Equalization is a planned overcharge of a flooded lead-acid bank, a few hours long, that brings every cell to the same full charge, mixes acid that has settled in layers and breaks up light sulfation. Do it only on flooded cells, with the room ventilated at the top, and take the lowest voltage the sources support, 15.0 V for a 12 V battery, because the sources that give 15.5 to 16 V exceed the corpus’s own limit of 2.5 V per cell.
Part of Battery Building and Maintenance
Periodic controlled overcharging of a flooded lead-acid bank to balance its cells and mix its electrolyte.
Weak Cells Stay Weak Unless They Are Pushed To Full
In a series string the same current flows through every cell, so a cell that charges more slowly than its neighbours ends each charge a little behind, and the gap grows. A deliberate overcharge lets the lagging cells catch up while the full cells gas harmlessly. The carrier lists three purposes: it breaks up minor sulfation, it equalizes charge between cells, and it stirs the electrolyte so that acid which has sunk to the bottom of the cell is mixed (stratification).
The Corpus Voltages Conflict
For a 12 V battery:
| Source | Equalizing voltage | Time and frequency |
|---|---|---|
| Carrier | 2.50 V per cell, 15.0 V | 2–4 hours, once monthly |
| Charging Systems | 15–16 V | 2–4 hours, monthly or when cells are unequal |
| Lead-Acid Construction | 15.5 V | 2–4 hours, monthly |
| Energy Storage | 15 V | 2–3 hours, every 1–2 months, flooded only |
Dividing by six: 15.0 ÷ 6 = 2.50, 15.5 ÷ 6 = 2.58 and 16 ÷ 6 = 2.67 V per cell. But Rechargeable Batteries says “never exceed 2.5 V per cell”. The sources therefore disagree, and the 15.5 and 16 V figures break that limit. Without altering any of them, the safer figure is the lowest, 15.0 V (2.50 V per cell). Treat higher settings as unverified until a cell of your own build is shown to tolerate them.
The same advice as for float applies: the correct voltage depends on the cells. A cell that gasses vigorously at 15.0 V and runs warm is already enough; one that stays undercharged with a persistent gap in specific gravity may need longer rather than higher voltage.
What You Need
- A flooded lead-acid bank — the energy-storage article restricts equalization to flooded, not sealed, cells
- An adjustable source or regulated charger — Charge Controllers
- A hydrometer and a voltmeter — to read each cell
- A thermometer or a hand on the case — to stop if the case becomes hot
- A well-ventilated space and a supply of distilled water
Method: Equalize A Flooded Bank
Step 1: Charge normally to full first, then read specific gravity and voltage of every cell and write them down. A difference between cells is the reason to equalize.
Step 2: Disconnect the loads. General knowledge, not from the other articles: the higher voltage can harm connected equipment.
Step 3: Raise to the equalizing voltage and hold it for 2 to 4 hours. The cells will bubble steadily.
Step 4: Watch temperature. A battery heating past 50°C is being overcharged: disconnect immediately (Charging Systems).
Step 5: Re-read every cell. If the spread has closed, stop. If one cell is still low after two equalizations, treat it as faulty rather than repeating (Desulfation Techniques).
Step 6: Top up with distilled water if the level has fallen (Water Level Maintenance) and return to float.
Worked Example: What A 3-Hour Equalization Costs In Gas And Water
Assume the worst case, a 100 Ah bank at 5 A (C/20), all of it going into gassing for 3 hours. Charge per cell = 5 × 3 = 15 Ah. At 0.42 L of hydrogen per ampere-hour (Water Electrolysis), each cell gives 15 × 0.42 = 6.3 L of hydrogen, and six cells give 6 × 6.3 = 37.8 L. The water lost, at 9 g of water split per faraday (general knowledge: 18 g/mol with two electrons each) and 26.8 Ah per faraday, is 9 ÷ 26.8 = 0.336 g per ampere-hour, so 15 × 0.336 = 5.0 g (5 mL) per cell. That is small per cell, but 37.8 L of hydrogen released into a small closed box is a serious explosion hazard.
Common Mistakes
- Equalizing sealed batteries — there is no way to replace the lost water
- Equalizing too often — every overcharge corrodes the positive plates a little
- Using the highest corpus voltage — it exceeds the corpus’s own per-cell limit
- Equalizing a bank with a dead cell — the bad cell gets hot while the good ones gas
Safety
Equalization is the deliberate production of explosive gas
By the arithmetic above, a 100 Ah bank can release about 38 L of hydrogen in three hours, and it rises and collects at the highest point of the room or in any lid, box or ceiling pocket. Hydrogen is explosive in air from 4% to 75% and burns with an invisible flame (Gas Hazards). Equalize only where the top of the space ventilates to the outside, with no switches, relays or open flames nearby, and do not use a flame to check the gas.
What’s Next
- Learn next: Maintenance and Reconditioning — the routine around equalization
- Combine with: Float Charging — the state to return to afterward