This paper presents a microcontroller-based, on-line battery state-of-charge (SOC) estimator, and monitoring system for sealed lead–acid batteries, used as back-up power supplies in various applications. The proposed SOC estimation system consists of a combination of the discharge time versus discharge rate data given in manufacturers’ data sheets, and coulometric measurements. Battery SOC monitoring tests have been carried out under both constant and dynamic load conditions, and for alternate charge–discharge cycles of lead–acid batteries of different capacities, and for different operating temperatures. The test results have shown that a very good accuracy in the estimation of available capacity is obtained for various practical operating conditions. Changes in battery characteristics which occur due to ageing are also taken into account by estimating the actual capacity of the battery, and adopting the corresponding recharacterization of the aged lead–acid cells.