Measurement of impedance of rechargeable batteries at very low frequencies reveals valuable information relating to battery behaviour and condition. Until now, successful measurements have been made in the laboratory with bench analysers. This requires batteries to be taken out of service for periods ranging from hours to days, which is impractical for units that are needed for continual use or that are difficult to remove for analysis (e.g. electric vehicles), or that are difficult or impossible to access (e.g. satellites, implanted medical devices, distant data centres). This manuscript describes a simple microprocessor-based circuit that draws current at a number of frequencies and provides voltage and current data that can be logged and used to calculate the impedance of batteries in situ. It is a good engineering example of a PIC-based electronic circuit design and verification including C-language code.