Lead-Acid Knowledge · Last reviewed 2026-09-15

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).

Direct answer: A 12V lead-acid battery is charged first at constant current (bulk), then held at a constant voltage around 14.4V (absorption) until full, then dropped to a float of about 13.5–13.8V to hold it without overcharging. AGM batteries use slightly lower voltages; equalization (≈15.5V) is a periodic flooded-only step, not for AGM or gel.

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.

The Author's Take

Position: Most lead-acid "sudden death" is really slow undercharging — a battery left below full for weeks sulfates, and no jump-start fixes that.

Reasoning: 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.

This is the author's editorial view, 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