Hello, Have you tried emailing Onyx first? I am not associated with the company. I live in Europe and the email address I have to contact Onyx Int for support is:
sales@onyx-international.com
If you email them your device's serial number hopefully they will be able to help you.
You can also post your query on their official forum, they reply pretty fast:
http://bbs.onyx-international.com/
I hope they will fix the battery for you. Good luck!!