Quoting from http://batteryuniversity.com/learn/a...ion_batteries:
A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger. Charge at a moderate temperature. Do not charge below freezing. Lithium-ion does not need to be fully charged; a partial charge is better. Chargers use different methods for “ready” indication. The light signal may not always indicate a full charge. Discontinue using charger and/or battery if the battery gets excessively warm. Before prolonged storage, apply some charge to bring the pack to about half charge. Over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.