I've always been a little surprised that you can top off a car's dead battery through the cigarette lighter, but apparently there's nothing stopping a second battery from doing a slow trickle charge back in. The Porta-Jump, a spare battery small enough to be left in the glove compartment, does this very thing. It's only $20, too, which seems a reasonable price, especially considering what a downer a dead battery can be. And since the simple electrical system in your car works both ways, once your alternator is turning fuel into electricity, you can recharge the Porta-Jump just by leaving it plugged in.

(Forgive any wide-eyed amazement that may be apparent in my post; I now live without a car, which is great except for when I totally miss driving. It's also been about a decade since I took any wistful walks through an auto parts store.)

Catalog Page [JCWhitney via Book of Joe via Coolest-Gadgets]