My Mophie just arrived. It’s a reserve battery in a slim durable case for iPhones and other smartphones. The Mophie is well-designed and you can easily move the phone in and out of it.
Mophie recommends putting the reserve battery on standby. When the smartphone battery is low, simply switch the Mophie to active and it charges the phone battery. It comes with its own charger cord and will charge both batteries.
They also sell portable batteries that can charge most any USB device.