Wireless charging has become popular because it enables you to power up compatible devices without the need for a cable. Qi is the dominant standard for compatible Android phones (up to 15W), while Apple iPhones rely on MagSafe charging (up to 25W). iPhones will also charge wirelessly on Qi chargers (7.5W) and Qi2 chargers (15W).

Some battery makers have built Qi or MagSafe-compatible wireless charging into the surfaces of their portable batteries. Such batteries mean you can leave the cables at home.

Another factor to consider is how quickly a power bank can charge your device. Battery output is measured in voltage and amperage. Amperage (or current) is the amount of electricity that flows from the battery to the connected device, while voltage is the amount of potential energy. Multiplying volts by amps yields wattage, which measures total power. To make devices charge faster, manufacturers either vary the voltage or boost the amperage.

Today’s devices support a wide range of rapid-charging technologies, such as Qualcomm Quick Charge, USB Power Delivery (PD), or proprietary fast-charge systems like OnePlus’ SuperVooc.

Quick Charge works by increasing voltage rather than amperage. This standard typically allows you to charge supported phones to 50% capacity in 5 minutes, which is especially helpful when you need power in a pinch.

Two compatible devices with Power Delivery can negotiate the fastest charging option available based on the charger, cable, and circuitry. It also allows power to flow in both directions.

The most common phones (those in Apple's iPhone and Samsung's Galaxy lineup) support charging rates of up to 45W. It’s best to look for batteries that get as close to your phone’s maximum rate as possible.

If you often forget to carry your backup battery when you need it most, consider a dedicated battery case instead. These cases combine the portability and protection of a case with extra battery capacity to keep your phone fully charged at all times.

There are several drawbacks. First, they have limited additional capacity. Second, they are limited in terms of the number of phones they support (mostly iPhones, Galaxies, and Pixels). Third, you can't charge much else with them. Stand-alone chargers might also allow you to charge a smartwatch or wireless headphones.