An exemplary embodiment includes a method for determining a time to recharge a rechargeable battery system including providing impedance data over a frequency range, said impedance data characterizing the operation of a rechargeable battery within a selected time window; analyzing said impedance data for predetermined behavior of said impedance data indicating an approaching state of discharge of said battery; and, determining from said predetermined behavior of said impedance data whether a recharge of said rechargeable battery is indicated.