A cargo drone can have enough nominal battery energy for a flight and still lack the electrical or thrust margin to finish it. Energy in watt-hours answers how much work the pack can supply. Loaded voltage, current capability and temperature determine whether it can supply the required power at the moment the aircraft needs it.
This guide is for electric multirotor logistics and delivery UAV projects. It explains how to connect motor data to a mission energy budget, keep reserve separate from usable capacity, and examine the low-voltage part of the mission. It does not prescribe a universal reserve percentage or flight-controller setting.
1. Separate nominal energy, usable energy and dispatch reserve
Pack capacity in amp-hours is not an energy value until voltage is included. A first label-based estimate is:
Nominal energy (Wh) = nominal pack voltage (V) × rated capacity (Ah)
For an illustrative 12S, 30 Ah pack using 3.7 V nominal cells, nominal voltage is 44.4 V and nominal energy is 1,332 Wh. A conventional 4.2 V-per-cell full-charge specification would correspond to 50.4 V. These cell voltages are assumptions for this example, not values to apply to another chemistry or a different cell specification.
Neither number establishes usable mission energy. Determine usable energy by integrating measured pack power over time under representative load, temperature, pack condition and approved operating limits:
Measured energy (Wh) = sum of [loaded voltage × current × interval in seconds] / 3,600
Then keep the mission reserve explicit. If testing establishes 1,100 Wh available from dispatch to the chosen operational limit and the mission plan protects 200 Wh as reserve, the planned mission has a 900 Wh budget. The 200 Wh is part of the 1,100 Wh; it must not be subtracted from nominal energy while losses and discharge limits are also applied a second time.
2. Build the aircraft mass ledger before choosing a larger pack
Use one all-up mass for each configuration. A useful ledger includes airframe, motors, ESCs, propellers, batteries, wiring, avionics, cargo equipment and cargo. Do not treat advertised payload as takeoff mass or leave battery mass outside the calculation.
Changing the pack changes the mass ledger, required rotor thrust and propulsion operating point. It can also change wiring, mounting and cooling requirements. Therefore, adding 25% nominal energy does not automatically add 25% flight time. Compare complete aircraft configurations, not battery labels in isolation.
After changing mass, return to the relevant multirotor motor data and find the new working thrust for each rotor. The companion thrust test data checklist explains how to compare combinations at equal thrust and keep the electrical measurement boundary consistent.
3. Calculate the mission in segments
A delivery aircraft usually has different power demands during takeoff, loaded transit, delivery waiting, unloaded return and landing. Payload changes, wind and route geometry can change those demands. A single hover-current value cannot represent every segment.
The table below is an invented planning example, not a recorded TORGATEK flight or a product performance claim. Power is total battery-side power, including propulsion and auxiliary loads. Each segment assumes constant average power only to make the arithmetic visible.
| Mission segment | Assumed average pack power | Duration | Energy |
|---|---|---|---|
| Takeoff and initial climb | 8,000 W | 1 minute | 133.3 Wh |
| Loaded outbound transit | 6,400 W | 3 minutes | 320.0 Wh |
| Delivery hover | 6,600 W | 1 minute | 110.0 Wh |
| Unloaded return transit | 4,800 W | 3 minutes | 240.0 Wh |
| Approach and landing | 5,500 W | 1 minute | 91.7 Wh |
| Total | Segment-specific values | 9 minutes | 895.0 Wh |
Each energy entry is power × minutes / 60. With the illustrative 900 Wh mission budget, only 5 Wh remains above the protected 200 Wh reserve. That is not a robust dispatch margin: small measurement errors, a longer approach or higher transit power can remove it.
One additional minute of delivery hover uses another 110 Wh. Mission energy becomes 1,005 Wh, leaving just 95 Wh of the assumed 1,100 Wh available. The aircraft would violate the example's 200 Wh reserve requirement, even though mission energy remains below the pack's nominal 1,332 Wh label.
The lesson is not that 200 Wh is the correct reserve for a cargo UAV. It is that the reserve must be tied to the actual recovery requirement, operating constraints and validated aircraft behavior. The route is acceptable only when the complete scenario, uncertainties and required reserve are supported by evidence.
4. Check the low-voltage power demand separately
A simple first-order electrical model is:
Loaded bus voltage is approximately open-circuit voltage minus current × effective series resistance
This model is useful for reasoning about voltage drop, but an operating battery is not a fixed resistor. State of charge, temperature, current history and aging affect its response. Measure voltage and current together instead of assuming one resistance value predicts every flight condition.
