Power Bank for Camping: Sizes, Phone Battery in the Cold and Solar Chargers

What do you need power for outdoors?

Before you buy or pack a power bank, list the devices that really need electricity on your trip and what each one is for. On most bushcraft and hiking trips there are only three, and one of them matters far more than the others.

• Phone: map and GPS, emergency phone, camera, weather forecast and contact with home. Because it is also your way to call 112, it is the one device that must never run empty. How to set it up as a navigation tool is explained in compass, GPS and map apps.
• Headlamp: in autumn and winter you may use it four to five hours every evening. Many modern headlamps have a built-in USB battery; others use AA or AAA cells. Details in headlamp batteries in the cold.
• Handheld GPS: usually runs on AA cells, which you can swap rather than recharge – a real advantage on long or cold trips. See GPS device or phone.
• Extras: a satellite messenger or personal locator beacon, a sports watch, a camera, a UV water purifier (see water purification), an e-reader. Each one adds to the power budget – decide which ones earn their place.

The phone's job list also tells you how to prioritise. Navigation and the emergency call come first, photos and music last. A charged phone is no replacement for map and compass, though: they work without any battery at all, which is why every guide in our navigation series treats them as the backbone and electronics as the helper.

How big a power bank for hiking? mAh, Wh and conversion losses

Power banks are sold by milliampere-hours (mAh), but mAh only describes charge at a certain voltage. The energy that matters is in watt-hours (Wh): Wh = mAh × volts ÷ 1,000. The printed mAh of a power bank refers to its internal lithium cells at about 3.6–3.7 V, while it delivers power at 5 V or more through the USB port. That is why you cannot simply divide 10,000 mAh by a 4,000 mAh phone and expect two and a half charges.

On the way from the cells to your phone battery, energy is lost: the voltage is converted up in the bank, converted down again in the phone, and some turns into heat in cables and electronics. As a rule of thumb you can use roughly 60–70 % of the printed capacity – the same figure we use in power management for navigation. In the cold it is less – the power bank calculator does the whole conversion for you.

The example is deliberately simple. Your own figures depend on your phone, your app, the signal situation and how often you look at the screen. The best data is your own: on the next day walk, note the phone's battery level in the morning and the evening. After two or three trips you know your daily need far better than any table – the trip planning guide has a simple pre-trip checklist where the figure fits in.

Power bank sizes for 1–5 days: rules of thumb

Using the method above with typical careful use – a phone as map and emergency phone in flight mode, a USB headlamp, about 8 Wh per day – gives these rules of thumb. They assume you start with fully charged devices. For your own devices, trip length and temperatures, use the power bank calculator.

Two small banks instead of one large one are worth considering on longer trips: if one fails or gets wet, you still have the other, and in a group you can lend one out. A large bank charges slowly from a wall socket – some take a whole night – so charge it the day before you leave, not the morning of departure.

Weight counts too. A 20,000 mAh bank typically weighs two to three times as much as a 5,000 mAh one; the pack weight calculator shows how it adds up, and the Kit Builder includes a power bank for multi-day trips. If you carry a handheld GPS or headlamp that takes AA cells, a few spare lithium cells can replace part of the bank – they are covered below.

How to save phone battery on a hike

The cheapest watt-hour is the one you do not use. A few settings and habits often stretch a phone from one day to two or three. The biggest consumers are the screen, the radio searching for a signal and apps that run in the background.

GPS and app settings

• Track recording: recording a track all day keeps the GPS working constantly. Record only when you need it, or choose a longer recording interval if your app allows it.
• Location for other apps: allow location only for the map app, and only while it is in use.
• Wi-Fi, Bluetooth, background refresh: switch them off unless you need them – a watch or satellite messenger may need Bluetooth.
• Battery saver mode: most phones have one; switch it on from the start of the day, not at 15 %.
• Photos and video: video and many photos with the screen on cost a lot. Take them, but count them as consumption.

The emergency reserve: how much battery to keep for calls

The rule of thumb across our guides is simple: keep 20–30 % of the phone battery purely for emergency calls, and treat that part as untouchable, like the last litre of water. In the cold, the higher end makes sense, because a cold battery can drop from half full to empty within minutes. The reasoning behind it is in making an emergency call.

• Set a turning point: when the phone reaches the reserve, it is no longer your map. Navigate by map and compass, and plan your route back so you do not need the phone.
• Agree check-in times: tell someone at home when you will report and what they should do if you do not. On a solo trip this matters even more; see safety when you go alone.
• In an emergency: call 112 (EU) or the local number, give your location, then keep the phone warm and in flight mode or off between agreed contact times so the battery lasts until help arrives.
• Weak signal: 112 can often be reached through another network when yours has no coverage, but do not rely on it – higher ground often helps. A text message sometimes gets through where a call does not.

