There's a myth floating around among laptop owners that lithium ion batteries in laptops must be discharged to zero every so often. Otherwise, the batteries will lose life. This belief stems from a characteristic common to older laptop batteries, namely nickel cadmium batteries. These batteries would lose capacity after charging as time went on if they were never totally discharged. (The rule of thumb now is no to discharge the NiCd batteries every three months to get the most out of them.)
Today, most laptops rely on lithium ion batteries, which do not have memories, according to Isidor Buchmann, founder of Cadex, a Canadian company that makes battery chargers and analyzers. Lithium ion batteries needn't be discharged to maintain their life, he says. In fact, they prefer partial rather than full discharges. Even so, users should run them down to zero every thirty charges or so. This recalibrates the battery's fuel gauge - the indicator on the laptop screen that shows battery power and time remaining.