When he comes home and wants to build robots with his 14-year-old daughter, Alexandra, he wants the batteries to be charged and ready. John Wozniak, a technologist at Hewlett-Packard responsible for testing batteries, had some advice for the owner of a battery-powered device: "You don't want to leave it sitting on the front seat of a car. Manufacturers now include special circuits in laptop batteries to watch for this and, if detected, shut down the system. Mr. Wozniak said that it was dangerous for home users to take apart their laptop batteries to replace the individual cells inside the plastic housing (a cost-saving measure described by some do-it-yourselfers on the Internet) because the fail-safe circuitry may be damaged in the soldering and unsoldering process. Mr. Wozniak, for instance, suggests disconnecting your battery if you use your laptop as a desktop replacement. The best solution, he says, is to leave the battery half-charged in a cool room and charge it completely just before using the laptop. James DeJager, the technical director for Kodak batteries, sees nothing wrong with frequent recharging, at least for the lithium-ion batteries that are now standard in many laptops and cellphones. He says he understands the argument for disconnecting the battery but says the circuitry can handle such constant charging. Furthermore, he said in an e-mail message, it was "better to keep the contacts clean and avoid the possible damage from repeated insertion/removal of battery."