Portable Power Banks: Capacity, Charging Speed and Travel Basics

Power banks are usually advertised by capacity in milliamp-hours (mAh). A 10,000 mAh power bank does not deliver 10,000 mAh to your phone. The rating refers to the internal cells at their lower voltage, and some energy is lost when converting to the charging voltage and as heat. Real-world usable capacity is noticeably lower than the number on the box.

A more useful comparison is watt-hours (Wh), which accounts for voltage. Many power banks list it on the casing. As a rough rule, a 10,000 mAh bank will recharge a typical smartphone about one and a half to two times, depending on the phone's battery size.

• 5,000 mAh: slim and pocketable; enough for a top-up during a long day.
• 10,000 mAh: the most common balance of size and capacity for phones.
• 20,000 mAh and above: useful for travel, several devices or tablets, but noticeably heavier.
• Laptop power banks: need both high capacity and high USB-C output.

Weight matters more than people expect. A large power bank you leave at home because it is heavy is less useful than a smaller one you actually carry.

Charging speed depends on the power bank's output, measured in watts. Phones often charge quickly at around 18 to 30 watts. Tablets and many laptops need more. Check the USB Power Delivery (PD) output on the USB-C port, and confirm it meets your device's needs. If a laptop requires 60 watts and the bank supplies 30, it may charge very slowly or only slow the battery drain while in use.

Also check the input speed: how fast the power bank itself recharges. A large bank with slow input can take many hours to fill.

USB-C is now the standard for most new phones, tablets and laptops. A USB-C port that works as both input and output keeps things simple. A USB-A port is still helpful for older cables and accessories.

If you charge two devices at once, check how the total output is shared; the fastest speed often applies only when one port is in use. Built-in cables reduce the chance of forgetting a cord but are harder to replace if they wear out. Wireless or magnetic charging is convenient but generally slower and less efficient than a cable.

Airlines and aviation authorities treat power banks as spare lithium batteries. Common rules, including US FAA guidance, are:

• Power banks must be carried in carry-on baggage, not checked luggage.
• Batteries up to 100 Wh are generally allowed without special approval.
• Batteries between 100 and 160 Wh usually need airline approval, and there may be a quantity limit.
• Many airlines now restrict using or charging power banks during the flight.

A clearly printed watt-hour rating on the casing makes security checks easier. Rules vary by country and airline and can change, so check your airline's current policy before you fly.

Lithium batteries store a lot of energy. Choose a power bank from a manufacturer that lists safety certifications and protection features such as over-charge, over-current and temperature protection. Avoid units that swell, get very hot, smell unusual or have physical damage, and never charge them on soft furnishings such as beds. Store power banks partly charged if you will not use them for a long time.

• Expecting the full mAh rating to reach your phone.
• Buying a low-output bank for a laptop.
• Packing a power bank in checked luggage.
• Choosing the largest capacity, then leaving it at home because it is heavy.
• Ignoring how slowly a large bank recharges itself.

Can I leave my phone plugged into a power bank all the time? Modern phones manage charging, but heat is the enemy of batteries. Avoid charging in hot pockets or bags where possible.

How long do power banks last? Like all lithium batteries, they lose capacity after many charge cycles. Replace a unit that holds noticeably less charge or shows any physical damage.

• Compare watt-hours, not only mAh.
• Pick a size you will actually carry.
• Match USB-C PD output to your most demanding device.
• Check input speed and shared-port behaviour.
• Confirm the Wh rating is printed on the unit for air travel.
• Look for listed safety protections and a clear warranty.