A phone that dies at 30% in a snowstorm, a camera that shows an empty battery after ten cold frames, a power bank whose 20,000 mAh sticker somehow charges nothing twice — none of this is bad luck. Lithium-ion chemistry behaves differently near and below freezing, and most of what looks like a dead battery is either a temporary voltage sag or the result of one genuinely dangerous habit: charging a cold cell. This guide explains the mechanism, the real math behind capacity stickers, the derating you should plan for, and the insulation routine that keeps every device alive until morning.
What cold actually does to a lithium cell
Inside a lithium-ion battery, charged lithium ions travel through electrolyte from one electrode to the other, and their mobility depends on temperature. As the temperature drops the electrolyte thickens, the cell's internal resistance rises sharply, and ion diffusion slows. Under load — screen on, radio transmitting, shutter firing — the voltage sags faster than the fuel gauge expects, so the phone's software misreads a cold, sluggish cell as an empty one and shuts down. Battery University summarizes this as increased internal resistance and reduced usable capacity at low temperatures (see their BU-501 discharge basics).
Temporary sag versus permanent damage
These are two different failures and confusing them is expensive. Discharge in the cold is mostly reversible: a phone that dies at −10 °C will often wake back up with most of its charge after ten minutes in an inner pocket. The ions were always there; they simply could not move fast enough. Charging in the cold is the irreversible one. Below about 0 °C the graphite anode cannot accept incoming ions fast enough, so metallic lithium plates onto its surface. This lithium plating permanently consumes cyclable lithium (real capacity loss) and can grow needle-like dendrites that pierce the separator between electrodes, causing internal shorts and, in the worst case, thermal runaway and fire. A cell that has been repeatedly fast-charged below freezing is damaged for life even if it looks fine afterward.
Manufacturer limits: Apple and the 0 °C line
Phone makers publish the numbers and cold users should know them. Apple's operating-temperature guidance puts the working range for iPhone, iPad and Apple Watch at roughly 0–35 °C (32–95 °F); below the lower bound devices may refuse to charge, shorten runtime, dim the display or shut down unexpectedly, while storage limits run colder but still exclude charging. The takeaway is not that the hardware instantly breaks at −1 °C — it will happily run while kept warm against your body — but that 0 °C is the charging boundary the manufacturer itself enforces, and so should you.
The mAh sticker lies: real 5V output from a power bank
The capacity printed on a power bank is measured at the 3.7 V nominal voltage of the internal cells, while your phone draws from the USB port at 5 V. Stepping voltage up wastes energy as heat, typically at 80–90% conversion efficiency in quality banks and worse in cheap ones. The honest output is:
- Delivered mAh at 5 V = sticker mAh × 3.7 ÷ 5 × efficiency
- With efficiency ≈ 0.85: factor is 3.7 ÷ 5 × 0.85 ≈ 0.629
- A 10,000 mAh bank therefore delivers about 6,290 mAh at 5 V — roughly 1.9 charges of a 3,278 mAh phone, not 3.0
| Sticker (3.7 V cells) | Cell energy | Real 5 V output at 85% | Phone charges (3,278 mAh) |
|---|---|---|---|
| 5,000 mAh | 18.5 Wh | ≈ 3,145 mAh | ≈ 0.96 |
| 10,000 mAh | 37 Wh | ≈ 6,290 mAh | ≈ 1.9 |
| 20,000 mAh | 74 Wh | ≈ 12,580 mAh | ≈ 3.8 |
Cable loss, a phone that is simultaneously transmitting on a weak signal, and cold-soaked cells all eat into these numbers further. Plan weight and charging stops from the 5 V column, not the sticker. The cold-weather power station calculator runs this conversion with the same 0.85 factor and then applies the cold derating below.
Cold derating: how much capacity really survives
A bare cell's usable capacity falls with temperature along a curve; the planning values used across this site are 100% at 25 °C, about 75% at 0 °C, about 55% at −10 °C and about 35% at −20 °C. The single most effective intervention costs nothing: body warmth adds roughly 15 °C of effective temperature. A bank or phone carried in an inner pocket at −10 °C behaves as if it were at +5 °C and retains about 80% instead of 55% — roughly half again as much energy from the same hardware.
| Ambient | Bare device | Inside inner clothing (≈ +15 °C) |
|---|---|---|
| 0 °C / 32 °F | ≈ 75% | ≈ 90%+ |
| −10 °C / 14 °F | ≈ 55% | ≈ 80% |
| −20 °C / −4 °F | ≈ 35% | ≈ 75% |
Notice that insulation beats buying a bigger bank: a 10,000 mAh bank on your body outperforms a bare 20,000 mAh one at −10 °C while weighing half as much.
