Upconvert of CC is complex. Downconvert from 12V to 8.4 V can be done with many simple electronic circuits, but anyway need a bit of experience. If you want, I can attach some electric diagram for this.
Thanks. I formed similar opinion about this. It looks like GH2 uses voltage measurement to decide it is AC or battery. And battery must always pass their complicated checks.
At least 8.4V. But other users who have conducted tests say 9+ is ideal. Also strange is that when I shoot in slow motion mode, it would say 'this battery cannot be used', but 24p is fine.
Just found tyampel citation - "you need to feed the coupler with 8.4 to 9.3 volts. Lesser voltage will result in a message "This battery cannot be used"."