Stationary battery — capacity test and internal-resistance (ohmic) test (IEEE 1188)

The guide on stationary battery-room ventilation covers the hydrogen hazard a charging lead-acid battery bank can create and the resulting ventilation requirements. This article covers a different, equally practical question: how do you establish whether a stationary battery bank — for example for a UPS or an emergency power supply — can still deliver its rated autonomy (back-up time) during an actual power outage? IEEE 1188 ("Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid Batteries for Stationary Applications") describes two complementary test methods for exactly that.

Two methods, two purposes

• Capacity (load) test: a full discharge of the bank at the rated discharge current, down to the specified end-voltage per cell or string, comparing the actual delivered ampere-hours (or back-up time) with the manufacturer's nameplate capacity. This is the definitive test — it directly measures what the bank can really deliver — but it takes the bank temporarily out of service and loads the cells heavily, which is why IEEE 1188 recommends it only periodically (in practice typically once every one to two years), not more often.
• Internal-resistance (ohmic) test: a quick, non-loading measurement of internal resistance (or impedance/conductance) per cell or per string, using an applied AC or DC pulse. This test can be performed much more frequently (IEEE 1188 cites a quarterly interval as common practice) because the bank is not discharged and therefore not additionally stressed.

Baseline and trending

A single ohmic reading on its own says little — the absolute value depends on the cell type, the capacity, and even the measuring instrument used. The reading only becomes valuable once a baseline is established at commissioning for each cell, and then periodically compared: a cell whose internal resistance has risen significantly relative to that baseline — or relative to the average of the other cells in the same string — points to beginning degradation and warrants closer investigation or replacement.

Replacement criteria

In practice, IEEE 1188 applies two, independently valid, thresholds to consider a cell or bank due for replacement:

1. Capacity below about 80% of the manufacturer's nameplate rating, established via a full capacity test.
2. An internal-resistance deviation of more than about 20% relative to the recorded baseline (or relative to the group average), established via the ohmic test.

Either criterion on its own is already sufficient reason for replacement; together they leave little doubt about a cell's condition.

Note: an ohmic measurement correlates mainly with the condition of the connections, cell terminals, and the surface layer of the plates — not directly and linearly with the remaining ampere-hour capacity deep within the active plate material. A cell can therefore show an apparently "normal" internal resistance while its actual capacity has already dropped due to a different degradation mechanism (for example sulfation or dry-out in VRLA cells). This is exactly why the periodic, full capacity test — despite more frequent ohmic screening — remains necessary.

Practical relevance

For the maintenance programme of a stationary battery bank (UPS, emergency lighting, generator starter batteries), it is important to combine both test methods: the ohmic test as a frequent, non-loading screening tool to flag deviating cells early, and the full capacity test as a periodic, definitive check that catches degradation mechanisms an ohmic test can miss. See also the guide on UPS battery types (VRLA/lithium) for the properties of the cell type itself, and the guide on periodic UPS testing for the broader periodic UPS inspection this battery test forms part of.

Common mistakes

1. Relying exclusively on ohmic/internal-resistance measurements and skipping the periodic, full capacity test — this leaves degradation mechanisms the ohmic test cannot detect unnoticed.
2. Not establishing a proper per-cell baseline at commissioning, which renders later percentage-deviation comparisons meaningless.
3. Comparing ohmic readings from different instruments or methods — absolute values are instrument- and method-specific; always compare against the baseline from the same instrument.
4. Performing a capacity test without first fully recharging or equalising the bank, producing an artificially low, misleading result that can lead to unnecessary battery replacement.

Related

• Stationary battery room — ventilation (hydrogen hazard)
• UPS battery types — VRLA/lithium
• UPS — periodic testing, load test, and leakage current
• UPS emergency-power continuity