The story below has some good information in regards to the EOS 5D Mark III and some third party batteries. Not all of them are created equal, and a few companies will support you in making sure you get batteries that are 100% compatible.
When Canon upgraded the firmware of the 5D Mark III to 1.2.2/1.2.3 they killed some of the functionality of aftermarket batteries.
- Each time you put a battery in it required you to acknowledge “Communication with the battery is irregular” and the camera would not register it.
My contact at Canon explained this was an effort to cut out counterfeit batteries claiming to be Canon batteries. I explained to him the 3rd party batteries I used were clearly NOT labeled Canon and in addition contained a chip to allow it to communicate properly. My contact explained that Canon would probably never make a change backwards on this situation.
Jon contacted the vendor that makes the batteries he likes: Maxtek/YoKool, they advised him that they now have a new battery that is 100% compatible to the new firmware. The new batteries he got directly from them do not have the communications issue and they will register with the camera.
Maxtek/YoKool only sells these batteries through Amazon. Maxtek/YoKool advises at least for the moment (until Amazon stocks reduce and have the new batteries) to make sure to order the batteries directly from them, not use the Amazon fulfilled link: See the correct batteries here.