How to choose a power bank for camping

A power bank's capacity is advertised at the cell voltage, typically 3.7 volts, while your phone charges at 5 volts or higher. Converting between them costs energy, and so does the heat the bank gives off while doing it. The practical result is that you get roughly 60 to 70 percent of the labeled capacity delivered into a device.

So a 10,000 mAh bank delivers about 6,500 to 7,000 mAh of usable charge. A modern phone battery is 3,000 to 5,000 mAh. That is two full phone charges from a 10,000 mAh bank, not three, and certainly not the four that a naive division suggests.

Work the other direction and sizing becomes simple. Count the devices, count the charges each one needs across the trip, multiply by the device battery size, and add 50 percent. That number in milliamp-hours is the bank you need.

Capacity decides how long the bank lasts. Three other numbers decide whether it is pleasant to use, and they are easy to miss on a product page.

Lithium-ion cells lose usable capacity as they get cold. The chemistry slows down, internal resistance rises, and a bank that delivers two phone charges at room temperature can deliver noticeably less at freezing. Below about 32 degrees Fahrenheit the effect is clear; well below that it is severe.

The capacity is not gone -- it comes back when the cell warms up -- but it is unavailable when you need it, which amounts to the same thing at 6 a.m. in a tent. The fix is behavioral rather than technical: keep the bank inside your sleeping bag overnight and in an inside jacket pocket during the day, not in an outside pack pocket.

Charging a cold battery is the more serious issue. Pushing current into a lithium cell below freezing can cause lithium plating, which permanently reduces capacity. Let a cold bank and a cold phone warm up before you connect them. Better banks refuse to charge when too cold, which is a feature rather than a fault.

We have not measured delivered capacity on these banks with a USB load tester. The 60 to 70 percent conversion figure is the well-established range for lithium-ion power banks at typical output voltages, and it is why the charge counts above are given as ranges rather than exact numbers. Rated capacities, output wattages, port configurations and weights come from manufacturer specifications.

The 100 watt-hour carry-on limit is the FAA figure, linked below, and the cold-weather behavior described is standard lithium-ion chemistry rather than a brand-specific claim. We publish no prices; power banks are discounted constantly and any number would be stale. Our criteria are at how we pick.