I also built a smaller version of what you guys have. I have a 8200 mah chargeblock that has a 12V 2amp output (basically the same as a rt charger) and it seems to work with my pro. I doesn't seem to charge as fast as the rt charger (I use it for travelling all the time) but it maintains and slow charges. so far so good. Cable was fairly simple to build. was going to get a Mcgyver one but 30 was a little steep.