The LiIon battery in modern laptops is designed to take a full charge. That said, holding off a few percent of 100 might extend the battery life a very short time, but also means that each individual use of the battery is shorter than designed. And all LiIon batteries start to decay over time, accelerating at two years after manufacture (not service date, manufacture date). So while one could theoretically extend the battery life by only charging to 80% (that's overkill, in my book, but we can use it since it's been mentioned), the extension of the life will be relatively immaterial as the battery will start to decay anyway, just based on time since manufacture. The current batteries in Macs are rated for about 500-1000 cycles (100-0-100), so even if you pressed the battery hard by doing that every day, the life of the battery would be about two years. If you stay in the 100-50-100 range, the battery will last about 4 years, but in that four years you get the same "battery time" overall. Using the numbers Kylebrod suggested (80-40-80) the battery should last about 5 years, but again, the "battery time" is about the same. So it's a wash, really. Use the Mac the way you need to, the battery will most likely not be any better or worse for wear unless you really abuse it, and if that's what you need to do, that's what a portable is all about and you should be ready to invest in replacing the battery when it's no longer functional for you.