If you're caught in an emergency, you can give your phone a little charge with a nine-volt battery, your house key, and a standard car charger.
At Instructables, user Ludvic demonstrates the simple procedure that allows a common household battery to give your phone a little juice. I was curious about the safety and effectiveness of the process, so I tried it myself. Sure enough, my phone's battery started to charge. Just plug your phone into the charger, hold the key to the contact on the side of the car charger plug, touch the end of the car charger plug to the positive side of the nine-volt battery, and touch the end of the key to the negative side.
It may not completely charge your phone's battery, but you should get enough power to make a quick emergency phone call. For more instructions, check out the link below.
Emergency Charge for Your Smartphone [Instructables]