Backup Batteries
The most prosaic option for keeping your cell phone charged is to utilize a spare battery. Many cell phones allow you to pop out and replace your dead battery with a fully-charged one, but not all phones have accessible batteries. In this instance, emergency packs are available that plug into a dock or mini-USB connector, or are part of a protective case. Keeping a spare in your purse or bag is convenient in an emergency if you cannot adequately use a larger backup device like a solar charger.
Solar Charging Devices
Using the power of the sun, these charging devices keep your cell phone powered and battery charged. Although these chargers often require precise placement relative to sunlight, most solar chargers are lightweight. However, solar chargers do not recharge batteries quickly, which must be remembered if a real emergency arises.
Hand-Crank Chargers
Hand-crank chargers use your own power to supply a small amount of current to your cell phone's battery. Often incorporating a built-in AM/FM and emergency-band radios, these chargers keep you informed until you can crank new life back into your phone. These units also often have a flashlight, collapsing neatly into a compact package you can stow in an armrest or in an emergency kit.
Large-Scale Emergency Backup
Sometimes you need emergency power for more than just your cell phone. Large-scale chargers operate on sealed battery packs, letting you recharge your cell and perform other tasks like re-inflate a car tire. These portable units are fantastic additions for the prepared homeowner or traveler, comprising a lot of juice in a relatively portable chassis. Smart versions of the unit even allow automatic polarity inversion in the event you attempt to jump-start your car and reverse the cables.