A power bank is a portable rechargeable battery pack that stores electricity to charge smartphones, tablets, laptops, and other USB-powered devices when no wall outlet is available.
When your phone dies mid-afternoon and you’re nowhere near a socket, that palm-sized brick in your bag is the difference between staying connected and going dark. A power bank—also called an external battery or portable USB charger—holds electrical energy in lithium-ion or lithium-polymer cells and delivers it through USB ports or wireless charging pads. They range from slim 5,000 mAh units that top off a phone once, to 20,000+ mAh bricks that can keep a tablet running for days. The right one comes down to capacity, portability, and safety certifications.
How Does a Power Bank Work?
A power bank operates in three straightforward stages. First, it absorbs energy from a wall outlet using a USB cable and adapter. Second, that energy is stored chemically inside its cells. Third, when you plug in a device, the bank converts stored energy back into electricity and sends it through the output port at the correct voltage.
Most banks use lithium-ion (Li-ion) or lithium-polymer (Li-Po) cells. Output voltages range from standard 5V up to 9V, 12V, or higher for fast charging protocols like Power Delivery and Quick Charge. The whole cycle—charge the bank, charge your phone—comes with some energy loss, which is why a 10,000 mAh bank won’t fully charge a 5,000 mAh phone twice. Roughly 60–70% of the stored energy transfers to your device; the rest is lost as heat and voltage conversion.
What Does mAh Actually Mean?
Milliampere-hours (mAh) is the unit you’ll see on every power bank label, and it tells you how much charge the battery holds. A 5,000 mAh bank is entry-level—enough for one smartphone top-off.
Watt-hours (Wh) is the more accurate metric for comparing capacity across different voltages, calculated as mAh × Voltage / 1000. Most banks list both. Be wary of models claiming absurd numbers like “50,000 mAh in your pocket”—those often use low-quality recycled cells and can be dangerous.
Are Power Banks Allowed on Planes?
Yes, but with strict limits. The FAA and ICAO restrict power banks to 100 Wh (roughly 26,315 mAh) per battery for carry-on luggage only—they are never allowed in checked baggage.
Power Bank Safety and Buying Tips
Lithium cells are highly flammable when damaged or poorly manufactured. Look for overcharge protection, short-circuit protection, and temperature monitoring built into the unit. In the US, power banks should carry UL 2056 and FCC Part 15 certification. Internationally, IEC 62368-1 is the principal global safety standard for portable IT equipment.
Practical rules: always use original or certified USB cables and wall adapters—cheap counterfeit cables increase fire risk. Never charge a power bank under a pillow or inside bedding. Inspect the bank regularly for cracks, bulges, or exposed wires, and replace it immediately if damaged. Stick with brands that provide warranty coverage and transparent specs.
If you’re shopping for a reliable portable charger that won’t break your budget, check out our picks for the best budget power banks—tested and vetted for real-world performance and safety.
References & Sources
- Standards Council of Canada. “Standard for Safety for Power Banks (UL 2056).” Outlines safety requirements for power bank certification in the US.
- Lenovo. “What Is a Power Bank?” Glossary entry covering how power banks work and their capacity ratings.
- Australian Competition and Consumer Commission. “Power Banks Guide.” Consumer safety guide on using and storing power banks.