The Sony PEG-UX50 was an impressive package when it was released in 2003. It had a 480x320 screen, keyboard, convertible screen, built-in camera, WiFi and Bluetooth. As with most portable devices, the short time the battery stayed charged left much to be desired. Sony sold an EB40 battery pack that piggy-backed the device, but reader [JAmerican] wanted to extend its charge even further. It took a little dancing around with the charging circuit, but he was eventually able to combine 4 batteries to construct a 3880mAh battery that keeps the UX50 running, even after 9 hours of use. Here's a site with a decent photo gallery of the UX50 (at the bottom).
http://www.hackaday.com/2006/02/14/ux50-long-life-battery/