Too Hot to Handle
Extreme temperatures are particularly harmful. Apple describes the "Notebook Temperate Zone" as between 50°F -95°F. Using your notebook below this temperature range shouldn’t damage the battery permanently, but you will notice that the battery holds less charge than normal. Fortunately, once the system is warmed up again, its performance should recover completely.
On the other hand, higher temperatures than those recommended by Apple can and will cause irreversible damage to your notebook’s battery. So it is important to make sure that your notebook doesn’t, "run hot." Built-in fans will spin into life should the machine get too warm (using up valuable battery life in the process) so it makes sense to try and optimize passive cooling as well. Apple’s notebooks are designed to be placed not on soft surfaces (like your lap) but onto something hard, like a tabletop. Little rubber feet on the underside of the machine maintain a gap through which air can flow. This flow of air carries away excess heat. A number of computer accessory manufacturers also produce laptop stands designed to improve this flow of air, in some cases by even using metal heat sinks or USB-powered fans to draw away excess heat.
Figure 1 Keep tabs on the number of charge cycles by looking over the Power section of the System Profiler. Properly cared for, Apple estimates a battery will retain about 80% of its charge capacity after 300 cycles.