Battery Health Testing with Impedance & Conductance: A Clear, Practical Guide

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.

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

Observation	Likely Meaning	Typical Next Step
Impedance trending up / Conductance trending down	General aging: grid corrosion, sulfation, dry-out (VRLA)	Increase monitoring frequency; plan replacement window
Sudden step change on one cell	Loose/contaminated connection, measurement error, or localized fault	Re-measure, inspect terminals/straps, re-torque, clean, retest
All cells drift together	Ambient temperature shift or system float setpoint issue	Verify temperature, charger settings, ventilation
Outlier stays abnormal after recheck	Cell-specific defect or accelerated aging	Schedule targeted replacement

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.