Why batteries deliver less energy in the cold, and how to plan for it

A battery in the cold gives less energy than it does at room temperature, and the device it powers may shut down early. The bank may still show charge, and the phone may still show a percentage, but the energy that reaches the phone falls. In the power budget this is a lower end-to-end efficiency, not a change in the stored watt-hours. Apple designs iPhone and iPad for ambient temperatures between 0 °C and 35 °C (32–95 °F), and says that using them in very cold conditions outside that range might temporarily shorten battery life and could turn the device off.

What the cold does inside the battery

A lithium-ion cell stores energy in a chemical form that releases it as current flows. Low temperature slows that chemistry and raises the cell’s internal resistance. Under load, the resistance drops the voltage at the terminals, and a device’s protection circuit may cut power when the voltage falls below its limit. The cell can still hold charge in that condition, so the bank or phone may show energy that it cannot deliver at the moment.

Cold reduces the energy a cell can deliver at that moment, and much of that capacity returns when the cell warms. That is why a phone that shuts down on a cold walk may run again after it is kept warm. Apple describes the cold loss as temporary: battery life returns to normal once the device is back within its operating range, and how quickly depends on how fast it warms.

The power budget does not model that temperature dependence directly. It takes the rated watt-hours as the starting point and applies one efficiency to them. Lowering that efficiency is a shortcut that stands in for the cold losses, and it is not a physical measurement of the cell.

This also explains why the same bank can behave differently on consecutive days. A bank left in a cold tent overnight starts the day colder than one kept in a warm pocket, so the first hour of use may give less than the afternoon.

Cold as a lower efficiency

The power budget tool has one input for losses, the assumed end-to-end efficiency. Cold losses are not a separate term in the model, so the simplest way to test a cold scenario is to lower that input. The methodology page describes the same simplified efficiency for the battery and solar paths, and it is an assumption, not a measurement. Treat a lower value as a planning scenario.

Start with the stored energy of a 10,000 mAh bank at 3.7 V:

stored = 10,000 ÷ 1,000 × 3.7 = 37.0 Wh

80% (tool default): 37.0 × 0.80 = 29.6 Wh usable
60% (cold scenario): 37.0 × 0.60 = 22.2 Wh usable
loss: (29.6 − 22.2) ÷ 29.6 = 0.25, so 25% less

The 25% is the scenario’s effect on usable energy, and it is the same whatever the cell’s size. A 20,000 mAh bank at 3.7 V holds 74.0 Wh, so it gives 59.2 Wh at 80% and 44.4 Wh at 60%, the same 25% reduction.

The efficiency input in the tool is the place to test the cold case. Set it to 60% and leave the rest of the trip unchanged, then compare the balance against the plan. The example below uses the 2 W, 3 hours a day, 3 day trip from the phone guide, with no solar input, so the cold effect is isolated.

80%: 29.6 − 18 = 11.6 Wh left
60%: 22.2 − 18 = 4.2 Wh left
50%: 18.5 − 18 = 0.5 Wh left

At 60% the margin shrinks from 11.6 Wh to 4.2 Wh, which is 7.4 Wh less, or about 64% of the original margin lost. At 50% the margin is almost gone. The bank still covers the demand in the cold scenario, but only just, and a single unplanned use of the phone can take the reserve. That is the practical reason to plan with a lower figure: the cold removes margin faster than it removes the bank’s headline energy.

Charging below freezing

Charging is stricter than use. Battery University’s guidance is that consumer-grade lithium-ion cells should not be charged below 0 °C (32 °F), because at freezing temperatures metallic lithium plates onto the anode, a permanent loss of performance and safety. It advises fast charging only between 5 °C and 45 °C. Apple notes that a very cold iPhone might not charge, or might stop charging. Warm the bank and the phone inside clothing or a sleeping bag before plugging them in, and follow the manufacturer’s stated charging range.

Wind and feels-like conditions

Cold on a trip is rarely still air. Wind carries heat away from the body and from the devices in a pack, so the conditions that matter for a battery are the air temperature and the wind together. The feels-like tool applies the wind chill formula used by the US National Weather Service and Environment and Climate Change Canada, and it applies only when the air is at or below 10 °C and the wind is faster than 4.8 km/h.

For a worked case, take −10 °C air and a 20 km/h wind. With V = 20 and T = −10, the formula gives:

V^0.16 = 20^0.16 = 1.615
WCI = 13.12 + 0.6215 × (−10) − 11.37 × 1.615 + 0.3965 × (−10) × 1.615
    = 13.12 − 6.215 − 18.36 − 6.40 = −17.9 °C

The wind chill is about −17.9 °C, which is a wind-chill temperature for exposed skin and not a battery temperature. The battery in a pack sits at a temperature the wind chill does not describe. The number tells you how cold the air feels, and the battery is inside the layers that the wind is removing heat from. The feels-like tool gives the same result for these inputs, and how wind chill is calculated explains the formula and its limits.

Practical steps that keep energy available

Each step protects the margin that the scenario shows is at risk.

• Keep the phone and the power bank in an inside pocket, close to the body, rather than in an outer pocket or a hip belt.
• At night, keep the bank inside the sleeping bag rather than in the tent’s open air or a pack left outside.
• Before charging a cold battery, check the manufacturer’s charging range and keep the battery within it.
• Lower screen brightness, close unused apps and turn off radios you do not need. Those are loads you control directly.
• Carry a spare bank, and keep it in the same warm place as the first.

Each step reduces demand or protects the energy that is available, and none of them changes the rated watt-hours. For planning, the efficiency scenario above is the figure to test against the trip, and the phone charges guide shows how the count changes when the efficiency falls.

Where cold changes the wider plan

Cold also affects other battery-powered items. A UV water treatment device runs on a cell that the cold affects in the same way, so its share of the margin can shrink too. The UV water treatment guide covers the limits of that method. For the other items to check before a cold trip, the offline trip planning checklist lists the planning steps in one place.