ecm-fitting

ECM Parameterization

Fit an Equivalent Circuit Model (R0 + N RC pairs, plus optional OCV) to cycling data and persist the result as a Parameterized Model.

When to use

• Build a fast surrogate from raw cycling data.
• Need an OCV curve plus R0 and RC dynamics from a pulse / drive cycle test.
• Save a fitted ECM to a project so it can be used in simulations.

Operation type

Authenticated fits run as background jobs on the platform's worker fleet (typically 10–60 s). Each fit_* call submits a job and returns a handle immediately; use wait_for_completion to block until the fit is ready.

The unauthenticated demo endpoint (fit_from_example) is synchronous and rate-limited.

Three input modes

Method	Source	Auth	Returns
client.ecm.fit_from_example(...)	Built-in demo dataset	optional	FitResults (sync)
client.ecm.fit_from_file(...)	Local file upload	required	EcmFitJob (async)
client.ecm.fit_from_measurements(...)	Measurements stored in the platform	required	EcmFitJob (async)

The active organization is resolved from the API key — no organization_id needs to be passed.

Quick start: fit from measurements

from ionworks import Ionworks

client = Ionworks()

# 1. Submit the fit (returns immediately)
fit_job = client.ecm.fit_from_measurements({
    "measurements": [
        # Each entry can carry per-measurement step + initial_soc filters.
        {"id": "meas-id-1"},
        {"id": "meas-id-2", "start_step": 5, "end_step": 50, "initial_soc": 0.95},
    ],
    "ecm_options": {
        "num_rcs": 2,             # 0–5 RC pairs (default 2)
        "fit_ocv": True,          # set False if data has an OCV column
        # Optional shared hyperparameters:
        # "capacity": 4.85,
        # "ocv_soc_curve": {"soc": [...], "ocv": [...]},
        # "bounds_capacity": {"lo": 4.0, "hi": 6.0},
        # "num_knots": 21, "num_knots_r0": 7,
        # "knot_schedule": [3, 5, 9, 21],
        # "clamp_boundary_knots": True, "clamp_max_ratio": 10.0,
    },
})

print(fit_job.job_id)

# 2. Wait for the worker to finish
result = client.ecm.wait_for_completion(fit_job, timeout=300)

print(f"RMSE: {result.rmse_mV:.2f} mV")
print(f"OCV provided: {result.ocv_provided}")
print(f"RC pairs: {len(result.rc_pairs)}")

result is a FitResults model with these arrays:

• time, data_voltage, model_voltage — downsampled traces for plotting
• soc, ocv, r0 — 200-point parameter grids over SOC
• rc_pairs[i].r, .c, .tau — per-RC-pair curves over SOC
• rmse_mV, num_rcs, ocv_provided — fit metadata

Fit from a local file

Accepts CSV, parquet, and any cycler format that ionworksdata can detect.

fit_job = client.ecm.fit_from_file(
    "data/pulse_test.csv",
    num_rcs=2,
    fit_ocv=True,
    initial_soc=0.95,   # optional
    capacity=4.85,      # optional, in Ah
)
result = client.ecm.wait_for_completion(fit_job)

You can also pass an open binary stream:

with open("data/pulse_test.parquet", "rb") as f:
    fit_job = client.ecm.fit_from_file(f, num_rcs=3)
result = client.ecm.wait_for_completion(fit_job)

To inspect a file before fitting (auto-detect format, get the time-series back without running the fit):

preview = client.ecm.detect_and_read("data/pulse_test.csv")

Polling manually

wait_for_completion is the recommended path. If you need lower-level access (e.g. driving your own progress UI), poll the job directly:

fit_job = client.ecm.fit_from_measurements(
    {"measurements": [{"id": "m-1"}]}
)

while True:
    job = client.job.get(fit_job.job_id)
    if job.is_terminal:
        break
    time.sleep(2)

if job.is_failed:
    raise RuntimeError(job.error)

from ionworks import FitResults
result = FitResults(**job.result)

Fit from a built-in example (no auth)

examples = client.ecm.list_examples()
example_id = examples[0]["id"]

# Optional: peek at the data
data = client.ecm.get_example_data(example_id)

# This call is synchronous (rate-limited demo endpoint)
result = client.ecm.fit_from_example(example_id, num_rcs=2)

The public /ecm/fit endpoint is rate-limited (60/min). RC-pair parameters are only included for authenticated users whose organization has ECM results access enabled.

Save fit as a Parameterized Model

Once you have a FitResults, persist it inside a project:

saved = client.ecm.save_to_project(
    name="ECM 2RC — pulse test",
    cell_spec_id="cell-spec-id",
    fit_results=result,
    description="Fitted from May 2026 pulse test",
)

print(saved.parameterized_model_id)

The new Parameterized Model can then be used as the parameterized_model in client.simulation.protocol(...).

