From the link to Calumet you provided, the battery shows only 2000mah---not 2600. 7.2V is what you want....
From the page cited:
Replacement BP-915 lithiu-ion battery for use with the Canon VL-10Li Video Light, ES4000, ES8400V, ES75, ES8200V, ES8100V, ES50, ES55, ES8000V, ES7000V, ES6500V, ES420V, ES410V, ES60, ES65, ES300V, XL1/XL1S, GL1, GL2 and other compatible camcorders.
The listed 7.2v is what's known as 'nominal' voltage. It's usually the voltage at which discharge becomes quite rapid, so it's the lowest usable voltage for that battery. As a battery charges, the output voltage increases. As it's discharged, the output voltage decreases. In this case, the fully charged output voltage of both
brands of batteries is 8.3v.
I have a DC voltmeter accurate to 1/100th of a volt. I used it.