A replacement 58 WH (watt-hour) 17" PowerBook battery costs $129 or $2.22/WH. The Battery Geeks have a 122 WH external battery pack for $189.99 or $1.55/WH. That's more than 30% cheaper. It should last twice as long as the internal battery which means fewer interruptions. You can charge it and your internal battery at the same time whereas you can only charge the internal battery in your laptop (very annoying). I may have to order one before my next long trip. Does anyone have any experience with these?