Cell phones can be charged a few different ways without a battery charger. You can go into your wallet or purse and buy a new battery, or you can go dangerous and hook up a low-voltage charger with alligator clamps to the battery and risk shocking yourself. Then again, you can do either of two reasonable things: use a USB data cable and your computer to charge the phone, or buy a Cellboost disposable charger to get some extra life out of your battery.
Charging a Phone with a USB Data Cable
A cell phone can be charged by connecting it to a computer with a USB data cable and siphoning off the system's power. Charging the phone with a USB data cable works the same way as using a normal charger, except a complete charge may take a little while longer (around three hours) because it's not connected directly to a power source like a wall socket or the outlet in the car. You can charge using a USB port with a laptop as well, but you'll do better if the laptop's power source is connected, so you don't kill the system's battery while you're trying to save the cell phone.
Cellboost, and anything similar, is a disposable charger that provides extra life to the batteries of electronic items, including cell phones, iPods, laptops, camcorders, and portable gaming consoles. It fits directly into your cell phone's charging port to provide an hour of usage before the cell phone dies, upon which the Cellboost charger becomes useless. The problems with Cellboost are that it can only be used once and it doesn't completely charge the cell phone. For people who frequently use their cell phones, Cellboost is an inadequate option, except in emergency situations.