Power in the Outdoors: Watt-Hours, Flights, Cold, and Safety

Power planning is arithmetic followed by discipline. The arithmetic is watt-hours; the discipline is not spending them on the wrong device at the wrong temperature.

Count watt-hours, not milliamp-hours

• A power bank's mAh rating is usually at 3.7 V cell voltage: Wh ≈ mAh × 3.7 ÷ 1000.
• A 20,000 mAh bank is about 74 Wh — but your phone will not see 74 Wh. Voltage conversion, cable loss and cold all take a share.
• Camera, drone and laptop batteries: read Wh, or V × Ah. "Two spare batteries" is not a power plan.

Flights and trains

• China domestic flights: power banks without a 3C mark, or under recall, are prohibited. Carry them in the cabin, never in checked baggage.
• Common limits: up to 100 Wh in carry-on; 100–160 Wh with airline approval; above 160 Wh prohibited. Always check the current CAAC and airline rules.
• Do not leave a power bank charging unattended in the air.
• International flights follow IATA and the airline; individual countries may be stricter.

Cold

• Lithium capacity drops sharply in the cold. Spare batteries go in an inside pocket, not an outside one or the roof of a car.
• Do not charge lithium batteries below freezing. Let them come back to room temperature first.
• Going from cold to warm: seal the device in a bag before opening it, or condensation forms inside.

Charging strategy

• Airplane mode, power saving, offline maps downloaded in advance.
• Power goes to life-support devices first: phone, headlamp, satellite communicator. Cameras get what is left.
• Solar panels are a supplement, not a supply. Cloud, shade and a poor angle will produce almost nothing.
• Use PD where supported, and avoid cheap cables and multi-port splitters that divide the current.
• Large portable power stations: ventilate, keep away from tents and naked flames, and follow the manual.

Safety

• Swollen, hot, smelly or loose-connector power banks: stop using them immediately. Do not charge them, do not fly with them.
• Do not bridge the output terminals with metal, and do not leave batteries in a hot car.
• Buy certified products, and watch for recall notices.
• Damaged cells go to e-waste collection, not the general bin.