You will need to remove the internal batteries from the jb3 to prevent charging mode (which draws too much at peak).
For the bazillionth time. You
don't need to remove the internals from the Jb3. I was the one who originally started the thread here a couple years ago about using the
Sima and I have never had a single problem running one or both batteries while on external power. I simply top off the batteries before I head out to to the shows. I have however forgot and the Jb3 charged for a few minutes before the internals were full and it returned to just running from the external source.
If you feel lucky/crazy enough to run your Jb3 without an internal. You better hope you have enough juice as if it runs out of external battery your recording will be lost.