depends on the battery... a Lipo you want to store at about 40-60% charged, never fully charged or discharged. Nimh or Nicad can be stored charged or discharged, but I normaly store them charged. After extended storage it will take a few cycles to get back to full performance.

With any type of battery you want to store them in a cool (not cold) dark place. Like a closet or drawer. If you garage is not heated, dont store them there, extreme temperature swings are hard on batteries of any type.