If so, what brand is it?
Mine refuses to work with a coupler from Kendrick Astro, and the Pegasus Pocket Powerbox.
The coupler will work with my 7D, but not with my 7D Mark II.
On the original 7D, I initially get a message "Cannot communicate with battery," but you have the option of 'allowing' it to use the battery, anyway.
The Mark II doesn't give you this option. It just doesn't start up.
If it's not one thing, it's another...