according to dewalt:
http://www.dewalt.com/dewalt-dna/featured-articles/best-practices-for-battery-life
5. DOES THE OUTSIDE TEMPERATURE AFFECT BATTERIES? HOW?Yes. If the batteries are too hot (105°F or higher) or too cold (below 40°F), the batteries will not take a full charge. Attempting to charge batteries outside the 40°F-105°F range can result in a permanent loss of run-time. When batteries are being charged and discharged, a chemical reaction is taking place, and if it is too hot or cold the chemical reaction is disturbed causing a loss of run-time.