Effective battery management systems (BMSs) for rechargeable battery systems require reliable measurements of state of charge (SoC), state of health (SOH), and charge capacity. Present internal state monitoring techniques are indirect and inferred from long-term measurement of current, open-circuit voltage (OCV), and temperature, which can only give an approximation of charge capacity and often require knowledge of the recent battery history to prevent unacceptable levels of inaccuracy. To improve the performance of the BMS, an alternative method of monitoring the internal state of Li-ion batteries using magnetic sensing is presented. A highly sensitive micromagnetic sensor design is investigated consisting of a single interdigital transducer (IDT) on a piezoelectric substrate, shunt-loaded with a magnetically sensitive giant magnetoimpedance (GMI) microwire. This design makes use of the coupling of the impedance characteristics of the GMI microwire to the IDT transduction process. The sensor can detect magnetic field changes of less than 1 Formula Omitted when operated at or near its peak sensitivity. With an appropriate experimental setup, the proposed sensor is used to monitor the internal state of a Li-ion cell used in plug-in hybrid electric vehicles (PHEVs). The initial characterization results confirm that the GMI-IDT sensor can be used to directly monitor the charge capacity of the investigated Li-ion battery.