Custom Battery Pack for Avionics Backup Systems: Reliable Power Design

A custom battery pack for avionics backup systems is an engineered lithium battery pack designed to provide emergency or supplemental power to critical avionics loads when the aircraft's primary power source is interrupted. Unlike an off-the-shelf battery, it is defined by the specific load profile, required backup endurance, voltage and current limits, charging and protection strategy, installation environment, and the documentation and qualification evidence required for the intended aircraft application.

Designing a reliable backup power source for avionics requires more than selecting a battery with the right voltage and capacity. It demands a complete engineering approach that connects the electrical load to the cell chemistry, pack architecture, battery management system, charger interface, mechanical integration, and validation process. This guide walks through each of those decisions, explains the critical distinctions that determine whether a pack will perform as expected, and provides a practical framework for specifying and sourcing a custom solution.

For a broader overview of where lithium battery technology fits across aerospace applications, see our guide to Lithium battery applications in aerospace.

What Should an Avionics Backup Battery Pack Do?

An avionics backup battery pack exists to maintain power to critical electronic systems when the aircraft's main electrical bus fails. These loads may include flight displays, navigation equipment, engine monitoring instruments, communication systems, or other electronics essential for continued safe operation.

A backup battery is not a propulsion battery, a starting battery, or a ground-power unit. It is sized and configured around a very specific question: which loads must continue operating, for how long, and under what voltage and current conditions?

That question shapes every subsequent design decision. The battery must deliver sufficient energy to support the defined critical loads for the required endurance, maintain acceptable voltage under load, withstand the installation environment, charge reliably when main power is available, and communicate or signal its status when necessary.

It is also important to distinguish between a battery pack and a complete backup power system. A battery pack provides stored energy with appropriate protection and management. A complete backup power system includes the battery, a charger matched to the battery chemistry and configuration, power-path switching that transfers the load when main power is lost, and the monitoring or control circuitry that manages the changeover. Understanding this boundary prevents a common sourcing mistake: specifying a pack when the system integration is still someone else's responsibility.

How to Define the Requirements for a Custom Avionics Backup Pack

The single most important step in developing a custom avionics backup battery is defining the requirement clearly. A battery manufacturer cannot design a reliable pack from a voltage and capacity figure alone. The engineering team needs to understand the load, the operating environment, and the intended use.

At minimum, the following inputs should be defined before contacting a manufacturer:

Requirement Category	Specific Information Needed
Critical loads	Which equipment must remain powered during backup operation
Backup endurance	How long the battery must support the load (minutes, hours)
Load profile	Steady current draw, transient pulses, startup inrush, and typical duty cycle
Voltage limits	Nominal operating voltage, minimum allowable voltage for the load, and maximum charge voltage
Current demand	Continuous current and peak current, including duration of peaks
Charging method	Available charging source, charge voltage/current limits, and charge time available
Temperature range	Operating, charging, and storage temperature limits
Mechanical envelope	Available space, mounting orientation, weight limit, and connector requirements
Documentation	Datasheet, test reports, transport documentation, and any qualification evidence required

A battery manufacturer's engineering team should ask for this information early in the process. In fact, custom battery pack design begins with a structured definition of voltage, capacity, continuous and maximum current, physical dimensions, connector type, charging method, temperature range, and estimated order quantity. This requirements-first approach prevents costly redesigns later.

Two specification distinctions are particularly important at this stage.

Capacity versus energy. Capacity is measured in ampere-hours (Ah) or milliamp-hours (mAh) and describes the total charge stored in the battery. Energy is measured in watt-hours (Wh) and describes the total work the battery can perform. A 12 V, 10 Ah pack stores 120 Wh, while a 24 V, 10 Ah pack stores 240 Wh — the same capacity but twice the energy. Runtime is determined by energy and load, not capacity alone.

Nominal voltage versus maximum charge voltage. Nominal voltage is the reference voltage of the pack, typically the midpoint of the discharge curve. Maximum charge voltage is the upper limit the pack can be charged to, and it is always higher than nominal voltage. For a lithium-ion cell, nominal voltage is commonly around 3.6–3.7 V, while maximum charge voltage is typically around 4.2 V. Confusing these values can lead to specifying the wrong charger or the wrong pack configuration.

Selecting Cell Chemistry and Pack Architecture

Once the requirement is defined, the next step is selecting the cell chemistry and pack architecture. There is no universally "best" chemistry for avionics backup — the correct choice depends on the specific trade-offs that matter for the application.

Li-ion Cylindrical Cells

Cylindrical lithium-ion cells, such as those in the 18650 or 21700 format, are widely used in custom battery packs. They benefit from mature manufacturing processes, consistent quality, and well-established supply chains.

