Electrolyte density in a lead-acid battery may vary by up to 20% during the discharge-charge cycle and this property can be used as an indication of the amount of stored energy. We describe the application of impedance measurements on a 10 MHz AT cut quartz crystal immersed in solutions of 0-59 wt% sulphuric acid for the purpose of determining the state-of-charge during battery operation. Changes in impedance magnitude, phase angle and equivalent circuit resistance were followed, all of which showed strong dependency on viscosity and density. Good agreement with the theoretically predicted behaviour of the motional resistance of thickness shear mode oscillators immersed in liquids was observed. These passive measurements of the crystal properties gave reproducible values, in contrast to active measurements involving determination of the frequency of a resonating circuit incorporating the crystal, which we have previously found to suffer from instability. Combining computer data acquisition and processing with appropriate calibration curves enabled direct monitoring of the discharge of a lead-acid battery.