A practical method of predicting state-of-charge (SOC) and state-of-health (SOH) of battery systems has been developed and tested for several systems. The method involves the use of fuzzy logic mathematics to analyze data obtained by impedance spectroscopy and/or coulomb counting techniques. Fuzzy logic provides a powerful means of modeling complex, non-linear systems without the need for explicit mathematical models. New detailed impedance date has been obtained on the discharge performance of primary lithium/sulfur dioxide cells. Earlier data, obtained by Rutgers co-workers on nickel/metal hydride and other systems, have been reviewed and re-interpreted using fuzzy logic methodology. Devices are being developed for several systems, which will predict the SOC and SOH of batteries without the need to know their previous discharge and/or cycling history.