The present invention provides a fully implantable cochlear implant system (FICIS) that allows various configurations of different modules to be combined so as to meet the needs of a particular patient, including very young patients. At least three main modules are used in the FICIS, including (1) a small implantable cochlear stimulator (ICS) module, with permanently attached cochlear electrode array; (2) an implantable speech processor (ISP) module, with integrated microphone and rechargeable battery; and (3) an external module. In one embodiment, the external module may comprise an external speech processor (ESP) module. In another embodiment, the external module may comprise an external battery charger (EBC) module. The ICS and ISP modules are configured to facilitate long time reliable use of the ICS module, e.g., for the lifetime of the patient, and low-risk, relatively easy replacement of the ISP module.