Non-intrusive monitoring of the state-of-charge (SOC) of lead-acid batteries by the use of wire wound coils is described. Coils were attached to the outside case of the battery, adjacent to the negative end plate, and excited using ac current of frequency in the range of 1-40 kHz. The change in inductance of the coils was monitored during battery cycling, as the metallic content of the electrode changed. Following correction for temperature changes, the technique is capable of estimating the SOC of a battery to an accuracy of +/- 10%. The inductance profiles that were obtained changed shape as the batteries aged and also with the exciting frequency. The origins of these changes are discussed. Crown Copyright