Just tried two Adorama power 2000 NP-F970 7.4V 7000mah is series.
When in series the total initial voltage is about 17volts on the EX3 screen.
Fully charged 17.1Volts on the EX3 screen and after 1 minute 15.8Volt.
I used de camera with these batteries for more than 6 hours.
I'd like to see a cheap little Vmount to EX mount adaptor which might stick out the back of the EX a bit but would allow the use of VMount batteries on them.