Because safety is of critical importance, the best way to charge a lithium polymer battery is with a digital charging station. The iCharger 206B Synchronous Balance Charger/Discharger is a highly rated device for getting a LiPo back to full strength.
LiPo batteries always have a partial charge and can explode if mishandled, and should be stored in a battery safeguard bag when not in use. Before charging the battery, check its voltage to ensure that it is within normal limits. A new 2S battery should register between 7.0 and 7.8 volts. The ideal level for a new 3S is between 10.5 and 11.7 volts. A new 4S should register between 14.0 and 15.6 volts in its proper range.