Abstract: Methods, devices, and systems for extending a useful life and/or an operable temperature range of a battery unit are provided. Specifically, a device is provided that includes a battery unit and a power management controller. The battery unit comprises a rechargeable lithium ion battery and a supercapacitor. The power management controller receives information relating to a measured or expected performance parameter of the rechargeable lithium ion battery, and, based on the information relating to the measured or expected performance parameter, routes received electrical power to the lithium ion battery and/or the supercapacitor, and/or provides electrical power from the lithium ion battery and/or the supercapacitor to a component of a computer system associated with the battery unit.