Measurements of charge-acceptance, internal resistance, voltage and self-discharge of a battery reflect its state-of-health (SOH). The galvanostatic non-destructive technique (GNDT) can be used to monitor the SOH of a battery by analyzing its impedance parameters, namely ohmic resistance, charge-transfer resistance and interfacial capacitance. In this technique, the battery is discharged galvanostatically at a substantially low-rate over a short duration, wherein the state-of-charge (SOC) of the battery is not affected. It has been possible to obtain charge-transfer resistance and double-layer capacitance values for both positive and negative plates of a commercial grade 6-V/4-Ah valve-regulated lead-acid battery during its dynamic discharge. The resistive components of the battery are found to be minimum at state-of-charge values between 0.2 and 0.9. The study shows that the optimum performance of the VRLA battery can be achieved at SOC values between 0.2 and 0.9. The ohmic resistance of the battery displays a linear variation with logarithmic values of its SOC. The technique provides an attractive tool for on-line monitoring of lead-acid batteries. (C) 2004 Elsevier B.V. All rights reserved.