Techniques for monitoring a battery of an implantable medical device are disclosed. First and second current sources are provided to draw currents having amplitudes of I1 and I2, respectively, from the battery. First and second voltage measurements, V1 and V2, are obtained when first and second combinations, respectively, of the first and second current sources are selectively activated. Battery impedance is determined using the current amplitudes I1 and I2 and the voltage measurements V1 and V2. The impedance measurement may be used to obtain an open-circuit voltage of the battery without the need to disconnect the battery from circuitry to which it provides power. Battery impedance and/or open-circuit battery voltage may then be used to determine an estimated time until an action is required involving the battery, which may include activation of an ERI or EOL indicator, or initiation of a recharge session.