Choosing num_rcs

RC pairs	When to use
0	Pure R0 + OCV (no transient dynamics)
1	Single time constant — short pulses, simple chemistries
2	Default — captures fast + slow transients (recommended)
3+	Complex relaxation behaviour; risk of overfitting

Fitting capacity from an OCV(SoC) curve

If you have a known OCV curve (e.g. from a separate slow-rate characterisation), pass it via ecm_options.ocv_soc_curve and the fitter will skip OCV fitting altogether — using the curve to interpolate OCV and (optionally) co-optimise capacity:

fit_job = client.ecm.fit_from_measurements({
    "measurements": [{"id": "meas-id-1"}],
    "ecm_options": {
        "num_rcs": 2,
        "ocv_soc_curve": {
            "soc": [0.0, 0.1, 0.2, ..., 1.0],
            "ocv": [3.0, 3.2, 3.4, ..., 4.2],
        },
        # Optional: search bounds for capacity (Ah). Only used when
        # capacity is None — when omitted, capacity defaults are used.
        "bounds_capacity": {"lo": 4.0, "hi": 6.0},
    },
})

Constraints:

• ocv_soc_curve.soc must be strictly increasing and lie in [0, 1].
• ocv_soc_curve.soc and .ocv must have the same length (≥ 2).
• bounds_capacity is only consulted when capacity is None; it requires hi > lo.
• Mutually exclusive with input data carrying an Open-circuit voltage [V] column — pass one or the other.

Validation runs locally in the SDK before the request hits the wire, so mismatched-length arrays or non-monotonic SOC values raise ValueError immediately.

Multi-measurement fits with per-segment SOC seeds

When you concatenate multiple measurements into one fit, each segment can carry its own initial_soc so the fitter starts each segment from the right state of charge — useful when measurements are recorded across different SoC ranges (e.g. one capacity check at full charge plus one discharge profile starting at 80 %):

fit_job = client.ecm.fit_from_measurements({
    "measurements": [
        {"id": "meas-fully-charged",     "initial_soc": 1.00},
        {"id": "meas-partial-discharge", "initial_soc": 0.80},
    ],
    "ecm_options": {"num_rcs": 2, "capacity": 4.85},
})

When initial_soc is omitted on a measurement and an ocv_soc_curve is provided in ecm_options, the service auto-seeds soc0 for that segment by inverting V[s] = OCV(soc0) − I[s]·R0(soc0) after a warm-up fit. Without a curve, single-measurement runs fall back to coulomb-counting.

Tuning knot resolution

For challenging traces (long relaxations, multiple time scales), bump the SOC-knot resolution:

"ecm_options": {
    "num_rcs": 3,
    "num_knots": 21,           # alpha + default beta resolution
    "num_knots_r0": 7,         # SOC knots for R0(SoC)
    "knot_schedule": [3, 5, 9, 21],  # multi-resolution refinement schedule
    # If R_rc(soc)=alpha/beta is built symbolically downstream, disable
    # boundary clamping (it costs fit quality at unobserved boundary knots).
    "clamp_boundary_knots": False,
    # Or relax the clamp ratio to preserve more SoC variation:
    # "clamp_max_ratio": 20.0,
}

knot_schedule must be a strictly increasing list of positive ints ending at num_knots. Defaults are auto-derived when omitted.

When fit_ocv=False

Set fit_ocv=False only if your input data already has an Open-circuit voltage [V] column (e.g. a pre-run pseudo-OCV characterisation). The fitter will use that directly and only optimize R0 + RC parameters.

For raw drive cycles, leave fit_ocv=True (the default).

End-to-end example

from ionworks import Ionworks

client = Ionworks()

# 1. Submit + wait for fit
fit_job = client.ecm.fit_from_measurements({
    "measurements": [{"id": "meas-1"}, {"id": "meas-2"}],
    "ecm_options": {"num_rcs": 2},
})
result = client.ecm.wait_for_completion(fit_job, timeout=300)
print(f"RMSE: {result.rmse_mV:.2f} mV")

# 2. Save as a Parameterized Model
saved = client.ecm.save_to_project(
    name="ECM 2RC — bench A",
    cell_spec_id="cs-123",
    fit_results=result,
)

# 3. Use it in a simulation
sim = client.simulation.protocol({
    "parameterized_model": saved.parameterized_model_id,
    "protocol_experiment": {
        "protocol": "...",  # UCP YAML
        "name": "1C discharge check",
    },
})
client.simulation.wait_for_completion(sim.simulation_id)

Error handling

from ionworks import IonworksError

try:
    fit_job = client.ecm.fit_from_measurements(
        {"measurements": [{"id": "bad-id"}]}
    )
    result = client.ecm.wait_for_completion(fit_job, timeout=60)
except ValueError as e:
    # Local validation failure (bad config shape)
    print(f"Config error: {e}")
except TimeoutError:
    print("Fit did not finish in time")
except IonworksError as e:
    # Server-side error (missing columns, no rows, fit failure, auth, etc.)
    print(f"API error ({e.status_code}): {e}")

Pass raise_on_failure=False to wait_for_completion if you'd rather get None back than an exception on failure — then fetch the job directly via client.job.get(job_id) to inspect the error.