Well as much as I want to, I want it to run it off its own battery. But this is what the seller had told me:
" And yes D-zero still drains the battery when it's on charge off mode. "
I don't really know how bad it is but I'm taking precaution since the parts you need for it is around $20 total with a decent 2800 mah USB battery anyway. Not bad if it means extended hours of listening through this portable setup!
Ah, gotcha! Makes sense now. To my knowledge (happy to be corrected if I'm mistaken), I don't believe the D-Zero pulls current from the phone when playing unless it's set to charge. I didn't listen to mine for any extreme listening sessions, as I recall, but I don't remember the battery draining any faster with the D-Zero hooked up (charge off) than without it. Of course, this was with the S3, which didn't have stellar battery life to begin with-- but that would seem to make it more susceptible to battery drain.
I'll keep my fingers crossed that you don't have any issues, but as you mentioned, it doesn't appear to be a big financial hurdle to covering your bases either way. Definitely keep us informed on how you get on ;).