How to Spec a Custom UAV Battery Pack: A Guide for Drone OEMs

Off-the-shelf packs are a starting point. Once your platform has real payload, endurance, and form factor requirements, they become the constraint. This post is a step-by-step framework for speccing a custom Li-Ion battery pack for UAV applications.

When does a custom pack make sense?

Three signals indicate you have outgrown off-the-shelf.

First: your endurance target cannot be met within the airframe's volume and weight budget. A standard pack leaves you 10–15 minutes short and adding a second pack pushes MTOW past the motor spec. That is a chemistry and configuration problem, not a procurement problem.

Second: your form factor is non-standard. Tight fuselage envelope, distributed dual-pack layout, or an unusual aspect ratio that no catalogue pack fits without airframe compromise.

Third: cycle life or thermal behaviour does not match field conditions. A pack rated for 300 cycles at 25°C that is operated daily in 35°C+ ambient or high-altitude low-temperature conditions will degrade ahead of schedule. Custom packs are specified for the actual operating environment.

If one of these applies, custom is the right path.

Cell chemistry: the first decision

Chemistry determines energy density, discharge capability, and cycle life. Get this right before anything else.

NMC 811 is the default for most performance UAV programs. At 250–270 Wh/kg and 400–600 cycle life, it hits the right balance for commercial drone development. Silicon anode cells are the call when weight is the program-defining constraint and budget allows. Current 21700 silicon anode cells reach up to 300 Wh/kg with 600-cycle life. LFP is the right choice when the platform runs multiple cycles daily and cycle life matters more than flight time per mission.

A note on cell format

21700 cylindrical cells dominate performance UAV packs. The format is mechanically robust, thermally predictable, and available from multiple suppliers, Samsung, Molicel, Amprius among them. The traditional argument for pouch cells was higher energy density, but that gap has closed. The Molicel M65A reaches 322 Wh/kg in a 21700 format. Silicon anode cells in the same cylindrical format reach 300 Wh/kg. Those figures cover most UAV energy density requirements without the structural complexity of pouch construction.

Pouch cells remain valid for two cases: the airframe geometry genuinely cannot accommodate cylindrical cells, or the energy density requirement exceeds what cylindrical cells can deliver. For most commercial drone programs, cylindrical 21700 is the right format.

Pack configuration: voltage, capacity, and weight

Voltage (series configuration)

Series cell count sets nominal pack voltage. Match it to your motor and ESC architecture from the start, changing it later means redesigning the power electronics.

Capacity sizing formula

Start with this:

Required capacity (Ah) = (Average power draw (W) × Flight time (h)) ÷ Nominal voltage (V)

Then multiply the result by 1.2–1.3. That factor covers two things: the 80% depth of discharge limit that preserves cycle life, and a safety margin for real-world variation in power draw.

Example: a platform drawing 1,200 W average on a 6S (22.2 V nominal) pack targeting 45 minutes of flight.

• Base: (1,200 × 0.75) ÷ 22.2 = 40.5 Ah
• With 1.25 margin: 40.5 × 1.25 = ~50.6 Ah

That is your target pack capacity before cell selection and parallel configuration.

Weight budget

Battery represents 25–40% of MTOW on a performance UAV. If your weight budget puts the pack above 35% of MTOW, revisit cell chemistry before locking the configuration. Moving from NMC 622 to NMC 811 recovers roughly 10–15% in mass for the same capacity. Moving to silicon anode cells recovers more. The chemistry decision and the weight budget are not independent, work them together.

BMS requirements for UAV applications

UAV BMS is not industrial BMS. Every gram on the BMS is a gram off payload or endurance. Specify these parameters before the pack design is locked.

Cell balancing. Passive balancing is standard for UAV packs up to 12S. It dissipates imbalance as heat during charging. Active balancing redistributes energy between cells and is more efficient, but adds weight and cost. Active balancing is only justified for large packs, above ~30 Ah, where tight cell-to-cell matching cannot be maintained over the pack's cycle life.

SoC accuracy. Specify ±2% or better. Mission planning and low-battery return-to-home decisions depend on accurate state-of-charge data. A BMS that drifts to ±5% SoC is a safety risk, not just a performance issue.

Communication interface. UART or SMBus for basic packs where the flight controller only needs voltage and SoC. CAN bus for platforms with full avionics integration, flight controllers, autopilots, and GCS systems that need real-time pack telemetry. Specify the protocol before enclosure design begins.

