If you've joined the gaming phenomenon that is Pokemon Go, you've probably already discovered that the game is brutal on your smartphone battery.

This weekend, I played for three hours and it completely drained my iPhone.

Turns out, searching for Pokemon in the tall grass of the real world takes a lot of power. The app is constantly pinging GPS, rendering cute Pokemon and urban environments, and the game requires the screen to be on for a lot of time.

The solution for any intrepid Pokemon hunter vying to catch them all is to buy a battery pack, which can double or triple the amount of time you can spend away from a plug.

Buying a battery pack can be confusing. Here's what you need to know along with some suggestions:

The most important stat is 'ampere count'

You'll want a battery pack with an amp count over 2 — typically, it will say "2.1A" or "2.2A" under the USB port. Although a port with an amp count of 1 will still charge your phone, anything with an amp count over 2 can "fast-charge" your phone, which means that you have to spend less time with a cord stuck into your phone and more time catching Pokemon.

Frequently, battery packs with two USB plugs will have one that supports "fast-charging," and one that only outputs 1A, like this $18 battery pack from Monoprice.

Bigger batteries hold more charge

The second most important stat on a battery pack is its capacity, which is measured in mAh. The number of mAh your battery pack supports is directly related to how much power it can hold.

Battery packs range from 1,000 mAh to some packs with 16,000 mAh or more. The iPhone 6 has a 1810 mAh battery, so a big battery can charge your phone several times over.

For example, this $30 AmazonBasics battery pack advertises that it can charge an iPhone 6s 9.3 times with its 16,100 mAh reservoir — and both ports are rated for 2.4A.

Consider a battery case

If you don't want to carry a heavy brick around while you hunt for Pokemon, your best bet might be a battery case, which snaps onto your phone.

If you use an iPhone, Apple makes a "smart battery" case for the iPhone 6S and iPhone 6S Plus that's pretty nice. It's smaller in terms of capacity than an external battery pack, but Apple says it can increase the battery life of an iPhone 6 up to "18 hours" on a cellular connection.

It's a bit pricey at $99, but it has the benefit of using Apple's own Lightning charging connector, and it also integrates with Apple's iPhone software to show you how much battery you still have left.

If you've got an Android phone, Mophie is a well-known and trusted brand, and it makes cases for most common Android devices, including the Galaxy S7.