How to Charge a Lead-Acid Battery
Lead-acid charging is a three-stage process — bulk, absorption, float — at specific voltages. The two classic mistakes are overcharging (gassing and water loss) and undercharging (sulfation).
The three stages
| Stage | What happens | Typical 12V voltage |
|---|---|---|
| Bulk | Constant current pushes charge in until the voltage rises to the absorption limit | rises to ~14.4V |
| Absorption | Constant voltage finishes the charge; current tapers down as the battery fills | ~14.4–14.8V (flooded) / ~14.2–14.4V (AGM) |
| Float | Low maintenance voltage holds the battery full without overcharging | ~13.5–13.8V (flooded) / ~13.4–13.6V (AGM) |
The two mistakes
Lead-acid chemistry punishes both extremes:
- Overcharging — holding the battery too high too long electrolyzes the water into hydrogen and oxygen (gassing), losing electrolyte and corroding the positive grid. In sealed/maintenance-free or AGM batteries the water cannot be topped up, so overcharging is especially damaging.
- Undercharging — if the battery never reaches full, lead sulfate crystals left on the plates harden (sulfation) and no longer convert back, permanently reducing capacity and cranking power.
Voltage depends on chemistry and temperature
The exact voltages depend on the battery type and its temperature. Flooded, AGM and gel each have their own recommended limits, and chargers compensate voltage with temperature — lower voltage when hot, higher when cold. Charging a battery at the wrong type's voltage is a common cause of early failure.
Charging and CCA
Charging is also where cranking performance is preserved. Sulfation from chronic undercharging raises internal resistance, which is exactly what lowers cold cranking amps (CCA) over time — see what CCA is and internal resistance.
How long does charging take?
Bulk charging follows the division — capacity to replace ÷ charger current — but the finish is slower: the charger switches to constant voltage and the current tapers, so the final 20% takes about as long as the first 80% (see the charging-time calculation with worked examples).
How to choose a charger
| Check | Rule |
|---|---|
| Size | ~10–20% of the Ah rating — a 100 Ah battery pairs with a 10–20 A charger |
| Type match | The profile must match the battery — flooded, AGM, gel or lithium |
| Voltage limits | Within the battery's absorption and float limits for its type |
Our Read
In my view: Most lead-acid "sudden death" is really slow undercharging — a battery left below full for weeks sulfates, and no jump-start fixes that.
What drives this: Sulfation is silent and cumulative; it shows up as a weak cold start long after the damage was done. The cheap fix is keeping the battery charged, which is why the float stage exists.
My view as an editor, not a purchasing guarantee.
Sources
- Battery Council International (BCI) — lead-acid charging and maintenance guidance.
- Manufacturer charging guides (Yuasa, Canadian Energy, Varta) — flooded vs AGM voltage limits.
- Cross-verified with charger documentation on bulk/absorption/float and temperature compensation.
Last reviewed: 2026-09-15