- 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.
- Equalize charge
- Duration: 24 hours
- Setpoint: 2.40 V/cell (≈ 14.4 V/unit for 12 V blocks with 6 cells)
- Float charge
- Duration: 3–7 days
- Setpoint: 2.30 V/cell (≈ 13.8 V/unit for 12 V blocks)
- 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.
- 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.
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).
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.
Battery capacity is temperature-dependent. Ratings are usually given at 25 °C (77 °F). If you test at a different temperature, adjust the planned discharge rate using the manufacturer’s temperature correction factors.
- Datasheet current for 1 h at 1.75 V/cell, 25 °C: ( I_{25} = 61.5 \text{ A} )
- Actual test temperature: 15.6 °C (60 °F)
- Correction factor (example): 0.93 at 60 °F
Adjusted one-hour test current:
[
I_{\text{test}} = I_{25} \times 0.93 = 61.5 \times 0.93 = 57.2 \text{ A}
]
Rule of thumb: Colder → less capacity (reduce test load), hotter → more capacity (increase test load), but always use the table from the battery’s datasheet.
For a constant-current test at the rated hour-rate:
[
\text{Rated Capacity (%)} = 100 \times \frac{t_{\text{actual}}}{t_{\text{specified at }25^\circ\text{C}}}
]
Where:
- ( t_{\text{actual}} ) = discharge time you achieved to EOD at the corrected load
- ( t_{\text{specified at }25^\circ\text{C}} ) = datasheet duration at 25 °C
If you sum ampere-seconds or ampere-hours during the run (e.g., via a battery monitor):
[
\text{Rated Capacity (%)} = 100 \times \frac{\text{Ah}\text{actual}}{\text{Ah}\text{rated at 25^\circ\text{C}}}
]
Keep the discharge mode consistent with the rating you’re testing (constant current vs constant power). Don’t mix.
- 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
- 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.
- Confirm ambient temperature and apply the temperature-corrected load setpoint.
- Start the timer and begin discharge.
- Log string voltage, current/power, and individual unit voltages at regular intervals (e.g., every 5–10 minutes for short tests, 30–60 minutes for long tests).
- Stop the test the moment the string reaches EOD under load (do not “coast” after EOD).
- Record the final time and immediately remove the load.
Safety: VRLA can heat up near end of discharge. Watch for abnormal unit voltages, temperature rise, or loose connections.
- 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.
- 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
- Battery: 12 V VRLA (6 cells), rated 100 Ah @ 20 h to 1.75 V/cell at 25 °C
- Test ambient: 20 °C; correction factor from datasheet table: 0.97
- Rated 20 h current at 25 °C: ( I_{25} = 5.0 \text{ A} )
Adjusted current:
[
I_{\text{test}} = 5.0 \times 0.97 = 4.85 \text{ A}
]
Discharge result: Reached EOD in 17.8 h.
Capacity:
[
\text{Rated Capacity (%)} = 100 \times \frac{17.8}{20.0} = 89%
]
Interpretation: At 89%, the string is below nominal but above a typical 80% replacement trigger—monitor trend or plan maintenance.
- 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.
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.
