The latest iPhones and iPads are now so power efficient that you can effectively say “Hey Siri” whenever you want and have instant hands-free access to your virtual assistant; but that’s not the case with some of Apple’s older devices. Those such as the iPhone 5, iPhone 5s, iPhone 6, and iPhone 6 Plus require that you be tethered to a power source for Hey Siri to work.
Albeit inconvenient, the chips in the aforementioned devices aren’t as power-efficient as those found inside of devices today; consequently, always-on Hey Siri would have an adverse impact on your battery performance. But if battery life is no object to you, then you can forcefully-enable Hey Siri on these handsets with a free jailbreak tweak dubbed HeySiri by iOS developer Julio Verene.