Is it possible to charge the SP4 from a portable powerbank? (i.e. like you can with a phone / iPad / etc.). I've had a bit of a search and other than suggestions to get an inverter for the car I haven't come across anything... I'm hoping there's an option... I have a powerbank with lots of fittings, is there a USB to SP4 power fitting out there somewhere? Would also work on a plane (for example) or anywhere offering a USB charging port...