I read recently about a portable charger that you could boost off a boat or car. It was lightweight and easy to stow onboard or in car. Here is one example. Amazon.com: 1byoneŽ New Release - The World's First Smart Portable Car Jump Starter. - Start Your Car Countless Times !!! 9000mAh 12V Multi-Function, UltraSafe Lithium Jump Starter, Emergency Auto Car Battery Charger Jump Starter With Portable Po