There are battery-less jump starters that use capacitors and the magic of DC-DC power electronics. Even if the vehicle battery voltage is too low to spin the starter, there's still a ton of chemical energy in there. The electronics in the jumper pack take power from the battery at say 11 volts and charge up the capacitor to 15 volts or so to spin the starter. Frickin magic.
Off topic as it's no use for the OP's needs but cool nonetheless.
