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AS 4086.2-1997
What Is Equalisation and Why Does Lead-Acid Need It?
In a lead-acid battery bank, individual cells age at different rates. Some cells have slightly higher internal resistance, accept less charge, and discharge more quickly. Over time, this imbalance causes the bank to perform as the worst cell — not the average. Equalisation charging is a periodic, deliberate overcharge that brings all cells to full charge simultaneously, gassing out stratification and reversing mild sulphation.
Equalisation is a maintenance requirement for flooded lead-acid batteries. It is not applicable to AGM or VRLA batteries (overcharge destroys sealed cells by forcing water loss through the safety valves) and not applicable to lithium batteries (cell balancing in lithium is handled by the BMS passively, not by overcharge).
Equalisation Procedure (AS 4086.2 Guidance)
Fully charge the battery bank using the normal bulk and absorption charge stages first
Increase the charge voltage to the equalisation voltage — typically 15.5–16.0 V for a 12 V nominal bank (2.58–2.67 V per cell)
Maintain equalisation voltage for 2–4 hours, monitoring cell temperatures and ensuring no cell exceeds 45°C
During equalisation, check specific gravity of each cell with a hydrometer — cells should converge to within 0.010 specific gravity of each other by the end of equalisation
After equalisation, reduce to float voltage and allow the battery to rest before returning to normal service
Equalisation Frequency Recommendations
Operating Condition | Recommended Frequency |
|---|---|
Normal cycling (full charge most days) | Every 1–3 months |
Partial state of charge operation (common in off-grid) | Every 2–4 weeks |
Battery bank showing cell imbalance >0.030 SG | Immediately — then weekly until balanced |
After battery has been in storage (discharged state) for >1 month | Before returning to service |
Engr. Jason Morales — Founder, SolarEnergyPH