Cold-weather flying and the voltage-sag trap

Why cold batteries lie

A lithium battery produces current through a chemical reaction, and cold reactions are slow. The energy is still in the pack; it simply cannot come out fast enough. The pack's internal resistance rises, and the harder you pull, the more voltage you lose to that resistance instead of to the motors.

The percentage you see is estimated largely from voltage. In the cold that estimate is made while the aircraft is sitting still and drawing almost nothing — which is the one condition where a cold pack looks fine. Ask it for a climb and the voltage drops sharply, the estimate catches up all at once, and the aircraft announces an emergency it was always heading towards.

What the cold actually changes

Pre-heating — the part most people do badly

A warm pack behaves like a normal pack. Getting there is not complicated, but there are two wrong ways to do it that are common.

1. Keep packs warm before you arriveInside jacket, inside the car cabin, in an insulated bag with a hand warmer. Warmth kept is much easier than warmth added.
2. Aim for room temperature, not hotSomewhere around 20–25 °C at the cells is the target. Hotter is not better and shortens pack life.
3. Never use direct heatNo hair dryer, no heater vent held against the pack, no hot water. Uneven heating stresses cells and can damage them permanently.
4. Use the aircraft itself if you mustPower up and let the aircraft idle on the ground for a minute or two. The internal draw warms the pack gently and evenly — slower than a heater, and safe.
5. Swap, do not stackKeep the next pack warm while the current one flies. A cold pack fitted to a warm aircraft is still a cold pack.

The first-minute rule

Even a properly warmed pack is at its most vulnerable in the first minute, when it is doing the most work and has not yet warmed itself through use.

1. Hover low for thirty to sixty secondsThree to five metres. Let the pack warm under gentle load before you ask anything of it.
2. Watch the voltage, not the percentageIf your app shows cell voltage, use it. Voltage under load is honest in a way the percentage is not.
3. Make the first climb gentleA full-power vertical climb on a cold pack is the classic way to trigger the step drop.
4. Abort on any sudden dropA percentage that falls several points in a few seconds is not a glitch. Land and change packs.

Margins that make sense in winter

Condensation — the damage that happens after landing

Bringing a cold aircraft into a warm room is exactly the same physics as fogging a pair of glasses, except the moisture forms on circuit boards, camera sensors and motor windings as well as the lens.

• Let the aircraft warm up in its bag, sealed, so the moisture condenses on the bag rather than on the electronics.
• Do not power up a cold aircraft indoors to "check something". Wait until it has reached room temperature.
• A fogged lens will clear on its own; a fogged sensor cavity may not, and the marks it leaves can be permanent.
• Wet snow on the airframe becomes water the moment you go inside. Brush it off outside, while it is still frozen.
• Silica packs in the case are worth the trivial cost through the winter.

The cold pilot matters too

Cold hands are slow hands. Fine stick control degrades long before you feel properly cold, and the moment you need precision is exactly the moment your fingers have stopped cooperating.

1. Fly with thin gloves you can actually feel throughThick gloves cause more crashes than cold fingers do.
2. Keep the controller warm between flightsIts battery sags in the cold too, and its screen dims and slows.
3. Keep the phone warm and out of the windPhones shut down in cold long before drones do, and yours is the only screen you have.
4. Set a time limit for standing stillJudgement degrades with body temperature, quietly. If you are shivering, the flying decision has already been made for you.

Where Fly4cast fits

Temperature, wind at altitude and the flyability of the next few hours all sit on the same screen, so a cold morning that looks fine from a window can be judged properly before you drive anywhere. Cold days are usually still flying days — they are just days with a smaller margin, and knowing the size of that margin is the whole job.