What These Tests Measure

Impedance (Ω)

• What it is: The battery’s internal opposition to AC current at a specific test frequency.
• What it tells you: As batteries age (e.g., corrosion, sulfation, dry-out), internal impedance typically rises.

Conductance (Siemens)

• What it is: The reciprocal of impedance—how easily the battery passes AC current.
• What it tells you: As batteries age, conductance typically falls and closely tracks usable capacity.

In field practice, new batteries can vary by about ±20% in their measured values. Because of this, trends over time are far more meaningful than comparing one battery’s absolute value to another’s on day one.

Why Baseline Matters More Than Absolute Numbers

Fresh batteries—even of the same make and model—often show noticeable spread in measured impedance/conductance. Using a single “good/bad” threshold on day one can be misleading. Instead:

• Right after commissioning, record each cell’s initial impedance or conductance value as the baseline.
• Trend each cell against its own baseline over months and years.
• Act on change, not just absolute values. A significant deviation from the starting value is a stronger signal of deterioration than a one-time reading.

The Link Between Conductance and Capacity

Numerous field studies have shown a strong correlation between conductance and discharge capacity. Practically, that means:

• Falling conductance generally indicates reduced capacity.
• For routine maintenance, conductance testing can serve as a reliable proxy for capacity trends, avoiding frequent time-consuming discharge tests.

Bottom line: Conductance trending lets you infer capacity changes without repeatedly performing full discharge tests.

Recommended Test Workflow

1) Establish the Baseline (Immediately After Installation)

• Measure and record every cell.
• Note temperature, state of charge, float voltage, and test instrument/model.
• Use consistent test points and probe pressure for repeatability.

2) Trend Consistently

• Test on a regular schedule (e.g., quarterly or semi-annually, aligned with site criticality).
• Keep conditions consistent (temperature, float state).
• Log results per cell in a table plus a simple trend chart.

3) Flag and Investigate Deviations

• Watch for rising impedance or falling conductance relative to baseline.
• Prioritize cells that deviate more than their peers.
• Confirm with complementary checks (float voltage, visual inspection, temperature, connection torque).

4) Decide on Maintenance Actions

• Re-torque or clean intercell connections if a few readings look anomalous.
• Equalize/condition per manufacturer guidance if sulfation is suspected.
• Replace individual cells that show persistent, significant deviation and/or fail corroborating checks.

Interpreting Results: Common Patterns

Instruments You’ll Encounter

• Impedance testers (e.g., classic Biddle type): inject AC and report internal resistance/impedance.
• Conductance analyzers (e.g., handheld analyzers widely used in the field): report conductance and often a derived “health” indicator.

Either approach works—just be consistent with the instrument and method you use over time so your trends remain apples-to-apples.

Practical Tips for Reliable Data

• Measure at float (fully charged, steady conditions).
• Control temperature where possible; note it when you can’t.
• Use the same test points and consistent probe pressure to reduce contact variability.
• Document everything (cell ID, date/time, instrument, operator notes).
• Visuals help: a simple line chart per string makes outliers obvious.

When to Do a Full Capacity Test

Impedance/conductance trending handles routine health checks well. Still consider a controlled discharge test when:

• Commissioning critical systems,
• After major events (deep discharge, heat incident),
• Before end-of-life planning or large-scale replacements,
• When trend data shows persistent anomalies you need to verify.

Conclusion

Impedance and conductance testing, used as a trending tool, offers fast, dependable insight into battery health. Capture per-cell baselines at installation, trend consistently, and investigate deviations. This approach reduces surprises, minimizes unnecessary full discharge tests, and helps you replace the right cell at the right time.