answer from Canon CPS:
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Thank you for contacting Canon product support regarding the battery options for your EOS 6D. I'm sorry that you're running into battery problems with some batteries.
Let me suggest that you take out the battery from the camera and clean the contact points in the camera casing, as well as on the battery itself, with a pencil eraser.
After leaving the battery and memory card out for at least 30 minutes, put them both back into the camera. It might even be a good idea to ensure that the battery is fully charged before placing it back into the camera's battery compartment.
Also, check the problem battery and see if it matches the logo appearance and style of the good battery. There are knock-off batteries that other companies sometimes try to pass off as Canon batteries, they look identical as the Canon ones with serial code, and these knock-off are the batteries which doesn't work like the originals, and might damage the camera, which is why we suggest that you stick with the authentic Canon batteries, make sure you have bought the batteries from Canon store or any reputable Canon store reseller.
Let us know if you have any other questions, and thanks again for choosing Canon.
Sincerely,
Nick from Canon CPS
Technical Support Representative
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So.. my brother's batteries are FAKE.. I don't know how and why...

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