What Size Battery Do You Need?

These days, "small" portable batteries have a capacity of around 5,000mAh, which means they easily fit into your pocket and still have enough power to fully top up your phone once.

Meanwhile, a 10,000mAh battery can give today’s flagship phoness two full charges. A 20,000mAh battery can charge two phones twice, or a phone twice and a tablet once. Some power banks have enough juice to power laptops. Of course, a higher capacity often translates to a heavier, larger, and more expensive battery.

Most companies advertise how many times their products can recharge popular phones, but if you want to calculate that number for yourself, RAVPower has a useful guide that can give you an estimate.

In the end, it’s best to assess your typical needs before buying. If your phone hits the red zone by mid-afternoon and you only need enough juice to get you to the end of the workday, a 5,000mAh battery should be plenty.

What's the Difference Between Power Input and Output?

You’ll find three types of ports on today’s portable batteries:

Generally speaking, you charge the battery itself via USB-C (input). Speeds differ depending on the battery you choose.

Nearly all batteries also include a standard USB-A port (output). This is for plugging in USB-A-to-Lightning or USB-A-to-USB-C cables for charging your iPhone or Android device. In many cases, the USB-C port for charging the battery itself functions as an output as well. Some batteries even include built-in output cables that save you the trouble of worrying about another wire.

The most important thing to do is match the battery’s output to your phone’s input. For example, if you have an iPhone, be sure the battery has a USB-C or Lightning connector or at least supports the cable you need.

Larger batteries with higher capacities might include a multitude of ports to support input and multiple outputs at the same time. You may see two USB-A or two USB-C ports, for example.

If you’ve recently picked up an iPhone 14 or iPhone 15, see our article on charging your iPhone.

What Is Fast Charging?

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 gives you wattage, the measure of 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 for power to flow both ways.

The most common phones (those in Apple's iPhone and Samsung's Galaxy lineup) support charging rates of up to 27W and 45W, respectively. It’s best to look for batteries that get as close to your phone’s maximum rate as possible.

Is Pass-Through Charging Safe and Useful?

Pass-through charging is another feature to consider; with it, you can charge your device and a portable power bank simultaneously. That's convenient if both your phone and backup battery are running on empty. You shouldn't encounter any safety issues if the manufacturer of the portable battery you buy advertises pass-through charging as a feature, but the power output to your phone or tablet might be slower in this mode.

How Does Wireless Charging Work?

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

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.

Should You Buy a Battery Case Instead of a Power Bank?

If you find that you often forget to carry your backup battery when you need it most, you should consider a dedicated battery case instead. These combine the portability and protection of a case with additional battery capacity to keep your phone topped off 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.

The Best Wireless Chargers

Why fumble around for a charging cable when you can rest your phone on a wireless charging pad? Whether you're an Android user or an iPhone fan, check out our roundups of the best wireless chargers and the best MagSafe chargers to help you cut the cord.

Ben Moore and Steven Winkelman contributed to this story.