Yeah that $500 battery will charge everything but your carNothing like what you see for USB devices will charge the SP3. Your only option that I am aware of is a UPS. Something like this
http://accessories.us.dell.com/sna/...T&cid=262075&lid=4742361&acd=1230980794501410
according to the chart, the 36 watt power adapter should be able to run for a couple hours. I haven;t done physic in a long time, but I think it's enough to recharge the surface pro 3's battery at least once, maybe twice if not completely dead.
edit: ^ never knew that battery existed
That cable for $99 is Highway Robbery but he's the only game in town at the moment.I bought the cord from mikegyver.com for $99 and the Intocircuit Power Castle PC26000 External Battery for $99. Works pretty well!
I'd rather just buy a cable at a fraction of the price..I am making a recommendation to buy the eTauro docking station )$89 Amazon. It comes with a standard (2.1 mm) power jack - same as used on many laptops. Use this jack and any 12 - 16 volt capable battery pack (Amazon has several from 2000 to 3200 mAh ones for under $130), and you are in business. Have not tested yet, but did get confirmation from eTauro that it would work. By the way, this is a great dock alterative. It only weighs 3 0z., and allows use of the kick stand at all angles.