Dan at RMG Enterprises in Sunnyvale recommended this portable battery for jump starting cars (including V8 trucks), so I got one. It'll also work to charge a laptop or cell phone. Apparently, the guys at RMG know the guy who invented this thing.
http://www.amazon.com/gp/product/B00FDYYK4A/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1