Cylindrical cells generally offer good energy density and current capability. Their rigid metal casing provides mechanical durability, and the standardized format simplifies pack construction. For applications where volume is not severely constrained and where mechanical robustness matters, cylindrical cells are often a strong choice.

Best suited for: applications requiring proven reliability, standardized formats, and a mature supply chain.

Li-Polymer Pouch Cells

Li-polymer (LiPo) pouch cells use a flexible aluminum-laminated pouch rather than a rigid metal can. This construction allows the cells to be manufactured in a wider range of shapes and thinner profiles, which can be a significant advantage when the battery must fit into a constrained avionics equipment bay or a custom-shaped enclosure.

The trade-off is that pouch cells are more mechanically vulnerable than cylindrical cells. They require careful handling, protection against puncture and impact, and mechanical support within the enclosure. A well-designed pack with a rigid housing and appropriate padding can mitigate these risks.

For applications that demand flexible form factors and higher energy density in a compact volume, the same pouch technology used in a lipo battery for drone applications demonstrates the design freedom this cell format provides — though avionics backup packs require very different protection, charging, and qualification considerations than high-discharge drone packs.

Best suited for: applications where the available space has an unusual shape, or where a thin, flat pack is required.

LiFePO4 Cells

Lithium iron phosphate (LiFePO4) cells offer a different set of trade-offs. Their nominal voltage is lower, typically around 3.2 V per cell, which means a given pack voltage requires more cells in series. They also have a lower energy density than other lithium chemistries, meaning a LiFePO4 pack will be heavier and larger for the same energy.

However, LiFePO4 cells are widely recognized for their robust thermal and chemical stability, long cycle life, and flat discharge voltage. These characteristics can be valuable in safety-critical or high-cycle applications.

Best suited for: applications where cycle life, thermal stability, or a lower-risk chemistry profile outweighs the weight and energy-density penalty.

Series and Parallel Architecture

The pack's voltage and capacity are determined by the series and parallel configuration of its cells.

• Series count (S) determines the pack's nominal and maximum voltage. Wiring cells in series adds voltage: a 1S pack is one cell in series, a 3S pack is three cells in series, and so on.
• Parallel count (P) determines capacity and available current. Wiring cells in parallel adds capacity and current-handling capability.

A 3S2P pack, for example, contains three cells in series for voltage, with two of those series strings wired in parallel for capacity. The cell-level ratings do not transfer directly to the pack level — a pack's voltage, capacity, current, and energy are the result of the complete configuration.

The correct configuration is driven by the load's voltage requirement and the energy/current demand derived from the load profile. This is a technical decision best made as part of the engineering design process, not by simply selecting a standard pack from a catalog.

It is also worth noting that the high-discharge requirements of UAV propulsion are fundamentally different from the endurance-focused profile of avionics backup power. A drone battery manufacturer designs packs for sustained high C-rate discharge and minimal weight, whereas an avionics backup pack prioritizes reliable energy delivery under a defined critical load, charging compatibility, and robust protection. An experienced fpv drone battery manufacturer may excel at high-rate pouch packs, but the engineering evidence required for critical aircraft electronics demands its own qualification path.

Protecting and Managing the Pack: BMS, PCM, and Monitoring

A custom avionics backup pack requires a protection strategy that matches its chemistry, configuration, and operating conditions. There are two distinct levels of protection:

PCM (Protection Circuit Module) provides essential protection functions: overcharge protection, over-discharge protection, overcurrent protection, and short-circuit protection. Some designs also include basic temperature protection. A PCM is a safety circuit, but it does not actively manage the battery's state.

BMS (Battery Management System) is a broader term that covers both protection and active management. A BMS typically includes the protection functions of a PCM plus additional capabilities such as:

• Cell balancing, which equalizes the state of charge across series cells. Without balancing, small differences in cell capacity or self-discharge can cause some cells to charge or discharge more deeply than others, reducing pack performance and usable life.
• Fuel gauging, which estimates state of charge (SOC) and remaining runtime.
• State of health (SOH) estimation, which tracks the pack's degradation over time.
• Communication interfaces such as SMBus, I2C, UART, CAN, RS485, or Bluetooth, allowing the battery to report status to the host system.

The choice between a PCM and a smart BMS depends on the system's needs. For a simple backup application where the aircraft system only needs a reliable voltage source, a well-designed PCM may be sufficient. For an application where the flight crew or maintenance system needs to know the battery's state of charge, remaining endurance, or health, a smart BMS with communication is necessary.

When specifying a custom pack, the engineer should define exactly which protection and management functions are required. Do not assume that a particular pack includes balancing or communication simply because it is described as having a BMS. These features are design choices, and each one has cost, complexity, and reliability implications.

