Systems and methods employ an external energy storage device (e.g., intelligent battery pack) interposed between a an information handling system (IHS) and its AC-to-DC adapter. The information handling system may be a fixed compute system, may lack an internal battery and/or may execute an operating system that does not have power management capability. The external energy storage device provides power and power state information to the IHS, e.g., that is based on state of charge (SOC) of the battery and/or absence/presence of AC power. IHS may perform one or more actions based on the power state information such as automatically save data and shutdown when notified AC power is lost and/or SOC is low, and may automatically restart and restore saved data when notified AC power is restored and/or SOC is sufficiently high. The external energy storage device may actively signal or command the IHS to act or may passively supply power state information and let the IHS decide how to act. The information may be provided as different power supply identifier (PSID) values or other types of analog values corresponding to different current levels on a single conductor of a single cable between the external energy storage device and IHS, or may be included in USB vendor-defined messages sent on the single cable.