I found the following information on the web regarding fully implantable CIs:
Implantable CI
A Canadian company called Epic Biosonics Inc. is developing a totally implantable cochlear implant (CI). It will be implanted under the skin behind the ear, in a position similar to where the internal electronics for current implants are placed. The speech processing will be performed entirely within the implanted component (unlike current implants, which require an external processor).
The Epic CI will have an internal rechargeable battery, which will need to be recharged about once a week. This will be done by wearing a small disc (attached to glasses or on a headband) for a few hours.
The Epic CI claims the following advantages over current Cis:
1. One of the big advantages of total implantation is the lifting of restrictions that currently protect the external processor. Swimming with the implant, for example, will not be a problem.
2. The Epic CI will be implantable in infants shortly after birth, because it does not depend on development of the mastoid bone.
3. The Epic CI will have 48 electrodes and will include technology that places the electrodes in close proximity to the appropriate nerve cells.
4. It has no magnet, so is compatible with MRIs.
Epic expects to perform their first complete implant around the end of next year, with full clinical trials to follow shortly. Commercial availability is still several years away.