Batteries lose most of their capacity long before they die

There is a settled way of talking about batteries in cold. It is worth asking how much of it survives contact with the detail.

What cold actually does to a cell

A battery produces current through a chemical reaction, and like most chemistry that reaction slows as temperature falls. The result is higher internal resistance, so the cell cannot deliver current as readily and its voltage drops further under load than it would when warm.

Devices interpret low voltage as low charge, which is why a headtorch or a phone can report empty while holding a substantial amount of energy. Warm the same cell and much of that apparent capacity returns, which is a useful demonstration that the energy had not gone anywhere. That distinction between capacity lost and capacity hidden is the single most practical thing to understand about batteries in winter.

Chemistry matters more than brand

Alkaline cells perform poorly in cold, losing usable capacity quickly as temperature falls and delivering less current when they do work. Lithium primary cells hold up considerably better in low temperatures, weigh less for the same energy and cost more, which is the trade being made. Rechargeable lithium-ion cells, used in phones and most modern torches, work in cold but lose runtime and should not be charged below freezing.

Charging a lithium cell below freezing can cause permanent internal damage, which is why devices with sensors refuse to charge until they warm up. Nickel-based rechargeables sit between the extremes and self-discharge more quickly in storage unless they are a low self-discharge type.

Keeping cells warm is the whole technique

The most effective measure is body heat, keeping spare cells and the devices you rely on in an inside pocket rather than in a pack lid. A device only needs to be warm while in use, so a torch carried inside a jacket and put on at dusk performs far better than one that has been cold all day. Insulating a battery pack in a sock or a small pouch slows heat loss and is enough to make a marked difference on a cold day.

Overnight, taking batteries into a sleeping bag keeps them functional for the morning, which matters most when the morning is the part that is dark. None of this is exotic, and it consistently outperforms carrying additional cells that are all equally cold.

Planning power for a cold trip

Assume less runtime than the manufacturer's figure, because those figures are quoted at comfortable temperatures with a fresh cell under a steady load. Carry a spare set of cells for anything that matters, and keep them warm and dry, since a spare that has been frozen all day is not much of a spare. Devices differ in how gracefully they fail: some torches dim progressively while others cut out, and knowing which yours does changes how you plan.

Locally, where a device takes both a rechargeable pack and standard cells, that flexibility is worth a small weight penalty on a long winter trip.

Solar charging is unreliable in short cold days at high latitudes, so it should be treated as a supplement rather than as the plan.

Storage and long-term health

Lithium-ion cells age faster when stored at full charge and when stored warm, so long-term storage is better at partial charge in a cool place. They also degrade with each charge cycle, which is why a pack that once lasted a weekend eventually manages a day even when it is warm. Alkaline cells left inside devices for long periods can leak and corrode contacts, which ruins the device rather than merely the battery.

Removing cells from anything going into storage is the least effort with the largest payoff in equipment maintenance. Batteries also have a shelf life, so the emergency torch that has not been opened for years should be tested rather than trusted.

None of this is uniform across a country the size of most of these.

Disposal and the part people skip

Batteries contain materials that should not go into general waste, and collection arrangements differ between countries and even between towns. Damaged or swollen lithium cells are a fire risk and need specific handling, which is worth checking locally rather than improvising.

Over a season, carrying dead cells out is part of carrying rubbish out, and they are among the more damaging items to leave behind in a wild place. Rechargeable systems reduce the volume of waste substantially over time, which is one argument for them beyond convenience and running cost. For any specific rules on transporting spare cells, particularly by air, check the current requirements with the carrier before travelling.

The takeaway

In the cold, keep the cells warm rather than carrying more of them. Most of that missing capacity is only hiding.

Go slowly enough to notice, and most of this stops being advice and starts being obvious.