If you have an iPhone 6s or later, or an iPad Pro (9.7-inch) you can use "Hey Siri" whenever you want. If you have an iPhone 6 or earlier, or another iPad or iPod touch model, you need to plug your device into power first.
ref: https://support.apple.com/en-us/HT204389
Thank you for clarifying. I completely forgot that the 9.7" Pro doesn't need to be plugged in.