So far they have been very careful not to show the back of the helmet. the only time I have seen the back is in the video from an earlier post from Hasbro. It was at the end where they quickly flipped it over before placing it on their heads. I did not see a compartment (at least not clearly). It was either poor resolution or a prototype (it is the same video that noted that audio was simulated).
The only other reference I have seen to the helmet's battery compartment is schematic images from a catalog.
If you go back to this post:
http://www.therpf.com/showthread.php?t=257970&page=4&p=4069091&viewfull=1#post4069091
You can see the panel in the back at the bottom when viewing the helmet from the side.