Protection thresholds. Thermal cutoff, overcurrent protection, and undervoltage lockout are non-negotiable. Define the specific thresholds in the spec document, do not leave them to the supplier's default settings.

Smart BMS with telemetry. Worth the weight penalty for commercial and fleet operations. Cycle count, temperature history, and per-cell voltage logs reduce field maintenance cost and enable predictive replacement. For single-platform development or research programs, the simpler BMS is usually the right call.

Form factor and mechanical integration

Define the pack envelope before approaching a supplier. Maximum length, width, and height. Connector location, whether it exits the front face, the side, or the end of the pack affects how the cable routes through the airframe. Mounting interface, rail, plate, and retention mechanism. All of this needs to be in the brief.

Centre of gravity matters as much as volume. A pack that physically fits but shifts CoG outside tolerance is unusable. For multirotor platforms especially, the battery sits on or near the CoG axis by design. If a custom pack is longer or shorter than the original, the mounting position shifts, and the CoG calculation must be re-run. Flag CoG sensitivity in your brief, it tells the supplier where there is and is not flexibility in the form factor.

Cable exit direction is consistently underspecified. A connector on the wrong face of the pack adds 15–20 cm of cable routing through a tight fuselage, which creates bend radius problems, weight, and assembly complexity. Specify exit direction explicitly.

Connector standard needs to be agreed before enclosure design is finalised. XT60 handles up to ~60 A continuous. XT90 handles ~90 A. AS150U handles 150 A and is the choice for high-current platforms. If none of these fit the harness architecture, custom contacts are an option, but they add lead time and cost.

For dual-pack redundant architectures: parallel packs require matched internal resistance. Not all cells maintain tight internal resistance tolerance over cycle life. Confirm with your supplier that the cell selected is suitable for parallel pack operation.

Certifications and transport compliance

UN 38.3. Required for air transport of any Li-Ion pack. Any reputable manufacturer will have this. Ask for the test report, not just the certificate, and confirm the report covers your specific pack configuration, not just the base cell. A UN 38.3 certificate on a 100 Wh cell does not automatically cover a 1,500 Wh pack built from that cell.

IEC 62133. Required for commercial UAV applications in regulated markets, particularly if the end product carries CE marking. Confirm with your compliance team whether this applies to your platform before issuing the RFQ.

ADR (road transport in Europe). Class 9 dangerous goods rules apply to Li-Ion packs above certain Wh thresholds when transported by road within Europe. Your supplier should handle documentation, but you need to be aware of the threshold and the packaging requirements.

If your platform requires DO-160 or MIL-STD-810, that needs to be in the specification document from day one, not retrofitted after pack design is complete.

What to send your supplier

An engineer who sends the following gets a real quote. One who sends "I need a UAV battery" gets a generic response and a two-week delay.

• Nominal voltage and capacity (Ah)
• Peak and continuous discharge rate (C rating or amps)
• Form factor envelope: max dimensions and weight budget
• Cell chemistry preference (or "open to recommendation")
• BMS requirements: communication protocol, SoC accuracy, telemetry output
• Connector type and cable exit direction
• Target cycle life and depth of discharge
• Certifications required (UN38.3, IEC 62133, other)
• Target volume: prototype only / pilot run / production scale
• Timeline: when is first article needed?

The more of this you can document before the first conversation, the faster a supplier can give you a real answer.

Key decisions: summary

• Chemistry first: NMC 811 for most performance UAV, silicon anode cells when weight is the program constraint
• Configuration: size capacity with 20–30% margin over the calculated requirement; verify against the weight budget before locking
• BMS: specify communication protocol and SoC accuracy before pack design is locked, changing these after enclosure design adds time and cost
• Form factor: nail the envelope and CoG budget before approaching a supplier
• Certifications: confirm UN38.3 is covered and covers your specific pack configuration; check IEC 62133 and DO-160 requirements with your compliance team before RFQ
• Timing: Dan-Tech delivers custom packs in as little as 3 weeks from confirmed spec — start the conversation early to protect your program schedule

Dan-Tech Energy builds custom Li-Ion packs for UAV and drone OEMs across Europe, from prototype to production. Browse our battery pack catalog, or use the ToolBox to configure your spec and get a direct answer on what is achievable and when. Lead times start at 3 weeks from confirmed spec.