I am very interested to see how this works out. Does the 12v by-pass the internal batteries?
Here's response from Doug on Battery set up:
Hi Chris, no worries, I'm not in any hurry. Yes, the way it is wired, it's either
the internal battery or the external battery. If they both worked together then
there would have to be a circuit that protects the batteries from each other. That
uses power and reduces runtime. If you have a weak internal then plug in an external
with no circuit protection, it will blow a fuse or, over time, cause both batteries
to swell and fail.