Phone battery in cold weather: why it dies and how to keep it going

Lithium-ion batteries – in phones, power banks and most headlamps – deliver less in the cold. The chemical reactions inside slow down, the internal resistance rises, and under load the voltage drops. The phone interprets the low voltage as an empty battery and shows a sudden fall or shuts down, sometimes with a fair amount of charge left. Most of that capacity comes back when the battery warms up; it is not lost for good.

Charging is the more sensitive part: many manufacturers advise against charging lithium-ion batteries below 0 °C, because it can damage the cells. Some phones and power banks refuse to charge when they are too cold.

• Never heat batteries next to the fire, on a stove or in direct sunlight on a dark surface – heat damages lithium cells far more than cold.
• Condensation: when you take a very cold phone into a warm tent, hut or car, moisture can condense on and inside it. Let it warm up slowly in a closed bag or pocket first.
• A short cable lets you charge with phone and bank in the same inner pocket.
• Touch screens and gloves: thin touch-screen gloves under mittens let you check the map without exposing bare hands to wind chill – see the wind chill calculator for how fast that matters.
• Lithium AA cells are much better in frost than alkaline or NiMH, which is one reason a GPS or headlamp on AA cells is popular for winter trips.

On a winter camping trip with 15 hours of darkness, power planning belongs next to the sleep system: plan for the larger power bank from the size table and keep everything warm. Cold also affects you – if your fingers are too cold to use the phone, look after yourself first; the warning signs are in hypothermia.

AA and AAA batteries: lithium, NiMH or alkaline?

Many headlamps, GPS devices and satellite messengers run on AA or AAA cells. Their great advantage: you can swap them in seconds and carry spares, with no power bank and no waiting. The type makes a big difference – the same comparison we use in the headlamp guide.

For spares, keep cells in their packaging or a small zip bag, never loose with keys and coins, and mark used cells with a pen so you do not mix them up. Used batteries go home with you and into a battery collection point – never into the fire, the latrine or the forest.

Solar chargers for camping: an honest assessment

A solar charger for camping sounds like unlimited free power, and on the right trip it is useful. But its output depends on factors you cannot control:

• Weather: under cloud a panel produces only a fraction of its sunny-day output.
• Sun angle and latitude: the lower the sun, the less energy arrives. In central and northern Europe, the short winter days with a low sun give very little; in summer, long days on open ground give much more.
• Shade: under a forest canopy most panels deliver almost nothing, and even partial shade on one cell can cut the output of the whole panel sharply.
• Orientation: a panel strapped to a pack while you walk through woods is rarely facing the sun.

So solar is useful mainly on long, sunny trips in open terrain: above the treeline, on Nordic fells in summer, on lakes and rivers by canoe, or in a base camp in a clearing. On a weekend in the woods, a slightly larger power bank is lighter, cheaper and more reliable.

Keeping electronics dry in rain and at the water

Water is the second big enemy of electronics after cold. Many phones are splash-resistant, but water rating is not a promise for every situation: seals age, a port full of water cannot charge, and power banks and cables are rarely waterproof at all. Pack them like your sleeping bag – the methods are in packing for rain camping.

• Water ratings: devices labelled IPX4 resist splashing water; IPX7 devices survive brief immersion under test conditions. Whatever the rating, do not open battery caps or covers in the rain – see headlamp problems in the field.
• River crossings and canoe trips: everything electronic goes into a dry bag before you step into the water; see crossing a river safely.
• If a device got wet: switch it off, dry the outside, do not charge it, and let it dry in a warm, ventilated place. Do not put it next to the fire.

Safe handling of lithium batteries and power banks

Lithium batteries store a lot of energy in a small space. Handled well they are safe; damaged, overheated or short-circuited they can overheat, release toxic fumes and catch fire. A few rules prevent almost all problems.

Heat is a risk in summer too: a power bank in a dark pack lying in full sun, or a phone left on a rock at midday, can overheat. In hot weather keep electronics in the shade and in the middle of the pack, as you would protect yourself – see heat and sun for the human side.

Charging in groups and with families

In a group, power is a shared resource like water and fuel. A little planning saves weight and prevents the classic situation where everyone's phone is low on the last day.

With children, the adults' phones are the emergency phones – children's devices come last. Give children their own simple headlamp with replaceable cells, so no one argues over the power bank at bedtime; more ideas in bushcraft with kids. Mark cables and banks with coloured tape so they find their way back to their owners. In a base camp, set up one charging point under the tarp, in a dry bag, where everything is charged and counted in the evening.

Before you go: power check and troubleshooting

Most power problems are found at home if you look. Ten minutes the evening before the trip are enough.

Power is one part of a well-prepared kit, not a replacement for skills. A full list of what belongs in the pack is in the bushcraft kit list, and if you are just starting out, the bushcraft for beginners guide shows how map, compass, headlamp and phone fit together. For walks that end after dark, plan the light with the daylight calculator and the tips in night skills.