Battery phase meter to determine internal temperatures of lithium-ion rechargeable cells under charge and discharge

Methods and systems to determine an internal temperature of a rechargeable lithium-ion cell based on a phase shift of the cell. Internal cell temperature may be determined with respect to an internal anode temperature and/or an internal cathode temperature. Internal anode temperature may be determined based on a phase shift of a frequency within a range of approximately 40 Hertz (Hz) to 500 Hz. Internal cathode temperature may be determined based on a phase shift of a frequency of up to approximately 30 Hz. A temperature sensor as disclosed herein may be powered by a monitored cell with relatively little impact on cell charge, may be electrically coupled to cell but housed physically separate from the cell, and/or may monitor multiple cells in a multiplex fashion. A rate of change in phase shift may be used to initiate pre-emptive action, without determining corresponding temperatures.