A power bank is a lithium-ion battery in a case with charging circuitry and one or more USB ports. Choosing one comes down to four questions: how much energy you need to carry, how fast it must deliver it to your devices, whether you will take it on a plane, and how to use and store it safely. Product pages tend to lead with a large milliamp-hour figure, which on its own says little about any of those. This guide explains how to read capacity and output ratings, what the Federal Aviation Administration (FAA) allows in your luggage, and the battery safety advice of the National Fire Protection Association (NFPA). It explains features, not individual models.
Capacity: mAh versus watt-hours
Most power banks advertise capacity in milliamp-hours (mAh). That number only describes energy when you also know the battery voltage, which is why airlines and the FAA use watt-hours. The FAA’s tip is simple: to calculate Wh, multiply the battery voltage by the amp-hours (Ah). Newer lithium-ion batteries have the Wh rating marked on them.
To convert a label yourself:
- Find the capacity in mAh and divide by 1,000 to get amp-hours.
- Find the battery voltage printed on the label or in the specifications (often listed as the cell or nominal voltage).
- Multiply the two. A label reading 10,000 mAh at 3.7 V works out to 37 Wh.
Use the voltage printed on your own power bank rather than an assumed figure. Note too that the energy a power bank can actually deliver to your phone is lower than its rated capacity, because some energy is lost in conversion and as heat; manufacturers that state a “rated” or “output” capacity give a more realistic figure.
How much capacity do you need?
Compare the power bank’s watt-hours with your device’s battery. Phone, tablet and laptop specifications often list battery capacity in watt-hours, or in mAh with a voltage. A power bank with comfortably more watt-hours than your phone battery can give a full recharge with margin to spare; charging a laptop takes far more. Bigger is not automatically better: higher capacity adds weight and size, and above 100 Wh the airline rules change.
Output: ports and charging speed
Charging speed depends on the power bank’s output, the cable, and what the device can accept. The key terms:
- USB-C is the connector shape. The USB Implementers Forum (USB-IF), which maintains the USB standards, stresses that USB Type-C is not the same thing as USB Power Delivery; a USB-C port does not have to support it.
- USB Power Delivery (USB PD) is the protocol that lets a charger and device negotiate higher power. USB-IF explains that USB PD was limited to 100 W before Revision 3.1, which extended it to up to 240 W over a full-featured USB-C cable. Each device takes only the power it requires.
- Output wattage per port, and in total when several ports are used at once, is what to compare. A power bank may advertise a maximum that drops when two devices charge together.
For a laptop, check the wattage its own charger supplies and look for a power bank port that can provide USB PD at a similar level. For phones, a modest USB PD output is usually enough. Matching cables matter as well; see charging cables.
Flying with a power bank
The FAA’s PackSafe guidance on lithium batteries sets out the US rules for passengers:
- Carry-on only. Spare lithium batteries, including power banks and phone battery charging cases, must be carried in carry-on baggage only. If your bag is checked at the gate or planeside, remove power banks and keep them with you in the cabin.
- Size limits. Lithium-ion batteries are limited to 100 Wh per battery. With airline approval, passengers may carry up to two spare larger lithium-ion batteries of 101 to 160 Wh.
- Protect the terminals from short circuit, for example by keeping the power bank in its retail packaging, a case or a protective pouch, or by taping exposed terminals.
- Airline rules may be stricter. The FAA notes that many airlines have stricter quantity and Wh limits, including how many power banks each passenger may carry. Check with your airline before travelling.
- Damaged or recalled batteries must not be carried in carry-on or checked bags unless the battery has been removed or made safe.
The FAA also asks passengers to notify the flight crew immediately if a battery or device is overheating, expanding, smoking or burning. More travel-specific advice is in travel gadgets.
Safety: buying, charging and storing
The FAA explains that all lithium-ion batteries can overheat and undergo thermal runaway, which can happen without warning if the battery is damaged, overheated, exposed to water, overcharged or improperly packed, or because of a manufacturing defect. NFPA’s lithium-ion battery safety advice for everyday devices is practical:
- Buy batteries that are listed by a nationally recognized testing laboratory and labeled accordingly.
- Use charging equipment compatible with your device, and stop charging once the battery is full.
- Stop using a battery that shows signs of damage, such as an unusual odor, excessive heat, popping sounds, swelling or a change in color.
- Do not store batteries in extremely hot or cold locations, or where they block the only exit from a room.
- Do not put lithium-ion batteries in household trash or recycling bins; take them to a battery recycling or household hazardous waste collection point.
In practice that also means not charging a power bank under a pillow or on a bed, not leaving it in a hot car, and not using one that has been dropped hard and now feels warm or misshapen.
Other features worth comparing
| Feature | Why it matters |
|---|---|
| Pass-through charging | Charges the power bank and a device at once; check the maker allows it |
| Recharge input wattage | Higher USB PD input refills the power bank faster |
| Built-in cable | Convenient, but a damaged cable means replacing the whole unit |
| Wireless charging pad | Convenient for phones; compare its wattage with the wired ports |
| Display of remaining charge | Easier to judge than indicator lights |
| Weight and size | Higher capacity adds both |
Wireless options are compared in wireless charging, and portable chargers sit alongside other carry-along tech in portable electronics. All everyday gadget guides are under everyday gadgets.
Questions to ask before buying
- What is the capacity in Wh, and is it printed on the case?
- Does it have a USB-C port with USB Power Delivery, and at what wattage per port?
- What is the total output when several devices charge at once?
- Is it listed by a recognized testing laboratory?
- Does it meet your airline’s limits if you fly?
