Batteries in the cold

Every battery has two electrodes – a positive and a negative terminal – and between them an electrolyte, a liquid or paste through which charged particles called ions can travel. When a device is connected, chemical reactions take place at the electrodes. The electrons flow outside through the device, the ions inside through the electrolyte. Only both together make a circuit.

Cold slows this process at several points. The reactions at the electrodes get slower, and the electrolyte becomes more viscous, so the ions move with more difficulty. Together this is called higher internal resistance. If the device wants a lot of current at once – for example a phone while transmitting or a bright headlamp – the voltage collapses. If it falls below the limit the device needs, it reports “empty” or switches off.

Most of the stored energy is still there; it just cannot get out fast enough. When the battery warms up again, the internal resistance drops and it delivers current again. That is why it is worth not throwing away cold, apparently empty batteries but warming them and testing them again.

Not every battery reacts the same way to cold. Ordinary alkaline batteries, the most common single-use cells, lose a lot of performance in the cold. Lithium single-use batteries are considered much more cold-tolerant and are therefore often chosen for winter trips. Rechargeable nickel-metal hydride cells lie somewhere in between. Which types a device accepts is in its manual – some devices are not designed for every battery type.

Lithium-ion batteries, as found in phones, power banks and many headlamps, also deliver less in the cold. For them an important additional rule applies to charging: they should not be charged below freezing. Metallic lithium can then build up on the negative electrode instead of being stored properly. This damages the battery permanently and can make it dangerous.

Manufacturers state their own temperature ranges for charging and using their batteries. These specifications always take precedence over general rules of thumb.

The simplest heat source is you. Carry your phone, spare batteries and power bank in an inside pocket, not in the outer pocket of the rucksack. At night they come into the sleeping bag, ideally in a small pouch so nothing gets lost. With headlamps that have a separate battery pack on a cable, you can wear the pack under your jacket.

The swap principle helps: two sets of batteries, one working in the device and one warming on your body. When the device dies, swap the sets. Also save energy: dim the screen, switch off unnecessary functions, and put the phone in airplane mode in areas without coverage, because the constant search for a network uses a lot of power.

When you bring a cold device into a warm room, moisture condenses on it – inside too. So let it warm up slowly in a sealed bag, and only charge a cold battery once it has reached room temperature.

Batteries contain a lot of energy in a small space. If a battery gets too hot, it can react uncontrollably, catch fire or burst. So never warm them at a campfire, on a stove, at a cooker, with a hair dryer or in a microwave. Body heat is slow but safe.

Damaged, dented or swollen batteries no longer belong in a device or on a charger. Put them in a safe place away from flammable things and take them to a collection point. Never open a battery and do not try to repair it.

Loose batteries in a pocket can short-circuit against keys or coins and get hot. So keep spare batteries in a box or with their terminals covered. And remember that great heat also does harm: a battery left in a car in the summer sun ages faster.

On a winter trip the phone is often map, lamp and emergency phone all at once. Precisely when you need it most – in cold, wind and darkness – its battery is at its weakest. Knowing this, you plan for it: keep the phone warm, carry a power bank and pack an independent lamp as well as a paper map.

During a power cut in winter the home also gets cold quickly. Torches and radios in a cold room or car run for less time than expected. So keep your emergency batteries in a place that does not get cold, and check them regularly.