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Capacity Testing vs Ohmic Readings for VRLA Batteries

Oct 21
4 min read

Updated: Oct 22

UPS and standby batteries can be checked in two ways: a capacity (discharge) test, and internal ohmic readings such as impedance, conductance or resistance. This guide sets out what two valve-regulated lead-acid (VRLA) battery makers, C&D Technologies and EnerSys, say each test can and can't tell you.

What a capacity test tells you

  • It's the true measure. EnerSys calls capacity discharge testing "the only true guide to state of health" (EnerSys PowerSafe SBS XL manual, §6). C&D says discharge testing to a published discharge rate and duration "will give a true measure of the battery's state of health" (C&D 41-6965, p. 4).

  • Start fully charged. EnerSys says to test only a fully charged battery, and a new battery only after its commissioning charge. It says to always complete a full discharge test, because partial discharges can lead to a false assessment of state of health (EnerSys SBS XL manual, §6).

  • Base the test on published data. EnerSys says the load and end-of-discharge voltage should be based on published performance literature, a temperature correction factor may be needed, and individual bloc or cell voltages should be logged at regular intervals during the test (EnerSys SBS XL manual, §6). C&D says the test load current should be derated for testing temperatures below 75 °F (C&D 41-6965, pp. 3–4).

  • Recharge afterward. EnerSys says to fully recharge the battery after a capacity test (EnerSys SBS XL manual, §6).

  • Follow the test protocol your maker names. For its PowerSafe DDmP125-33 LP systems, EnerSys refers to the latest version of IEEE 1188 for capacity test protocol (EnerSys DDmP125-33 LP manual, §19.2.1, p. 34).

What ohmic readings tell you

  • What they are. C&D describes internal ohmic testing as measuring internal resistance, conductance, impedance or admittance to judge the condition of a battery's internal components. It calls this "a trending and troubleshooting tool". AC test methods measure without discharging the battery and with little if any interruption in service (C&D 41-7271, pp. 3–4).

  • Why they move. C&D says grid corrosion, shedding of active material, strap corrosion or drying of the electrolyte all show up as an increase in a battery's resistance. As cells age, average impedance gradually rises (C&D 41-7271, pp. 2, 4).

  • What they can't do. C&D says neither impedance nor conductance testing can replace the actual performance capacity test, because neither gives a clear indication of every problem that can make a battery fail. They can alert you that some cells need a closer look (C&D 41-7271, p. 5). EnerSys says capacity testing can be complemented by trending ohmic measurements, and that the correct way to use ohmic readings is as a trending tool over time (EnerSys SBS XL manual, §6).

  • Some failures only show up this way. For its DDmP125-33 LP series, EnerSys notes that some internal failure modes can't be detected by cell or unit voltage measurements. It says IEEE 1188 recommends a quarterly internal ohmic measurement, compared with a baseline or average value (EnerSys DDmP125-33 LP manual, §19.3, p. 35).

Readings depend on the meter and the temperature

  • Compare like with like. C&D says impedance depends on test frequency, so different test equipment gives somewhat different results (C&D 41-7271, p. 3). Its ohmic reference sheet says its values are approximate, may vary up to ±20%, are for reference only, and can't be used for a warranty claim. It says initial readings taken at installation should be used as the reference (C&D Ohmic Reference Values, Current VRLA Products).

  • Record the temperature. C&D says to measure and record battery temperature with every impedance or conductance reading, because cooler temperatures greatly increase impedance. Values taken over time must be normalized for temperature to be comparable (C&D 41-7271, p. 8).

  • Take readings at the post. C&D says to take ohmic readings directly from the battery post whenever possible (C&D 41-6965, p. 4).

Set a baseline, then trend it

The makers agree on the method but differ on timing:

Document

When to set the baseline

When to look further

C&D 41-6965 (C&D High Rate Max, Pure Lead Plus and Pure Lead Max VRLA UPS batteries)

After 3 months on float charge, then trend each unit over time

Not stated in this bulletin

C&D 41-7271 (C&D VRLA products)

Compares against the value when new

If impedance or conductance varies more than 50% from the value when new, evaluate further, including a performance capacity test

EnerSys SBS XL manual

Take "initial" readings at installation, then "baseline" readings after about 6 months in service

Yearly readings; if a reading varies more than 50% from baseline, evaluate further, including a performance or capacity test

Use the method in your own battery's manual. If you test with a CELLTRON conductance tester, see How to Test UPS Battery Strings with a CELLTRON Conductance Tester for how Franklin Grid Solutions says to set reference values.

Quick checks that aren't a capacity test

C&D lists open-circuit voltage, on-charge current acceptance, on-charge float voltage and momentary load tests as integrity tests. It says these tests "do not determine or assure the battery's actual capacity", but they may indicate a need for replacement or for a complete performance capacity test (C&D 41-7264, p. 1).

Keep records

EnerSys says a complete recorded history of the battery's operation is essential, and that good records show when corrective action may be needed (EnerSys DDmP125-33 LP manual, §19.3, p. 34). For the wider maintenance picture, see UPS and Standby Battery Testing and Maintenance and When to Replace a UPS Battery.

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Sources (checked 2026-10-21)

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