As batteries become a major component of numerous advanced vehicles, significant efforts have been allocated towards characterizing and estimating battery energy capability over the lifetime of a vehicle. Currently, battery State of Charge (SOC) is one of the primary values used for this characterization; however SOC usage has several issues when implemented in Electric Vehicle (EV), Hybrid Electric Vehicle (HEV), and Plug-In Hybrid Electric Vehicle (PHEV) systems. One of the main issues with reporting battery SOC as a characterization of battery energy capability is that it only gives a percentage of the energy available to the operator. SOC does not accurately represent the true capability or capacity of the battery, and thus fails to account for the impact to capability with respect to battery size, age, and recent operational history. These inaccuracies of SOC characterization are magnified as it is incorporated into the control systems of EVs, HEVs, and PHEVs, which are typically configured to process only characterizations of the “rechargeable” Energy Storage System (ESS) in units of energy. As such, significant attention has been given of late to calculations and algorithms related to the assessment of the energy remaining in an ESS . In this document, these techniques are referred to as State of Energy (SOE) estimation. This is because this term is directly tied to energy efficiency. This paper seeks to identify issues and opportunities in the estimation of battery energy for EVs, HEVs, and PHEVs.