Charging, Power Path, and Integration with the Aircraft Bus

A backup battery does not operate in isolation. It is connected to the aircraft's electrical system through a charging path, a load path, and, in many cases, a switching circuit that manages the transition between main power and backup power.

Charger Matching

The charger must be matched to the pack's chemistry, series count, charge voltage limit, and charge current limit. Using a charger designed for a different chemistry or a different series configuration can overcharge the pack, reduce its life, or create a safety hazard.

For lithium-ion and LiPo cells, charging typically follows a constant-current/constant-voltage (CC/CV) profile. The charger applies a constant current until the pack reaches its maximum charge voltage, then holds that voltage while the current tapers to a termination level. The exact voltage and current limits are cell-specific and must be respected.

For a custom avionics backup pack, the charger should be considered part of the battery system, not an afterthought. The Integrated Battery Back-up System sold for general aviation, for example, pairs a LiFePO4 battery with a matched charging system, switching circuitry, and battery state monitoring as a complete unit. This illustrates an important point: the battery's long-term reliability depends as much on its charge management as on the cells themselves.

Power-Path Switching

When the main aircraft bus fails, the backup pack must take over the load quickly and cleanly without damaging the downstream electronics. The power-path switching circuit manages this transfer.

In its simplest form, switching may involve a diode-based OR-ing circuit so the higher of the two voltages (main bus or battery) feeds the load. More sophisticated designs use MOSFET-based switches or contactors, often controlled by the BMS or a supervisory controller, to transfer the load and prevent reverse current flow into a failed bus.

The switching design must address several questions:

• How fast must the transfer occur?
• Can the avionics tolerate a brief interruption during changeover?
• Is reverse current protection required?
• How is the transition between charge and discharge managed?
• How is the backup system disconnected during maintenance?

Connector and Wiring

The connector and wiring are critical but often overlooked integration points. They must match the pack's current rating, maintain correct polarity, withstand the vibration and temperature conditions of the installation, and remain serviceable. A connector rated below the pack's peak current is a reliability risk regardless of the battery's quality.

Mechanical, Thermal, and Environmental Requirements

The electrical design is only half the engineering challenge. The pack must survive the mechanical and thermal environment of an aircraft installation.

Mechanical Enclosure

The enclosure protects the cells from physical damage, provides the mounting interface, and supports the pack's internal structure. For pouch cells, the enclosure also supplies mechanical compression that prevents swelling or movement. The design must account for vibration, shock, and crash loads, as well as the available installation space and weight limits.

Thermal Management

Lithium cells generate heat during charging and high-rate discharge. In an enclosed avionics bay, heat must be dissipated through the enclosure, mounting interface, or forced airflow. The pack's expected operating temperature range must be defined in the requirement, and the design must maintain cells within that range.

A critical distinction is the difference between charge temperature, discharge temperature, and storage temperature. A cell may be capable of discharging at -20°C, but its charging limits and storage limits may be different. These are separate specifications, and each should be evaluated for the specific application.

Ingress Protection and IP Ratings

The enclosure may also carry an ingress protection (IP) rating, which defines its resistance to solids and liquids. An IP rating is specific to the enclosure design and assembly — it is not a general property of a battery chemistry. Do not accept generic "waterproof" language in a specification; demand the actual IP rating and the test conditions that support it.

Thermal Runaway as a System-Level Concern

Lithium batteries carry an inherent risk of thermal runaway, a self-sustaining chain reaction that generates intense heat and can lead to fire. Thermal runaway is not a single failure mode — it can be initiated by internal defects, mechanical damage, overcharge, over-discharge, or external heat sources, and its progression depends on the cell chemistry, state of charge, and pack construction.

In aviation installations, thermal-runaway mitigation is a system-level safety consideration. It involves the cell's intrinsic safety characteristics, the BMS's protection thresholds, the pack's thermal management, the enclosure's ability to contain fire or vent gas, and the aircraft installation's ability to detect and survive a failure event. The FAA has published extensive research on lithium battery hazard analysis that illustrates the complexity of this issue. A custom battery supplier should be able to explain how their pack design addresses these risks for the specific application.

Testing, Documentation, and Compliance Evidence for an Avionics Backup Pack

One of the most common misunderstandings in battery sourcing is confusing the different types of testing and compliance evidence. Each serves a different purpose, and each has a different scope.

UN38.3 Transport Testing

UN38.3 is the United Nations standard for the safe transport of lithium batteries. It requires a defined set of tests — including altitude simulation, thermal cycling, vibration, shock, external short circuit, impact/crush, overcharge, and forced discharge — depending on the cell or battery type. Compliance is demonstrated by a test report and summary, and the framework is defined in the UN Manual of Tests and Criteria, Section 38.3.

