An implantable stimulator system, comprising: (a) an implantable stimulator circuit configured for implanting within an organism: (b) an external circuit configured for retention sufficiently proximal said implantable stimulator circuit for communicating power to said implantable stimulator circuit, and data to and/or from said implantable stimulator circuit; (c) wherein said implantable stimulator circuit comprises: (i) a first inductive coil configured for receiving power from the external circuit as a power coupling; (ii) a second inductive coil configured for communicating data with the external circuit as a data coupling; (iii) a data link circuit configured for communicating data through said data coupling; (iv) a flexible epidural electrode array comprising a plurality of electrodes; (v) a multi-channel stimulation circuit having a high voltage output stage configured for multiplexed driving of the plurality of electrodes in said epidural electrode array; (vi) a multi-channel stimulation recording circuit; (vii) a bio-impedance characterization circuit utilizing a time-domain method configured for determining bio-impedance for an electrode-tissue interface at the plurality of electrodes to determine an equivalent circuit model that is utilized to ensure no violation arises on stimulator compliance voltage and electrode water window; wherein said bio-impedance characterization circuit is configured for measuring over potentials of said plurality of electrodes, tissue resistance, double layer capacitance, and charge transfer resistance of said equivalent circuit model when a current stimulus is applied to the electrodes; and (viii) a controller circuit and memory configured for storing stimulation parameters and recorded stimulation data, and for controlling stimulation of the plurality of electrodes, based on said equivalent circuit model determined by said bio-impedance characterization circuit, and communication with the external circuit. (a) an implantable stimulator circuit configured for implanting within an organism: (b) an external circuit configured for retention sufficiently proximal said implantable stimulator circuit for communicating power to said implantable stimulator circuit, and data to and/or from said implantable stimulator circuit; (c) wherein said implantable stimulator circuit comprises: (i) a first inductive coil configured for receiving power from the external circuit as a power coupling; (ii) a second inductive coil configured for communicating data with the external circuit as a data coupling; (iii) a data link circuit configured for communicating data through said data coupling; (iv) a flexible epidural electrode array comprising a plurality of electrodes; (v) a multi-channel stimulation circuit having a high voltage output stage configured for multiplexed driving of the plurality of electrodes in said epidural electrode array; (vi) a multi-channel stimulation recording circuit; (vii) a bio-impedance characterization circuit utilizing a time-domain method configured for determining bio-impedance for an electrode-tissue interface at the plurality of electrodes to determine an equivalent circuit model that is utilized to ensure no violation arises on stimulator compliance voltage and electrode water window; wherein said bio-impedance characterization circuit is configured for measuring over potentials of said plurality of electrodes, tissue resistance, double layer capacitance, and charge transfer resistance of said equivalent circuit model when a current stimulus is applied to the electrodes; and (viii) a controller circuit and memory configured for storing stimulation parameters and recorded stimulation data, and for controlling stimulation of the plurality of electrodes, based on said equivalent circuit model determined by said bio-impedance characterization circuit, and communication with the external circuit. (i) a first inductive coil configured for receiving power from the external circuit as a power coupling; (ii) a second inductive coil configured for communicating data with the external circuit as a data coupling; (iii) a data link circuit configured for communicating data through said data coupling; (iv) a flexible epidural electrode array comprising a plurality of electrodes; (v) a multi-channel stimulation circuit having a high voltage output stage configured for multiplexed driving of the plurality of electrodes in said epidural electrode array; (vi) a multi-channel stimulation recording circuit; (vii) a bio-impedance characterization circuit utilizing a time-domain method configured for determining bio-impedance for an electrode-tissue interface at the plurality of electrodes to determine an equivalent circuit model that is utilized to ensure no violation arises on stimulator compliance voltage and electrode water window; wherein said bio-impedance characterization circuit is configured for measuring over potentials of said plurality of electrodes, tissue resistance, double layer capacitance, and charge transfer resistance of said equivalent circuit model when a current stimulus is applied to the electrodes; and (viii) a controller circuit and memory configured for storing stimulation parameters and recorded stimulation data, and for controlling stimulation of the plurality of electrodes, based on said equivalent circuit model determined by said bio-impedance characterization circuit, and communication with the external circuit.