  Modeling of Li-ion cells is used in battery management systems (BMS) to
determine key states such as state-of-charge (SoC), state-of-health (SoH), etc.
Accurate models are also useful in developing a cell-level digital-twin that
can be used for protection and diagnostics in the BMS. In this paper, a
low-complexity model development is proposed based on the equivalent circuit
model (ECM) of the Li-ion cells. The proposed approach uses online impedance
measurement at discrete frequencies to derive the ECM that matches closely with
the results from the electro-impedance spectroscopy (EIS). The proposed method
is suitable to be implemented in a microcontroller with low-computational
power, typically used in BMS. Practical design guidelines are proposed to
ensure fast and accurate model development. Using the proposed method to
enhance the functions of a typical automotive BMS is described. Experimental
validation is performed using large prismatic cells and small-capacity
cylindrical cells. Root-mean-square error (RMSE) of less than 3\% is observed
for a wide variation of operating conditions.
