This disclosure relates generally to relates to the field of estimation of remaining useful life (RUL) in lithium based batteries, and, more particularly, to estimation of remaining useful life in lithium based batteries based on coupled estimation of a state of charge (SOC) and a state of health (SOH) during charging/discharging in constant current (CC) and constant voltage (CV) modes. The disclosed RUL estimation technique considers the inter-dependency of SOC-SOH and influence of internal-external parameters/factors during the coupled estimation of SOC-SOH. The coupled estimation of SOC and SOH is based on a reduced order physics based modelling technique and considers the influence real time environment obtained using real-time dynamic data obtained by several sensors during the coupled estimation of SOC-SOH.