Every year, people rely more and more heavily on their smartphones and other electronic devices to help them remember things, handle their finances and stay safe as they navigate the modern world. With this increased dependence, it is more important than ever to always have access to a reliable power supply.

In some cases, letting one’s phone die might just mean that the picture-taking will have to stop for a little while and Twitter will have to wait until later. But in some situations, it can be a big problem. For those who rely on their phones as digital wallets, health monitoring functions, controlling smart home devices, or navigating, a dead phone might jeopardize one’s comfort and even safety. Power banks are an increasingly popular solution for ensuring a constant source of backup power.

How Do Power Banks Work?

A power bank is essentially a portable battery that can be carried around outside the home and used to charge electronic devices. They are charged up by being plugged into a typical wall outlet. Once they’re ready, they can be put in a bag or pocket and taken on the go. Most of these portable chargers have a USB output, making them compatible with the charging cables of most rechargeable devices.

One of the main features that distinguishes different power banks from one another is their storage capacity. The key unit here is the milliampere-hour (mAh) rating. This rating tells users how much juice the power bank can hold, and it determines how many times a charger can recharge a particular device.

Here are the battery capacity ratings for a few popular smart phones:

• Samsung Galaxy S10: 3,400mAh
• Google Pixel 3: 2,915mAh
• iPhone X: 2,716mAh

These numbers indicate how many times a particular power bank can charge a particular device. For example, the iPhone X, has a 2,716mAh battery, and so it can be recharged several times by a power bank with a 10,000mAh rating. More precisely, since 10,000/2,716 = roughly 3.7, a 10,000mAh bank could fully charge the iPhone X about three and a half times. This estimate is a little high, because the charging and storage process is not 100 percent efficient, and some power gets lost in the process. But it gives a good ballpark figure.

Keeping Your Power Bank on You

When comparing power bank models, the charge rating is important to keep in mind. But it’s not the whole picture. For those buying one of these things to serve as a portable phone charger, the portability of the device needs to be balanced against its functionality.

For example, a 30,000mAh charger might sound great—it could easily keep a smartphone charged for over a week. But some 30,000mAh models weigh almost 2 lbs. and are the size of a biology textbook! This cumbersome design just isn’t very practical for on-the-go needs.

So, the key here is to balance power needs against the portability of the device. In order to get the most use out of the portable battery, a model with a slender, lightweight design can easily be kept at all times in the car, purse, backpack, or pant pocket.

Popular Power Banks for Sale

Here are some of the best power banks currently on the market:

• Anker PowerCore+ mini 3350 - The PowerCore+ mini 3350 from Anker is one of the most portable power banks you can find. Rated at 3,350mAh, this charger is designed to provide 1-2 charges for a typical smartphone. It’s a little larger than a tube of lipstick, and weighs just 3 oz. Ultra-portable, great for single-outing use.
• Anker PowerCore 10000 - Anker also makes of the leading models in the 10,000mAh range: the PowerCore 10000. With a 10,000mAh rating, the PowerCore 10000 is suitable for charging a single device for several days, or can be used for multiple devices on a single day (keeping the whole family charged up, for example). It’s a little smaller than a wallet, and weights just 6.4 oz, offering a nice balance of portability and energy output.
• MAXOAK 50,000mAh Power Bank - For a high-storage power bank geared for very large groups and/or very long trips, the MAXOAK 50,000mAh Power Bank is a solid option. It’s much bulkier than the two Anker chargers above, weighing in at 2 lbs. 12oz, it isn’t designed for being carried around in pockets. But it’s 50,000mAh battery and six output ports can keep dozens of devices charged for a big single-day event, and can keep a pair of users topped off for over a week during an extended camping trip, for example. It is even capable of charging laptops and other devices that use non-USB cables.