Capacity Testing of VRLA Batteries: A Clear, Practical Guide

1) Prepare the Battery Bank

1.1 Assemble and baseline

• Assemble the battery at its installation site with the final inter-cell links, cables, and torque per manufacturer spec.

• Record open-circuit voltage (OCV) for every unit before any charging.

• Ensure interconnects and cable gauges match the intended operating configuration.

1.2 Charge protocol before testing

1. Equalize charge

   • Duration: 24 hours

   • Setpoint: 2.40 V/cell (≈ 14.4 V/unit for 12 V blocks with 6 cells)

2. Float charge

   • Duration: 3–7 days

   • Setpoint: 2.25 - 2.30 V/cell (13.50 - 13.80 V/unit for 12 V blocks)

3. Log voltages

   • Measure and record each unit’s voltage during both equalize and float. Look for outliers.

Tip: Stabilized, fully floated batteries yield more reliable capacity numbers and reduce scatter test-to-test.

2) Define Test Settings (Load, Time, and End Voltage)

2.1 Choose the test type

• Constant Current (A): Hold current constant; commonly used for telecom DC plants.

• Constant Power (W/cell or W/unit): Hold power constant; typical for UPS where bus voltage varies during discharge.

2.2 Set the End-of-Discharge Voltage (EOD)

Use the manufacturer’s recommended EOD per cell. Common values for VRLA are 1.75–1.80 V/cell. For 12 V blocks (6 cells), multiply by 6 (e.g., 1.75 V/cell → 10.5 V per 12 V unit).

2.3 Pick the rated duration

Choose the rated time from the datasheet (e.g., 1 h, 3 h, 10 h, 20 h). Your target load comes from the table for that duration and EOD.

3) Test Setup & Instrumentation

3.1 Equipment checklist

• Load bank sized for the required current or power at the system voltage

• DC ammeter (for current) and digital voltmeter (for each unit and string total)

• Timer (or data logger)

• Battery cell monitoring system (optional but recommended)

• Torque tools per hardware spec

3.2 Wiring and verification

• Connect the load bank across the full string (e.g., 48 VDC systems with hundreds of amps may require 10 kW+ loads).

• Verify polarity, shunt orientation (if used), and that metering reads correctly at a small pilot load before the real run.

4) Interpret and Report Results

4.1 Pass/fail guidance

• Many standards accept strings ≥ 80% of rated capacity as “serviceable,” but follow your organization’s threshold.

• If the string fails:

  • Identify weak units (lowest per-unit voltages under load, abnormal IR/conductance if measured).

  • Replace weak units and retest after the full charge protocol.

4.2 Minimum report contents

• Battery model, count, configuration, installation date

• Charge history before test (equalize/float durations and setpoints)

• Temperature and correction factor used

• Test mode (constant A or constant W), EOD, rated duration

• Measured time to EOD and calculated Rated Capacity (%)

• Per-unit voltage logs (at intervals)

• Observations, anomalies, corrective actions

5) Practical Tips & Common Pitfalls

• Do not skip float stabilization (3–7 days). Under-charged strings under-perform.

• Hold the setpoint steady. For constant-current tests, keep current within ±1–2%.

• Measure individual units. Weak blocks often hide behind “OK” string voltage.

• Apply the correct EOD. A higher/lower EOD can inflate/deflate measured capacity.

• Use the right correction table. Use the manufacturer’s factors for your exact model.

• Document everything. Good logs make trend analysis and warranty claims easier.

Conclusion

A good capacity test is mostly good preparation: proper charge, correct temperature correction, the right EOD, and disciplined logging. With those in place, the math is simple—and the result is a trustworthy Rated Capacity (%) you can compare across years to plan maintenance and replacements.