Miniature Jump Starter

zakimodo said

I think the key to how "well" these work is that they have some level of smarts inside of them.

Most of the ones I have seen take a bit of warm up time before they are ready to actually jump the car. My guess is that the box is getting some baseline for the battery charge. As soon as it sees the voltage tank (when to go to turn the engine over) they deliver their "jump". The spike they provide has been compared to the discharge of energy observed when you short a out a battery.

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They do have battery management electronics in most today, but that isn’t what makes them work. It is all battery chemistry & design. They have to have a very low equivalent series resistance so that they can deliver a large amount of current.

The one I have says it is rated for 26 watt hours. That would be about 93KW seconds (26x 3600). A watt second is a joule of energy. So my battery pack has 93KJ of energy.

300 amps at 12 volts to start a car is about 3.6 KWatt (12 x300). Let’s say you crank your car for 5 seconds. 3.6 KW x 5 seconds is 18 KJ.

So my battery pack has 93 KJ of energy and it takes 18 KJ to start my car. This means I might be able to start my car about 4 times at 5 seconds a try.

This is an over simplification, but gives you a good idea of how it works.

On a safety note, high voltage electrical energy levels above 50 joules are considered lethal. Things start getting kinetic at about 100 joules. Not quite the same here, but you get the idea. If not handled properly, it is a potential bomb.