about how much charging can i expect to be able to do out in the field from my full size car's battery and still be able to drive back home? i dont want to be stranded but i need to charge my batteries!

it depends on what you plan to charge

just check car battery with a voltmeter once in a while and you should be ok whole day

+1, it all depends. I've run 2 chargers off of my truck battery charging 7-8 5000mah 2s packs with no problems.

I've charged my 2s 5000 packs probably 20 times on the truck battery.... started right up. I do bring the jumper cables just in case.