  Traditional equivalent circuit models (ECMs) have difficulties in estimating
battery internal states due to the lack of relevant physics, such as the
lithium diffusion in active particles. Here we configure a circuit network to
describe the lithium diffusion and define it as a new high-level circuit
element called diffusion-aware voltage source. The circuit representation is
proven equivalent to the discretized diffusion equation. The new voltage source
gives the electrode potential as a function of the surface concentration and
thus automatically incorporates the diffusion overpotential. We show that an
ECM with the proposed diffusion-aware voltage sources (called "shell ECM") can
reproduce the single particle model simulation results, making it a trustworthy
easy-to-implement substitute. Furthermore, the simplest shell ECM consisting of
a single diffusion-aware voltage source and a resistor is validated against
experimental constant-current discharges at various rates. The diffusion-aware
voltage source can be used to measure diffusivity by fitting the diffusion
resistance against experimental data. The viability of the shell ECM for
onboard usage is confirmed by implementation into a battery management system
of WAE Technologies. By tracking the internal concentration states, the shell
ECM demonstrates robustness to dynamic applied-current profiles.
