Modern EV battery packs controllers are no longer just voltage / current measurement systems. A new feature being integrated into advanced BMS is in-situ EIS (Electrochemical Impedance Spectroscopy) of each cell. This allows the BMS to probe the internal impedance spectrum of every cell (resistive + reactive components), giving richer insight into each cell’s health, temperature, aging, and allowing more precise range or safety estimation.

However, validating this EIS function is technically challenging if you rely solely on real battery packs. That’s where simulation equipment becomes essential. With it, you can flexibly emulate the EIS responses per cell, test edge cases, and validate the BMS algorithms under ideal conditions.

 

In the sections below:

1. We explain the EIS theory (with diagrams).

2. We show how the EIS is applied at different scales (cell, module, pack).

3. We highlight phase shift and impedance concepts.

4. We explain how your simulator works, and why it’s uniquely useful.

 

1. Basics of EIS: Excitation, Response & Impedance

When doing EIS, you apply a small or large AC (alternating current) perturbation superimposed on the DC operating point, across a frequency sweep, and observe the voltage/current response of the whole pack or each individual cell. Because of the internal behavior of the cell (electrochemical reaction, diffusion, double-layer), the response is offset in amplitude and phase relative to the excitation.

· If you excite with a sinusoidal current, the cell’s response voltage will follow the same frequency, but the phase difference and amplitude attenuation depend on the impedance of internal processes.

· Impedance Z is a complex quantity: Z = Z’ + j Z’’. The real part (Z’) is resistive, the imaginary (Z’’) is reactive (capacitive or inductive behavior).

· As frequency varies, different internal processes dominate: at high frequency, you might see mostly ohmic + double-layer effects; at lower frequency diffusion or kinetics effects emerge.

You can plot results in Nyquist plots (imaginary vs real) or Bode plots (magnitude, phase vs frequency).

Above are example EIS plots and equivalent circuits (source: general EIS references) to help visualize how amplitude & phase shift evolve with frequency. 

Also consult standard references:

· Gamry’s “Basics of Electrochemical Impedance Spectroscopy” for how impedance of resistors, capacitors, inductors combine in series/parallel 

· BatteryDesign’s EIS overview 

· Concise definition of EIS in Li-ion systems 

These sources confirm that what we’re describing is consistent with standard theory.

2. Where to Inject the Excitation: Cell, Module, or Pack Level

In a battery system, the EIS excitation and measurement can be applied at different scales:

· Cell-level: Each cell is excited individually, and its response measured.

· Module-level: A group of cells in series or parallel is excited and measured collectively, which may obscure individual cell behavior.

· Pack-level: The entire pack is excited, and the internal cell responses are inferred or multiplexed.

In many BMS designs, the pack-level injection is preferred for minimal additional hardware, but it requires that the BMS or external generator can resolve responses to each cell accurately.

3. Phase Shift, Synchronization & Precision

One of the trickiest parts is phase shift — that is, the response of each cell is not exactly in-phase with the excitation current. The BMS detects this phase delay to derive reactive components of impedance. Small errors in phase (e.g. 0.1°) or amplitude (e.g. 0.1 mV) can significantly affect the calculated impedance.

 In our system, we simulate:

· A reference excitation signal that the BMS sees (so BMS knows when the excitation occurs).

· Synchronized per-cell voltage responses, with configurable phase offset and amplitude.

· Mixed DC + AC signal: the baseline DC voltage plus a small AC ripple on top.

 Because the BMS algorithm may measure amplitude ratio and phase angle versus excitation, your simulator must maintain very tight synchronization across all channels.

4. How Our EIS Simulation Equipment Works

Our solution is to provide a hardware + software system that simulates EIS behavior per cell under the BMS control of the excitation signal. Key attributes:

· Supports up to 256 channels via cascading modules

· Each channel can output DC + AC (small ripple) voltage, with precise amplitude & phase

· Frequencies from <1 Hz to 1 kHz or more

· Phase shift resolution: <0.1°, amplitude resolution: <0.1 mV (for AC); DC precision better (e.g. 1 mV)

· Tight synchronization across all channels

· Controlled and configured via Ethernet, USB or EtherCAT so the test software can command the simulator

· With integrated CAN and CANFD bus I/O’s the BMS can communicate with the simulator

· Support for trigger handshake: the simulator receives a digital or CAN-based trigger and outputs the excitation reference plus cell responses

· Ability to script test sequences: different frequency, amplitude, and phase profiles over time

· Real-time interfaces to MATLAB / real-time controllers to adjust simulation on the fly based on cell simulation models and create closed loops with the BMS

With this EIS simulation equipment, you can do open-loop EIS testing (preset sequences) or closed-loop (where the BMS sends back results and the simulator adjusts). Also, you can test degraded or edge conditions easily — which is impossible or very slow with real battery packs.

5. Why This Simulator Is Crucial for EIS-BMS Validation