It charges cell phones, but it also jump starts cars.
 

It charges cell phones, but it also jump starts cars.

http://www.ebay.com/itm/Portable-Pow...=STRK:MEAFB:IT

Has anyone seen one of these? Does it really jump start cars?

The big (and very heavy) ones really will start a car or a tractor. They have a sealed lead acid gel cell inside. Issue can be when the gel cell can't be recharged anymore...you need to get a replacement that fits or buy a complete new unit. In the US, MCM Electronics and some other vendors carry compatible gel cells.

The ones I am talking about are a little bigger than smart phones and use Lithium-Ion batteries.

Define "jump start". The way I use the term it means that you can clamp it on the terminals of a dead battery and start a car. Something "a little bigger than a smart phone" can't even hold cables big enough to carry the current for that.