Power banks: sizing a battery from what you actually use

Battery capacity is only comparable in watt-hours. A bank printed as 10,000 mAh is rated at the 3.7 volt cell level, so it holds about 37 Wh, and only around 85 percent of that reaches your devices. Work out what you actually consume in a day, multiply by the nights out, add the losses, and the right bank is usually smaller than the one most people carry.

Why watt-hours are the only unit worth planning in#

Milliamp-hours measure charge, not energy. The figure means nothing until you attach a voltage to it, and the voltage the marketing number uses is the bare cell voltage of a lithium-ion cell, nominally 3.7 volts. Multiply and you get energy:

10,000 mAh x 3.7 V = 37,000 mWh = 37 Wh.

That is the honest capacity of the pack, though not what you get out of it. The cell sits near 3.7 volts and your device wants 5 volts, or 9, 12 or 20 volts if the two ends negotiate a USB Power Delivery contract. Every conversion step costs energy as heat: the boost converter in the bank, cable and connector resistance, and the charging circuit in the phone. The bank also spends a trickle on its own controller and display. About 85 percent of rated energy is a fair planning figure, with cheap electronics and long thin cables at the worse end.

The last column is where the disappointment lives. A 10,000 mAh bank against a 4,000 mAh phone looks like two and a half charges on paper; after conversion losses it is closer to two, and less again if the phone runs warm. Nothing is faulty, the arithmetic was done in the wrong unit.

What your kit actually draws in a day#

Devices are cheap to run compared with the fear people carry batteries against. Three figures cover most backcountry loads:

• A phone in airplane mode, used for map checks and photographs, costs a few watt-hours over a full day.
• The same phone left on a cellular network at the edge of coverage, with the screen used for navigation, costs roughly three times as much, because the radio transmits harder the worse the signal gets.
• A headlamp running at 100 lumens draws around 1.1 watts, so an hour of use is about 1.1 Wh.

The ratio matters more than the absolute numbers. Turning the radio off changes the daily draw by a factor of three at zero grams, which no bank can match. Our power budget calculator totals a device list this way and returns a bank size.

The five-step budget, worked twice#

We use the same sequence every time. It takes two minutes on paper.

1. List every device and give each a watt-hours-per-day figure. Use the table above, or measure with the protocol further down.
2. Sum them into a daily draw.
3. Multiply by the number of days you are away from a socket. Count the day you leave if you start on a partial charge.
4. Divide by 0.85 to convert what the devices need into what the bank must hold.
5. If nights are near or below freezing, divide by 0.8 again, then round up to the next bank size and check the total against the 100 Wh flight limit.

Worked example one, disciplined use. Phone in airplane mode at 4 Wh per day plus a headlamp at 2 hours a night, 2.2 Wh. Daily draw 6.2 Wh. Four nights out: 6.2 x 4 = 24.8 Wh. Divide by 0.85: 29.2 Wh of rated capacity. A 10,000 mAh bank at 37 Wh covers it with about 20 percent spare.

Worked example two, same trip, radio left on. Phone at 12 Wh per day plus the same headlamp: 14.2 Wh per day. Four nights: 56.8 Wh. Divide by 0.85: 66.8 Wh. Now add a freezing forecast, divide by 0.8: 83.5 Wh. That is a 26,800 mAh bank, about 500 grams of it, for exactly the same trip and exactly the same gear.

The gap between those two numbers is the whole argument: most people solve a settings problem by buying mass. Airplane mode, low screen brightness and downloaded offline maps halve a phone budget before you spend anything.

Choosing a bank size#

Working from the two consumption models above, at 6 Wh per day for disciplined use and 15 Wh per day for heavy navigation:

In freezing conditions, take about 20 percent off the day counts. Two smaller banks often beat one large one: they split between people, one can charge from a panel while the other charges a phone, and a failure costs you half the trip rather than all of it. The penalty is a second case and a second cable.

Cold weather, and why the bank sleeps in your quilt#

Lithium cells lose usable capacity as they get cold: internal resistance rises, voltage sags under load, and the bank hits its cutoff with energy still chemically present. Near freezing the shortfall is on the order of 20 percent, and it is temporary. Warm the bank and the capacity returns, because nothing was consumed.

That is the entire basis for sleeping with the bank. A power bank in the foot of your quilt or against your thigh stays within a few degrees of body temperature instead of dropping to ambient, which keeps the chemistry in the range where the rated capacity is available. The same logic covers the phone, the satellite messenger and spare lithium cells. At bedtime the electronics go inside with you, along with the water filter, which cracks if it freezes. See quilts against sleeping bags for how much interior volume you have to work with.

Discharging in the cold is safe, just inefficient, so using a frozen bank to run a light is fine. Charging is the operation with the temperature floor.

