Possible to charge a dead car battery with one of those survival flash lights that you shake back and forth?

Question

I've heard an urban myth that if your car is stuck somewhere with a dead battery, you cold start rubbing the negative battery post with nylon cloth to build up static electricity that will eventually charge the battery. I'm sure this is remotely true.

But how about one of those survival flash lights that you shake back and forth to generate electricity? If you take the flash light apart and connect the positive from the flash light to the positive of the battery and the negative to negative........ how many days do you think one will have to be standing there shaking the flashlight back and forth in order to charge the car's battery enough to make it spin the starter?

Answer 1

It depends on how dead the battery is. If you shake the flashlight (or read a book) for an hour, the battery might recover enough on its own to start the engine. If it's exhausted, no amount of charging will enable it to start the engine.

A full charge for a car battery is somewhere in the range of 100AH (100 ampere hours). If you charge it continuously with a current of 1A, it will be fully charged in about 100 hours.

A human pedaling a bicycle can produce a sustained power output of about 300 watts. Pedaling a generator with 50% efficiency would give you about 1.25A, so you could fully charge a car battery in about 80 hours, or two full work weeks. If the battery isn't too fully discharged, a couple hours of pedaling might be enough to get you started. Shaking the flashlight probably can't even generate enough to compensate for leakage.

If the car has a manual transmission, one person can push it fast enough on level ground to kick-start it. That's a much more effective way to use human power to start a car.

Answer 2

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