Check the actual wire gauge of your jumper cables. Don't be fooled by thick insulation. Copper "looking" cables aren't necessarily solid copper. Far from it.
In a jumper type situation, its all about the difference in voltage between the jumper battery and the battery being jumped. The new lithium batteries are pretty good at initial surge voltage which helps a lot in a jump situation.
I was totally amazed once when my li-ion portable battery booster actually jump started an older GM V8 diesel with totally dead wet cell batteries. (2-12 volt starting batteries from the factory)
Chum lee
