to widely publicized incidents both within the aviation industry as well as other non-aviation industries, it is not uncommon for customers further down the value chain to request the opportunity to witness some of the testing that is considered to represent greater risks. In some cases, the DER/FAA may also wish to witness certain tests. Such monitoring may be done onsite or remotely through commonly available meeting applications.
Unlike many other standards, the total number of samples required for RTCA rechargeable battery test regimes is relatively small (by its very nature, non-rechargeable battery testing requires larger sample sizes). This is achieved by specific samples being assigned to specific tests (very significant reuse), the sequential order of the testing being defined for each sample, and the number of replicates for any given test kept to a minimum. On balance, the testing takes longer than some other regimes since much of the testing is run in series instead of parallel.
Conduct of the test regime requires that all samples be “conformed” prior to the start of any testing. This means that all test samples are verified to ensure that they are in the correct state for testing and are not damaged in a way that might negatively impact the test. The QTP is the reference for defining the correct pre-test state. Pre‑test documentation will also include pictures. Execution of certain tests may require video of testing in progress in addition to the various parametric measurements called for in the test descriptions. Finally, post-test, the units are inspected with any anomalies being documented in writing and with pictures.
Formal report generation can be extensive due to the significant number of tests involved as well as the supplemental data and photo requirements. Having a report template developed at the beginning of the process can minimize the reporting effort required at the end of the test. It also helps identify key test aspects that must not be overlooked. Some labs will go a step further and develop lab-specific checklists or data sheets. These documents may be included in the QTP and/or report template.
Any negative findings will require some degree of analysis and corrective action once it has been established that the finding was attributable to the product itself and not the result of a test anomaly. Once the corrective actions have been implemented, a recovery test plan will be developed between the product manufacturer, their customer, and the FAA representative or their designate. It is possible that the implemented changes may require that other non-failed tests be repeated if there is a potential that the changes may have an impact on those test outcomes. Once again, a revision to the report will be generated that appends the existing report with the new data.
- The method of compliance for aerospace battery applications in the United States is specified in the regulations and supporting guidance published by the FAA.
- The relevant FAA guidance document types include Advisory Circulars and Technical Standard Orders.
- Such regulations reference industry-developed test standards available from sources like RTCA, UL, and IEC.
- Common chemistries such as lithium-ion, NiCd, NiMH, SLA, and non-rechargeable lithium are included.
- The testing may include cells, battery packs, or the supported device (EUT).
- The process for complying with such standards is formally documented in a QTP that serves as an agreement with the manufacturer, their customers, and the FAA. It also provides the detailed test plan and reporting requirements for the test laboratory conducting the test program.
- The testing uses a minimum number of samples overall because it is sequential in nature. But this usually equates to a longer test duration than some other standards that utilize parallel testing.
- There are many nuances to FAA compliance, so it is imperative that the specific requirements for a given product are thoroughly researched and verified prior to beginning what is a rather extensive compliance effort.