You may not be recharging it frequently enough. Li-ION batteries shouldn't be too deeply discharged. Try re-charing every 2-3 days rather than just once a week. Also, do not recharge in an unheated area.

From Battery University (http://www.batteryuniversity.com):


-Do not discharge lithium-ion too deeply. Instead, charge it frequently. Lithium-ion does not have memory problems like nickel-cadmium batteries. No deep discharges are needed for conditioning.

-Do not charge lithium-ion at or below freezing temperature. Although accepting charge, an irreversible plating of metallic lithium will occur that compromises the safety of the pack. Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help.