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Why Test Battery Capacity?
A battery's nameplate capacity is measured at the factory under ideal conditions — typically at 25°C, discharged at a C/10 rate to a defined endpoint voltage. In the field, the actual usable capacity depends on temperature, age, charge history, and the actual discharge rate. Without periodic capacity testing, a system owner may believe they have 200 Ah of storage when the actual usable capacity has fallen to 120 Ah — an invisible 40% performance loss.
Standard Capacity Test Procedure (AS 4086.2)
Pre-condition: Fully charge the battery using the normal charge profile. Allow to stand at rest for 1 hour to stabilise.
Discharge: Apply a constant current load equal to C/10 (i.e., for a 200 Ah battery, apply a 20 A load).
Record: Note start voltage, time, and temperature. Sample voltage every 30 minutes.
End condition: Stop discharge when terminal voltage reaches the manufacturer's specified cut-off voltage (typically 10.5 V for a 12 V battery).
Calculate: Capacity (Ah) = discharge current × discharge duration (hours). State of Health (SoH) % = (measured capacity / rated capacity) × 100.
Interpreting Test Results
State of Health | Meaning | Action |
|---|---|---|
90–100% | New or near-new condition | No action required |
75–90% | Normal ageing | Monitor annually; adjust autonomy calculations |
60–75% | Moderate ageing — noticeable performance loss | Plan for replacement within 2–3 years; verify system still meets autonomy requirements |
Below 60% | End of useful life for most applications | Replace immediately or accept severely reduced autonomy |
LiFePO4 Capacity Testing
For lithium batteries with a BMS, State of Health can often be read directly from the BMS via its software interface. The BMS tracks cumulative Ah throughput and compares measured cell capacity to rated values. However, a direct physical capacity test remains the most accurate verification method, particularly for warranty claims or system sale purposes.
Engr. Jason Morales — Founder, SolarEnergyPH
