Experts' Tips for Purchasing the Best Backup Battery to Keep Your Devices Charged

— -- A dying phone is many people’s worst nightmare.

Experts say that backup batteries could be a solution to that problem. There are various name brands in all different sizes on the market, which can make the decision process difficult.

According to Mark Smirniotis, a writer and researcher at The Wirecutter, the cheapest backup battery option is not always the best choice. Smirniotis separates backup batteries into categories by physical size and also factors in many things including cost, shape and features of the battery pack.

“Batteries all tend to look the same when you're scrolling through listings, but what we've found is it's not just about capacity or how many times it can charge your phone,” Smirniotis told "Good Morning America." “There's other things that go into it as well.”

"GMA" also teamed up with Gary Kwok, Manager of Application Engineering at Cadex Electronics, which is a leader in the battery testing industry. They put 17 different popular backup batteries through a capacity test.

“This is the Cadex C7400-C battery analyzer,” Kwok said. “We use it to test the capacity of different type of batteries, different chemistries.” The test specifically looks to see how much of the battery’s listed capacity actually delivers to the device. Engineers at Cadex have found that these claims can differ from the actual amount delivered, which can be an issue.

Gary Kwok from Cadex Electronics tested three of each backup battery model. Then, Mark Smirniotis from The Wirecutter analyzed the results and determined his top picks.

If you’re in the market for a pocket-sized backup battery, The Wirecutter says that the Amazon Basics Portable Power Bank with Micro USB Cable 2,000 mAh (currently listed for $4 and requires an adapter to use with an iPhone) and the Incipio OFFGRID 1500 mAh for iPhones ($40) are its top picks. The Wirecutter says that these backup batteries are about the size of a credit card and will provide a few extra hours on your phone.

For small-sized backup batteries, The Wirecutter says that the EasyAcc Power Bank PB6000CB ($17) is their top pick. They say that the EasyAcc is about the size of a deck of playing cards and has enough power to provide a smartphone with two or three full charges.

When it comes to the medium-sized backup batteries, The Wirecutter says the Anker PowerCore 15600 is their top pick. They say this backup battery has enough power to recharge a tablet almost twice or to recharge a smartphone five times.

For the large-sized backup batteries, The Wirecutter chose the RAVPower 26800mAh Xtreme Portable Charger for their top pick. They say that this backup battery has the ability to charge a smartphone every night for a week.

For some, the best backup battery may be in the form of a battery case. Apple recently released their iPhone 6s Smart Battery ($99). In their buyer's guide to battery cases, published before the release of the iPhone case, the Wirecutter chose the Anker Battery Case ($40) as their top pick for iPhones 6 and 6 Plus.

While much of the buying decision is based on how you plan to use the backup battery, experts suggest that consumers do their homework and check to see if the device has been tested and reviewed before they invest in one of these “lifesavers.”