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Question: Where is the internal battery?

I just received the HF M31. The manual says that an internal battery keeps the date-time data and eventually needs to be replced. However, the manual does not describe its location. I check the camcorder and could not it either. Please let me know whete this internal battery is.
I have also a Canon elura 85 camcorder and the location of the internal battery is readily found.
Thanks, hst