Extra battery for iphone? (not an iphone case)?

I remember seeing this thing that a girl had in her "what's in my bag" video of a rechargeable battery that you connect to your iphone to recharge it. I don't wanna buy the rechargable cases, so? Does anyone know what the battery is called?

i kind of looked like this
image

A extra iphone battery or back up iPhone battery. Buy them on amozon or Marshall's and things like that.

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