The iron rule: never charge below 0 °C
If you remember one rule from this guide, remember this: do not feed charging current into a lithium cell at or below 0 °C. Plating begins within minutes at high charge rates, and the dendrite damage accumulates invisibly until a future charge ends in a swollen cell, a puffed pocket or a fire. Phones and reputable banks include temperature sensors that block charging, but cheap banks, some cameras charging over USB, and cells that have just been warmed from −20 °C to a surface reading of +1 °C while their core is still frozen defeat that protection.
How to charge safely in winter camp
- Warm the device and the bank first, ideally to 5–10 °C, inside clothing or a sleeping bag; charge only while both stay warm.
- Charge in an inner pocket while moving, with cable and bank both under layers — never from a bank dangling outside the pack.
- A chemical hand warmer helps, but wrap it in a sock or hat: never stick it directly against a cell. Localized overheating degrades lithium just as surely as cold, and condensation from direct warming invites corrosion.
- Avoid fast-charging protocols in the cold; slow charging at a warm temperature is far gentler than a fast pulse into a marginal cell.
Device-by-device field tactics
Treat warmth as a resource you carry on your body, and allocate it by consequence:
- Phone: inner chest pocket, not a hip pocket or pack strap. Download maps for offline use, switch to low-power mode, and use airplane mode in weak signal areas — a searching radio is the biggest single power draw. Sleep with it in the foot of your sleeping bag on the coldest nights.
- Camera: keep the body inside your coat until you shoot; carry spare batteries exclusively in inner pockets and rotate them. Condensation is the secondary enemy: seal a cold camera in its bag before entering a warm tent so moisture forms on the bag, not the optics and circuit board.
- Headlamp: disposable lithium primaries (for example Energizer L92 cells) hold voltage far better in cold than alkalines, which can leak as they weaken. Carry the headlamp and spare cells into the sleeping bag overnight.
- Overnight: every lithium device that must work in the morning goes into the sleeping bag with you. A boot bag inside the tent is not warm enough at −20 °C.
Solar panels in winter: peak hours, not daylight hours
A panel's wattage is a laboratory rating measured under bright, perpendicular, cool standard conditions. Real daily harvest follows wattage × effective peak-sun-hours × a derating factor of about 0.65–0.75 for angle, heat, cable loss and charge-controller efficiency. Clear weather offers roughly 4–6 peak-sun-hours at most latitudes, but winter mountains subtract heavily: the low sun never climbs overhead, valley walls shade panels for hours, and short days leave little margin. As a worked example, a 20 W panel seeing 4.8 effective peak hours at 0.70 derate yields 20 × 4.8 × 0.70 ≈ 67.2 Wh in a day — about 1.8 charges of a 10,000 mAh bank, before cold derating. NREL's solar resource data and the U.S. Department of Energy's solar glossary (linked in the sources panel) explain the peak-sun-hour model. Tilt panels steeply toward the low sun, keep new snow off the glass, and remember the harvested energy still cannot go into a freezing-cold bank — warm it before charging.
Storage, self-discharge and travel rules
Lithium cells self-discharge only a few percent per month at room temperature, but the rate climbs with heat and with being kept fully charged; long-term storage at roughly 40–60% state of charge and cool temperatures is the standard recommendation. For travel, airlines treat power banks as spare lithium batteries: they belong in carry-on baggage only, never checked luggage, and banks up to about 100 Wh (a 10,000 mAh bank is ≈ 37 Wh; 20,000 mAh ≈ 74 Wh) generally fly without airline approval, while larger ones need it. Tape or cap exposed connectors so keys and coins cannot short them. At home, never leave spare banks in a freezing car or a hot dashboard — both extremes age cells quickly.
The cold-weather power routine
- Before the trip, compute real 5 V capacity and expected charges from the sticker, not from the marketing claim.
- Charge everything fully indoors; start warm and start full.
- Move phone, camera batteries, bank and headlamp into inner-pocket and sleeping-bag positions.
- Switch the phone to low-power mode, offline maps and airplane mode when signal is weak.
- Never charge a cell at or below 0 °C; warm it first, keep it warm through the charge.
- Keep hand warmers separated from cells by a layer of fabric.
- Budget solar from effective peak-sun-hours and a 0.65–0.75 derate, not from panel wattage.
- Carry banks in carry-on luggage, protected terminals, 40–60% charge for long storage.
Run your own battery budget — true 5 V capacity, cold derating, solar harvest and headlamp runtime — with the cold-weather power station calculator before the trip, so the number you pack against is the one the thermometer will actually deliver.