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Stationary battery internal resistance trend

Paste the internal resistance, impedance or conductance readings of each cell to see which ones depart from baseline, which need replacing and how the string is aging.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Battery string

Changing technology resets the limits to its defaults. For conductance, limits apply to the equivalent resistance (1 ÷ conductance): a 50% resistance increase equals a 33% conductance drop.

Readings by cell
See what was read

Battery strings carry hazardous DC voltage, very high short-circuit current and release hydrogen. Measure with insulated gloves and tools and no ignition sources, per NFPA 70E Article 320 and the manufacturer manual.

—Cells beyond limits
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Each cell deviation from reference

Worsening in the latest reading (resistance or impedance increase, or conductance drop), with the warning and replacement limits.

Tap the chart to see the values.

Trend across readings

String average and critical cells at each reading, compared with the average reference.

Tap the chart to see the values.

Cells furthest from reference

In order of worsening, with deviation from the reference and from the present string average.

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Method, assumptions and references

Equations used

Deviation from reference: resistance or impedance d = R_latest ÷ R_ref − 1; conductance (equivalent resistance) d = G_ref ÷ G_latest − 1
Status: d ≥ replacement limit → replace or capacity-test; d ≥ warning limit → watch; equivalent conductance drop = 1 − 1 ÷ (1 + d)
Deviation from present average: R ÷ mean(R) − 1; spread = standard deviation ÷ mean; trend = latest reading mean ÷ first reading mean − 1

Assumptions and limits

  • Default limits: VRLA warning 20% and replacement 50%; vented 50% and 100%. They follow test-instrument maker guidance (20% to 30% screening for VRLA, 50% to 100% for flooded) and EPRI data in which cells with impedance 50% above nominal mostly had less than 80% capacity. Confirm with the battery manufacturer.
  • Ohmic testing is comparative and does not replace a capacity test. Compare only readings from the same instrument, at the same points and with the string in the same state (float, similar temperature).

Standards and technical documents

  • IEEE Std 1188-2005 (R2010): Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications
  • IEEE Std 450-2010: Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications
  • IEEE Std 1491-2012: Guide for Selection and Use of Battery Monitoring Equipment in Stationary Applications
  • IEC 60896-21 e IEC 60896-22: Stationary lead-acid batteries — Valve regulated types
  • EPRI. Stationary Battery Guide: Design, Application, and Maintenance (TR-100248)

U.S. regulations

  • NFPA 70E Article 320: safety requirements related to batteries and battery rooms
  • NFPA 70 (NEC) Article 480: storage batteries
  • OSHA 29 CFR 1910.305(j)(7) and 1910.151(c): battery installations, ventilation and eyewash

Frequently asked questions

Why measure battery internal resistance?

To find weak cells before they fail in an outage, without discharging the string. Internal resistance rises as a cell loses capacity, and in a series string the weakest cell limits everyone’s run time. The calculator compares each cell with its baseline and with the others, and shows the trend across readings.

Where do I get baseline values?

Ideally the commissioning reading, taken once the string has stabilized on float; IEEE 1188 recommends establishing a baseline. Without it, use the manufacturer value for the model, measured with the same kind of instrument, or the string average in the present reading, which highlights cells that stand out.

How do I read the example result?

The example string has 16 12 V monoblocs with an average baseline of 4.20 mΩ. Cell 7 rose from 4.19 to 6.55 mΩ, 56% above baseline: past the 50% replacement limit. Cell 12 is 24% above, on watch. The others rose about 6% in three years, normal aging.

What increase calls for replacement?

There is no single code number. Tester makers suggest investigating at 20% to 30% for VRLA and 50% to 100% for flooded cells. EPRI data show cells 50% above nominal impedance mostly have less than 80% of capacity, the replacement point in IEEE 1188 and IEEE 450. Confirm with the manufacturer and, when in doubt, run a capacity test.

Do resistance, impedance and conductance give the same answer?

They measure the same thing differently: resistance and impedance in mΩ rise with aging; conductance in siemens (mhos) falls. Values are not interchangeable between instruments. For conductance the calculator converts the limits: a 50% resistance increase equals a 33% conductance drop.

How often should I measure?

IEEE 1188 calls for quarterly internal ohmic readings on VRLA batteries; for vented batteries, at least yearly readings are common practice. Always measure at the same terminal points with the string on float. Cells on watch deserve monthly readings until the trend is clear.

What does the calculator not do?

It does not measure actual capacity, does not replace a discharge test and ignores temperature, float voltage and connections. A sudden jump on a single cell may just be a loose connection: retorque and measure again before replacing.

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