Battery Voltage Sag on a Cold Morning Mapping Mission
A 6 kg fixed-wing RPAS is tasked with a 45-minute mapping sortie at 100 m AGL. Ambient temperature at launch is 2 °C, and the battery pack was stored overnight in an unheated vehicle. The pre-flight check shows pack voltage at 96% of nominal, and the autopilot's remaining-capacity estimate reads 92%. Six minutes into the flight, at the far end of the mapping grid, the voltage alarm triggers under climb and recovers when the aircraft levels off. The aircraft is 3.5 km from the launch point, with a 15 kt headwind forecast for the return leg. The autopilot's endurance estimate still shows 28 minutes remaining.
1. Interpret the voltage alarm and the discrepancy with the capacity estimate.
Read the worked answer
The alarm is a voltage-sag event, not necessarily a depleted pack. Cold cells have higher internal resistance, so voltage droops under load (climb) and recovers at lower current (level flight). The capacity estimate is a coulomb-counting or model-based figure and lags the real electrochemical state; it does not account for the cold-induced resistance rise or for the reduced usable capacity at 2 °C.
The key judgement is that the 28-minute estimate is optimistic. The pack was cold-soaked, its true usable capacity is below nominal, and the sag under load indicates the margin is thinner than the display suggests. Treat the alarm as a warning that the reserve is being eaten faster than modelled, not as a transient to ignore.
2. The aircraft is 3.5 km out with a 15 kt headwind on return. What is your decision, and what numbers drive it?
Read the worked answer
Turn back now. The relevant numbers are the return distance, the headwind, and the reduced usable capacity:
- 3.5 km at, say, 15 m/s groundspeed into a 15 kt (about 7.7 m/s) headwind gives an effective ground speed of roughly 7 m/s, so the return leg alone could take about 8 minutes.
- Add the time already flown (6 minutes) and the fact that the pack is cold-soaked and sagging under load; the displayed 28 minutes cannot be trusted.
- You also need a reserve for the descent, landing pattern, and a go-around.
Given the alarm, the cold pack, and the headwind, continuing the grid would leave no defensible reserve. Abort the mapping, climb only as needed to clear terrain, and return on a direct track to preserve energy. Land and reassess the pack before any further sortie.
3. What pre-flight changes would have prevented this situation?
Read the worked answer
The root cause is a cold-soaked pack flown without accounting for temperature effects:
- Warm the pack to within its operating range before flight (insulated storage, warm vehicle, or heated box) and verify it has been at temperature long enough to stabilise.
- Recompute the endurance estimate using the actual pack temperature and a conservative usable-capacity figure, not the nominal 100%.
- Set the alarm thresholds for voltage-under-load rather than a single low-voltage trip, and brief an abort point at the far end of the grid.
- Reduce the planned sortie duration to leave a larger reserve when the temperature is near or below freezing.
A pre-flight voltage reading at 96% of nominal on a cold pack is a flag, not a green light.