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The internal memory battery in the XF105 is not replaceable. It isn't like the older cameras that had a watch style battery that could be replaced once it was drained. The memory battery is either soldered onto the main board of the cmaera, or it's actually not a battery at all and the memory information is stored on a chip on the main board. There's no way to change this battery with this model camera without a complete break down and soldering on either a new battery or a new chip.
Canon has tried several different methods of storing memory and settings. Some like the GL2 have an option to store the info on a memory card, and some of the much older pro models have a removable coin battery. Unfortunately, now what we have is a proprietary memory battery. If it's not holding a charge, then there's really no other option than to send it in to the nearest Canon service center and have it replaced.
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