Battery Technology Reference · Last reviewed 2026-09-15
A newly assembled plate is not yet a battery plate — its paste is a raw mix that has not become active material. Formation passes a controlled current through the assembled cell, electrochemically converting the positive paste to lead dioxide and the negative to spongy lead. The charge profile and duration are carefully controlled, because under-formation leaves capacity unused and over-formation wastes energy and damages the plates.
Discharge converts lead, lead dioxide and sulfuric acid into lead sulfate and water. Charging pushes current the other way and reverses that — the sulfate returns to lead on the negative, lead dioxide on the positive, and acid is regenerated. The profile matters: a correct charge fully reverses the reaction without overcharging, which would split water into gas (water loss) and corrode the positive grid.
| Stage | What happens | Purpose |
|---|---|---|
| Bulk (constant current) | High current restores most charge | Fast recovery |
| Absorption (constant voltage) | Current tapers at the voltage limit | Finish without overcharge |
| Float (low constant voltage) | Tiny current holds full charge | Standby maintenance |
The correct charge voltage differs by technology, which is why a charger must match the battery. Typical 12V absorption voltages (approximate, temperature-dependent):
| Technology | Typical absorption voltage | Note |
|---|---|---|
| Flooded | ~14.4–14.8 V | Higher to mix acid and avoid stratification |
| AGM | ~14.4–14.7 V | Sensitive to overcharge — keep within the limit |
| EFB | ~14.4–14.7 V | Similar to AGM, no equalisation charge |
Overcharging and undercharging are the two chronic killers: overcharge gasses off water and corrodes the grid; undercharge leaves sulfate to harden into the crystals that cause sulfation. The correct profile — and a charger matched to the battery technology (flooded, AGM or EFB) — keeps the battery in the narrow band between the two.
Position: Most premature lead-acid deaths are charging errors, not manufacturing defects — and the difference between a battery that lasts 5 years and one that dies in 2 is usually the charger, not the brand.
Reasoning: The chemistry is reversible but only within a window; overcharge and undercharge both push it outside that window and leave permanent damage. A charger matched to the battery's technology keeps it in the window, which is why charging is the highest-leverage maintenance decision.
This is the author's editorial view, not a purchasing guarantee.
Return to Battery Technology Reference · World Battery Hub. Informational, not purchasing advice.