Current draw of Surface Pro when charging

I want to use an external battery to recharge recent Surface Pro models.  The batteries I'm considering break the circuit at 6A and cannot be reset without access to AC power, which would defeat the purpose in my application.  Has anyone tested the maximum current drawn from the main charging point by recent model Surface Pros?  Could a Surface Pro possibly trip such a battery?  Has anyone had experience with a battery that would work in this application?