A cochlear implant does not have an internal battery within a person's body.
It is powered by an external battery that transmits power through the skin by magnetic induction.

Thus the components of a cochlear implant underneath the skin do not need to be removed before cremation.
The external earpiece (microphone, processor and battery-powered transmitter) should be removed from the body before cremation.
Even if the external earpiece is not removed, the battery in the external earpiece is not likely to explode in a cremator.

For further information:
Wikipedia article