I walked across the street and borrowed a "hot pack" from the car repairplace. Slapped that on and the old Benz fired right up. I will need to search eBay for a good deal one of those...
For probably less money you could carry a spare charged battery around in your trunk. Not as handy, no, but holds much more juice, and you already have the jumper cables. A used battery at the U-Pull is $12 here, but that's a crap shoot. (The Frankenheap is on its second year with just such a battery.) -- Jim