Power Bank Buying Guide: Capacity and Flight Rules

The short answer

This power bank buying guide comes down to three numbers: capacity, output and size. For most phones, a 10,000 mAh power bank gives about two full charges and weighs about as much as a phone. If you fly, pick one under 100 Wh and keep it in your carry-on bag. Never pack it in checked baggage.

Read on for how to check each number, so you buy the right one the first time.

How to read mAh and Wh

Every power bank shows its size in mAh, which means milliamp hours. A bigger number means more stored energy. A 20,000 mAh unit holds twice as much as a 10,000 mAh unit.

Airlines use a different unit, watt hours, written Wh. The two are linked by a simple sum. Multiply the mAh by the battery voltage, then divide by 1,000. Most power banks use cells rated at 3.7 volts.

Here are three examples. A 10,000 mAh unit is about 37 Wh. A 20,000 mAh unit is about 74 Wh. A 27,000 mAh unit is about 100 Wh. So you can use the same sum on any label.

Some brands print the Wh on the case. If yours does not, do the sum yourself and write the answer on a piece of tape. A gate agent can then check it in a few seconds.

Sources: US Federal Aviation Administration, Pack Safe, Batteries, lithium: watt hours are the battery voltage multiplied by the amp hours. The 3.7 volt figure is the usual rating for lithium-ion cells.

Why the label is not the whole story

A 10,000 mAh power bank will not give you 10,000 mAh into your phone. Some energy is lost as heat when the voltage is raised for the USB port. In practice, expect well under the rated figure to reach your phone.

That is why a 10,000 mAh unit gives a typical phone about two charges and not three. Look up your phone's battery size in its spec sheet. Divide the usable energy by that number to guess your charges.

Treat any brand that claims it delivers the full label as a warning sign.

Match the size to your day

Think about how you travel before you choose.

• Day trips and commuting: 5,000 to 10,000 mAh. It slips into a pocket and covers one full charge or more.
• Weekend away: 10,000 to 20,000 mAh. It can keep a phone going for a few days with light use.
• Phone plus tablet: 20,000 mAh or more. Check that it is under 100 Wh if you plan to fly.

Output matters too. A port marked 1 amp charges slowly. A port of 2 amps or more is better for modern phones. Look for two ports if you share the battery with a friend.

Check which cables you need. A micro USB input is common on cheaper models, and it can take a long time to refill a large unit. Plan to charge it overnight.

Flight rules for power banks

A power bank holds a lithium battery, and airlines treat it as a spare battery. The rules below come from the US Federal Aviation Administration (FAA). Each airline can add its own limits.

The main rule is simple. Spare lithium batteries, and that includes power banks, must travel in your cabin bag. They are not allowed in checked baggage. The FAA names power banks, also called portable rechargers, on its Pack Safe page.

The size limits are as follows.

• Up to 100 Wh: allowed in carry-on without special approval.
• Over 100 Wh and up to 160 Wh: needs the approval of your airline, and a limit of two spares per passenger is common.
• Over 160 Wh: not covered by these allowances, so leave it at home.

Protect the metal contacts. Keep the unit in its pouch, tape over the ports, or place it in a plastic bag. This stops a short circuit if it touches keys or coins.

Do not rely on one site for the rules. Each airline can set tighter limits, such as a cap on how many units you may carry. Check your airline's page before you leave home, and again at the airport counter if you are unsure.

Sources: US Federal Aviation Administration, Pack Safe, Batteries, lithium (faa.gov). Check your airline for its own rules.

What to check before you buy

Use this short list in the shop.

1. Find the mAh and the Wh on the case. If there is no Wh figure, work it out.
2. Look for a brand name, a model number and a serial number. A blank case is a bad sign.
3. Check the input and output ratings printed in small type. They should be clear and spelled correctly.
4. Ask about protection. Good units guard against overcharge, overheating and short circuits.
5. Feel the weight. A large number on a very light case often means the capacity is overstated.
6. Keep the receipt. A shop that offers a warranty cares about what it sells.

Be careful with very cheap units. A unit that promises a huge capacity at a tiny price is unlikely to deliver it. Spend a little more on a known brand with clear labels.

Never use a power bank that is swollen, hot to the touch or damaged. Stop using it and do not take it on a plane.

Before you pack

Charge the power bank the night before. Put it in your carry-on, with the ports covered. If you have more than one, check that the total fits your airline rule. This is general information, not safety certification for any single product.

With a few minutes of sums, you can pick a unit that keeps your phone alive and passes the security check without a fuss.

Sources: US Federal Aviation Administration, Pack Safe, Batteries, lithium.

Sources

• US Federal Aviation Administration, Pack Safe: Batteries, lithium