UN38.3 is not an aircraft installation approval. It confirms that the battery has been tested for safe transport. It says nothing about whether the battery meets the electrical, environmental, or reliability requirements for installation in an aircraft.

Product Safety Testing (IEC, UL, and Similar Standards)

Product safety standards such as IEC 62133, UL 1642, or UL 2054 apply to specific battery designs and constructions. A test report or certificate is issued for a defined model or product variant, and it is only valid for that specific construction. Passing one model does not certify a different model with different cells, a different series configuration, or a different enclosure.

When requesting product safety evidence from a supplier, specify the exact model and request the certificate or report with the model number, test standard edition, issuing body, and validity scope.

Aircraft Installation Approval (FAA, EASA, and TSO Context)

Approval to install a lithium battery in an aircraft is a system- and installation-level decision, not a battery-level certification in most cases.

The FAA's policy on lithium battery systems explains that TSO authorization is one potential path, but the actual approval basis depends on the aircraft category, the installation, the system design, and the applicable regulatory framework. The EASA lithium battery FAQ similarly describes a case-by-case approach for rechargeable lithium battery installations, depending on aircraft category and the applicable certification basis.

For an experimental aircraft, a modification shop, or an OEM holding a supplemental type certificate, the approval pathway is defined by their specific design and regulatory context. The battery manufacturer's role is to supply a well-characterized pack with verified specifications and test evidence — not to assert that the pack is "approved" as a universal aviation product.

Datasheets, Test Reports, and Traceability

For any custom avionics backup pack, the buyer should expect the following documentation:

• A pack-level datasheet specifying voltage, capacity, energy, current limits, temperature limits, dimensions, weight, and interface details.
• Model-specific test reports covering any safety or environmental testing performed on the exact pack construction.
• A UN38.3 test summary for transport purposes.
• Batch-level traceability linking cells, assembly records, and test results to the specific pack serial number.
• A change-control agreement requiring the manufacturer to notify the buyer of any change to cells, materials, construction, or manufacturing process.

Traceability is particularly important in aviation applications, where a fleet-level issue may need to be traced to a specific cell batch or manufacturing shift.

For adjacent aerospace applications that rely on similarly rigorous engineering and high-reliability power design, a Lithium battery for satellite equipment follows its own qualification standards — but shares the same principles of cell selection, protection, thermal management, and documentation that apply to critical avionics backup systems.

How to Choose and Work with a Custom Avionics Battery Manufacturer

The choice of a custom battery manufacturer is an engineering decision, not just a procurement decision. The supplier's design process, documentation discipline, and willingness to work within a defined requirements framework matter as much as their manufacturing capability.

When evaluating a potential partner, consider the following criteria:

Evaluation Area	What to Ask
Engineering process	How does the manufacturer capture requirements? Do they ask about load profile, current, temperature, mechanical constraints, and charging?
Design transparency	Will they share the cell selection, S/P architecture, BMS design, and protection thresholds?
Prototype and validation	How is a prototype verified against the requirement? What tests are performed, and what evidence is provided?
Documentation	What datasheets, test reports, and compliance documents are provided for each model?
Traceability	How are cells, batches, and serial-numbered packs tracked?
Change control	Will the manufacturer notify the buyer of any change that could affect performance or reliability?
Commercial terms	What is the MOQ? What is the lead time for prototypes and production? What is the warranty scope?

A reputable manufacturer will welcome these questions. Gloflux, for example, positions its custom battery manufacturing around the ability to tailor cells, battery packs, and BMS configurations to the specific application. The company's custom-pack design process begins with a structured engineering discussion, not a price quote.

It is also worth verifying that the manufacturer distinguishes clearly between what it can build and what it cannot promise. A supplier that claims universal aviation approval or a single "safest chemistry" should be treated with caution. A supplier that acknowledges the limits of its specification data and offers to provide exact model evidence is more likely to be a reliable engineering partner.

Getting Your Avionics Backup Pack Sourced — Next Steps

A custom avionics backup battery pack is a project, not a catalog item. It begins with a requirement definition, moves through engineering design and prototyping, and continues through validation, documentation, and production.

The next step is straightforward: prepare a written requirement covering the critical loads, backup endurance, voltage and current limits, charging method, temperature range, mechanical space, connector, and documentation needs. Share that requirement with the manufacturer's engineering team for feasibility assessment, and ask for a proposal that includes the design approach, prototype plan, test evidence, and documentation package.

Defining a clear requirement before contacting a manufacturer significantly improves the chance of receiving an accurate, useful response — and it is the first step toward a reliable, well-documented avionics backup power solution.

For more information on Gloflux's battery products and custom solutions, review the Gloflux product range.