What is the best way to store LabQuest units over summer vacation?

The best treatment is to simply box up the LabQuest for the summer, and leave it disconnected from the charger. Over the summer, the battery will slowly lose its charge, but will be undamaged. When you return in the fall, we recommend that you charge all LabQuests for a full 8 hours before using them. When you first turn it on, it may take as long as 3 minutes for the LabQuest to reboot if the battery completely lost charge.

Exposing the battery to temperatures over 35°C (95°F) will significantly reduce its lifespan. If your school storage area is unusually hot when the air conditioning is off for the summer, you may want to keep it in a location that remains cooler

More details on the longevity of the battery can be found at:
http://www.vernier.com/til/1768.html