At a hypothetical constant 6,600 W battery-side demand, the arithmetic is:
| Measured loaded voltage | Power demand held constant for illustration | Calculated pack current |
|---|---|---|
| 46.0 V | 6,600 W | 143.5 A |
| 42.0 V | 6,600 W | 157.1 A |
| 40.0 V | 6,600 W | 165.0 A |
Current increases by about 15% between 46 V and 40 V in this constant-power example. For the same resistance, the associated I squared × R loss would increase by about 32%. These are arithmetic comparisons, not predictions that a particular aircraft will hold power or thrust at 40 V. Actual motor speed, controller limits, thrust and temperature can change as voltage falls.
Tyto Robotics' published propulsion testing methodology describes how battery sag can affect repeatability. For a supplier test request, specify a representative loaded-voltage range and ask for working-thrust data at its lower end. Do not compare a fresh-pack peak test with a late-mission requirement as if they were the same condition.
5. Do not mistake remaining charge for remaining capability
A pack can retain charge while reaching a voltage, current or temperature limit that makes the required aircraft power unavailable. Conversely, a brief voltage dip alone does not quantify the energy left for the route. Keep these checks separate:
Energy: Can the mission and required recovery be completed within measured usable watt-hours?
Power: Can the battery and distribution system supply each mission segment without exceeding their limits?
Propulsion: Can the installed motor/ESC/propeller combination provide required thrust at the lowest relevant loaded voltage?
Thermal behavior: Do temperatures remain within the specified limits for the required durations and installation?
Control and recovery: Does the aircraft retain the authority and validated recovery behavior required by the operation?
ArduPilot's battery failsafe documentation treats voltage and estimated remaining capacity as separate inputs and describes low and critical response layers. Its current-limiting and voltage-scaling documentation also explains that battery protection can affect available throttle and control behavior. These features do not create additional battery energy or prove that the propulsion system has sufficient margin.
Set protective behavior using the aircraft's current documentation and validation process. Extending endurance by disabling protection or copying thresholds from an unrelated aircraft does not resolve an undersized power system.
6. Make the reserve depend on recovery, not a convenient percentage
Separate the energy needed to finish the planned route from the energy needed if the delivery cannot be completed. Consider the actual recovery path, payload retained after an aborted drop, an approach repeated at the destination and the conditions that the operation permits. A reserve calculated for an empty return can be inadequate when cargo remains attached.
Use conservative, measured segment power where the scenario calls for it. Do not assume that a return leg is cheaper simply because the outbound leg was expensive: wind direction, groundspeed and payload state may change the relationship. There is no single headwind percentage that safely corrects every route.
Keep uncertainty visible beside the energy budget. Record sensor accuracy, duration variability, pack-to-pack differences and the representative battery condition. A plan with only 5 Wh of headroom cannot absorb even a modest error without consuming the protected reserve.
7. Request a validation package before freezing the design
For a discussion with a motor supplier, provide all-up mass by payload state, rotor count and layout, propeller envelope, voltage range, installation environment and segment durations. Ask for data that connects those conditions to the candidate motor product, rather than assuming its headline rating establishes mission suitability.
Document the exact motor, ESC, propeller and battery configuration.
Obtain working-point thrust, current and temperature records across the intended loaded-voltage range.
Characterize pack usable energy under representative load and approved operating limits.
Validate installed-aircraft power and temperatures through a staged, appropriately controlled test program.
Reconcile measured mission energy, recovery requirements and protective behavior before approving the configuration for the intended operation.
Log total pack voltage/current, elapsed time, payload state, flight phase, relevant component temperatures and the battery's identification and condition. Check sensor calibration before interpreting a small efficiency improvement. Keep the same measurement boundary when comparing bench records and flight logs.
Frequently asked questions
Can I estimate endurance from battery Ah and hover current?
It can provide a rough comparison when the assumptions are explicit, but it omits changing voltage, mission phases, usable capacity and reserve. A segment-based watt-hour budget is more transparent for a cargo mission.
Will a higher-voltage battery automatically increase flight time?
No. Higher voltage can reduce current at the same electrical power, but the complete motor, ESC, propeller, pack mass and wiring configuration must be compatible. Compare measured aircraft energy and performance, not voltage alone.
Why can two identical battery labels produce different results?
The label does not establish the packs' condition, temperature, loaded voltage or usable energy under your mission. Compare logged discharge behavior and documented pack history under equivalent conditions.
Plan the propulsion requirement with TORGATEK
For a new delivery platform, contact TORGATEK with the mass ledger, battery architecture, propeller limits and mission profile. Ask which motor combinations have relevant operating-point evidence and which require additional testing. A useful selection starts with the power and thrust needed near the end of the mission, not only the best number at takeoff.
Engineering note: All numerical examples are hypothetical planning calculations, not measured endurance, battery settings or operating approval for a TORGATEK product or aircraft.
