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If you, like me, tend to blow through your monthly smartphone data allowance in a week or two, and have a hard time keeping your iPhone's battery above 20%, you'll want to know about a feature in Apple's latest operating system, iOS 9, that could be taking a toll on your phone's battery life and data usage.

It's called "Wi-Fi Assist," and it's meant to help you by automatically switching you over to cellular data when wifi connectivity is low. The feature is great if, for example, you're in the middle of watching a great YouTube video and your wifi suddenly gives out out.

But if your phone is downloading big files over cellular connections—including any streaming music, podcast, or video-heavy apps like Snapchat or Vine that you have running in the background—you could be in for a shock when your monthly phone bill arrives. And since cellular downloads also use a lot of power, you could also find your battery giving out sooner than you expected.

If you've already upgraded to iOS 9, here's how to turn off Wi-Fi Assist:

Step 1: Go into your iPhone's settings menu.

Step 2: Select "Cellular"

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Step 3: Scroll all the way down, past all of the apps, to the bottom of the screen.

Step 4: Find the switch that says "Wi-Fi Assist"

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Step 5: Turn it off.

Congratulations! Now you'll only go over your data plan by $20 next month, instead of $50.