Flying with lithium batteries#

The rules are set internationally and enforced per airline, and they are simpler than the airport signage suggests. Batteries not installed in equipment, which includes every power bank, must travel in the cabin. Under the ICAO instructions that IATA publishes for airlines, batteries up to 100 Wh need no prior approval, those between 100 Wh and 160 Wh need the operator's approval and are usually limited to two per passenger, and above 160 Wh they are not accepted in passenger baggage at all.

This is why 26,800 mAh is such a common bank size: at 99.2 Wh it is the largest round capacity that stays under the threshold. If the bank is marked only in mAh, write the watt-hour figure on it, because doing the multiplication in front of a security officer in a hurry is a poor use of both your mornings.

Measure your own daily draw#

The device figures above are a model. Yours will differ, and one evening at home replaces the guess.

1. Buy an inline USB power meter, the kind that sits between the bank and the cable and reads watt-hours cumulatively. This is domestic equipment, not lab equipment.
2. Charge your phone to full, then use it for a normal trail day at home: airplane mode, the offline map open for the minutes you would really look at it, the same photographs, the same messages queued.
3. After 24 hours, recharge it through the meter and read the total watt-hours. That is your daily draw plus the charging loss, which is exactly the number the budget needs.
4. Repeat with cellular on. The ratio between the two runs is the biggest single lever you have.
5. Do the same for the headlamp at the brightness you actually walk at.

Write the results into your gear list. Three devices measured once beat any table on any website, including this one.

Common mistakes#

Buying on milliamp-hours. Two banks with the same printed capacity can differ in delivered energy by 10 percent or more depending on converter efficiency and idle draw. Watt-hours are comparable; mAh alone are not.

Assuming rated equals delivered. Sizing a trip on rated capacity leaves you about 15 percent short on the last day, which is always the day you need the map.

Leaving the bank in the vestibule. It drops to ambient overnight, and the 20 percent cold shortfall arrives at breakfast.

Storing banks full or empty for months. Lithium cells age fastest at high state of charge and high temperature, and a flat cell can self-discharge into an unrecoverable state. Store at about half charge somewhere cool, and top up twice a year. Our gear storage guide covers where in the house that is.

Solving a power problem with more battery. Measurement usually shows the fix is a setting, a headlamp mode or an offline map, and weight is the last resort. If the trip genuinely runs long, compare a bigger bank against a panel in solar for camping. A vehicle trip with a fridge is a different problem entirely, covered in 12 volt power and camping fridges.

Frequently asked questions#

How many mAh do I need for a weekend of camping?#

For two nights with a phone in airplane mode and a headlamp, a 5,000 mAh bank, about 18.5 Wh, is enough with margin. For two nights with cellular navigation running, budget around 30 Wh, so 10,000 mAh. The variable is not the trip length, it is whether the phone radio is transmitting, which roughly triples the daily figure.

Why does my 10,000 mAh power bank not fully charge my 4,000 mAh phone twice?#

Because the useful comparison is in watt-hours and about 15 percent is lost to conversion. The bank holds 37 Wh, delivers about 31.5 Wh, and the phone battery is 14.8 Wh. That is 2.1 charges, and heat during fast charging can push it below two. The bank is behaving normally.

Do power banks lose charge in cold weather?#

They do not lose stored charge; they temporarily lose access to it. Near freezing, expect on the order of 20 percent less usable capacity because internal resistance rises and the pack reaches its cutoff voltage early. Warm the bank against your body and the missing capacity returns. Nothing is permanently damaged by discharging cold.

Can I take a power bank on a plane?#

Yes, in carry-on only, never in checked baggage. Up to 100 Wh needs no approval, 100 to 160 Wh needs the airline's permission and is typically limited to two, and anything above 160 Wh is refused. A 26,800 mAh bank is 99.2 Wh, which is why that size is so common in travel ranges.

Is it safe to charge my phone from a power bank overnight in a tent?#

Charging itself is fine, but two details matter. Do not start a charge when the cells are below 0 degrees C, and do not bury a charging bank under insulation where it cannot shed heat. In practice, warm the pair inside your quilt for half an hour, then charge with the bank somewhere it can breathe.

How should I store lithium batteries between trips?#

At roughly half charge, cool and dry, and not inside a hot vehicle or a loft in summer. High state of charge and high temperature both accelerate capacity loss, and a completely flat cell can fall below the voltage where a charger will safely restart it. Check them every few months and top up to about half.

Should I carry one big power bank or two small ones?#

Two small ones for a group, one big one for solo weight saving. Splitting the capacity gives redundancy, lets two people charge at once, and keeps each unit comfortably under the flight limit. The cost is roughly the mass of a second case and connector set, which is usually 40 to 60 grams.