Having a second battery is one of the best ways to maximize the usefulness of any notebook computer. Apple sells spare batteries for its notebook computers, as do several third-party manufacturers. In some cases, the alternate brands offer batteries with more power capacity than the one that shipped with your computer when you bought it, making them especially attractive purchases. But if you have two or more batteries, how do you store any batteries not in use?
Storing a completely discharged battery for a long period of time (i.e. over a month or more) is one of the worst offenses. A certain amount of power is needed for the battery’s built-in circuitry to remain "smart," that is, monitor its own status and safety. Normally, batteries keep this energy reserve tucked away even when the notebook declares that the battery has run out of power. After a while, even this energy reserve is exhausted, and the battery dies. In such a state, it might not be recognized by the charger inside your notebook, forcing you to buy a replacement.
Oddly enough, storing batteries at full charge isn’t a good idea either. Apple recommends storing batteries at 50% charge. And the best place to store the battery isn’t inside the computer. Neither of these issues matters much for short-term storage, but if you’re tucking away your notebook computer for more than a couple of weeks, remove the battery first. If you happen to live somewhere warmer than the "Notebook Temperate Zone", consider storing the battery somewhere cool, such as a basement or refrigerator. Conversely, storing batteries somewhere that is likely to get exceptionally warm, like your car, would be hazardous to battery health.
As well as keeping stored batteries cool, Apple also recommends that every six months, batteries should be checked and if necessary, charged up to their 50% level.
Figure 3 Store spare batteries somewhere cool with approximately 50% charge