Porta-Jump: Tiny Cube Jump Starts Your Car
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.)
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Clever, but there's a small problem...
You'd better hope that you don't forget about it after you've tucked away your little battery in the glove box or trunk. Especially if you park outside in cold weather. Batteries drain a lot faster than you'd expect in cold weather.
Trying to jump your car with a dead battery is a sure-fire recipe for a sheepish and defeated call to AAA.
However, if you tossed this guy in the trunk (assuming it's not vapor-producing) and connected it to a solar trickle charger sitting on the rear deck of your car, well, you'd have an always charged backup battery. Of course, if you're going to go to that effort, you might as well connect the trickle charger to your main battery and be done with it.
I independently thought up this exact same idea yesterday. I guess I'd better think of something else.
The reason that nobody thinks of using the cigarette lighter is that the little thing can't carry much current. Cigarette lighter plugs are connected to the battery through a fuse, probably in the 10-15 amp range. Clearly, using jumper cables that can easily carry 50 amps or more is going to result in getting your battery back in a state to start you car a lot faster.
This toy probably isn't much more than a gel cell, a fuse, _MAYBE_ a current limiting resistor, wire and connector. But hey, it does the job...