A battery monitor control system (150, 200) comprising: a) one or more primary switches (Q, Q 2, Q 4) adapted to be connected to two or more terminals (T+, T 0, T-) of a string of one or more battery units (102, B 1, B 2, B 3, B 4) connected in series; b) a load resistor (R L) in series with said one or more primary switches; c) a current sensor (R S) in series with said one or more primary switches; d) two or more lead wires (112, 118, LW 1, LW 2, LW 3, LW 4, LW 5) adapted to be connected to two or more busbars (104, 106, BB 1, BB 2, BB 3, BB 4, BB 5) electrically connecting said battery units in said string; and e) a digital signal processor adapted to: 1) control (V g1, V g2) said one or more primary switches to produce a ripple current through said string of battery units, said ripple current having a ripple current wave form comprising a reference frequency; 2) measure a ripple voltage wave form (V B1, V B2, V B3, V B4) across each of said battery units via said lead wires; 3) measure said ripple current wave form (I L) from said current sensor; 4) determine a real portion (R B1, R B2, R B3, R B4) of a complex impedance of each of said battery units using said measured ripple voltage wave forms and said measured ripple current wave forms; and 5) output said real portion of said complex impedance of each of said battery units such that changes in said real portion of said complex impedance of each battery unit can be tracked over time wherein said primary switches are controlled by a pulse width modulated sine wave. a) one or more primary switches (Q, Q 2, Q 4) adapted to be connected to two or more terminals (T+, T 0, T-) of a string of one or more battery units (102, B 1, B 2, B 3, B 4) connected in series; b) a load resistor (R L) in series with said one or more primary switches; c) a current sensor (R S) in series with said one or more primary switches; d) two or more lead wires (112, 118, LW 1, LW 2, LW 3, LW 4, LW 5) adapted to be connected to two or more busbars (104, 106, BB 1, BB 2, BB 3, BB 4, BB 5) electrically connecting said battery units in said string; and e) a digital signal processor adapted to: 1) control (V g1, V g2) said one or more primary switches to produce a ripple current through said string of battery units, said ripple current having a ripple current wave form comprising a reference frequency; 2) measure a ripple voltage wave form (V B1, V B2, V B3, V B4) across each of said battery units via said lead wires; 3) measure said ripple current wave form (I L) from said current sensor; 4) determine a real portion (R B1, R B2, R B3, R B4) of a complex impedance of each of said battery units using said measured ripple voltage wave forms and said measured ripple current wave forms; and 5) output said real portion of said complex impedance of each of said battery units such that changes in said real portion of said complex impedance of each battery unit can be tracked over time 1) control (V g1, V g2) said one or more primary switches to produce a ripple current through said string of battery units, said ripple current having a ripple current wave form comprising a reference frequency; 2) measure a ripple voltage wave form (V B1, V B2, V B3, V B4) across each of said battery units via said lead wires; 3) measure said ripple current wave form (I L) from said current sensor; 4) determine a real portion (R B1, R B2, R B3, R B4) of a complex impedance of each of said battery units using said measured ripple voltage wave forms and said measured ripple current wave forms; and 5) output said real portion of said complex impedance of each of said battery units such that changes in said real portion of said complex impedance of each battery unit can be tracked over time