Lead-Acid Float / Absorb / Equalize Calculator

Compute correct float, absorb, and equalize voltages for flooded, AGM, and gel lead-acid batteries at any temperature. For UPS, solar, and automotive battery bank charging.

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Pick battery type (flooded, AGM, gel) and number of cells in series.
  2. Enter ambient temperature for correction.
  3. Tool returns float, absorb, and equalize voltages.
Input
°C
Presets
Voltage Profile
Float V
—V
Absorb V
—V
Equalize V
—
Temp Corr
—mV

Show Work

Enter values.

Reference (per cell @ 25°C)

Flooded Float
2.25 V/cell
Maintenance.
Flooded Absorb
2.40-2.45 V/cell
Active charge.
Flooded EQ
2.58-2.67 V/cell
Monthly desulfation.
AGM Float
2.25-2.30 V/cell
Slightly higher.
AGM Absorb
2.40-2.45 V/cell
No EQ allowed.
Gel Absorb
2.35 V/cell max
Overvolt damages.

History of Lead-Acid Charging

Gaston Plante invented the lead-acid battery in 1859. The three-stage charge profile (bulk / absorb / float) emerged from telecommunications central-office standby power practice in the 1930s-50s, where flooded lead-acid banks powered switching equipment 24/7. AGM (Absorbed Glass Mat) batteries, invented in 1972 for military aircraft, are sealed and cannot be equalized; gel cells (1957) similarly. Temperature compensation (-3 mV/C/cell) is codified in IEEE 485 for stationary battery applications.

About This Calculator

Pick battery type (flooded/AGM/gel), number of cells in series (6 for 12V, 12 for 24V, 24 for 48V), and ambient temperature. The tool returns float, absorb, and equalize voltages at your temperature, using -3 mV/°C/cell compensation.

AGM and gel batteries should NEVER be equalized — the high voltage dries out the electrolyte or damages the gel. Flooded batteries benefit from monthly equalize charges for best life. Everything runs client-side.

Frequently Asked Questions

Float vs absorb?

Float: continuous maintenance voltage (13.5 V for 6-cell 12V battery) — keeps battery at 100% with minimal water loss. Absorb: higher voltage (~14.4-14.8 V) for active charging. Charger stays in absorb until current tapers, then drops to float.

Equalize?

Periodic high-voltage (15.5-16.0 V) charge for flooded batteries to mix electrolyte and desulfate. Should NOT be done on sealed AGM/gel — overvoltage damages them permanently.

Temp compensation?

All voltages shift -3 mV/°C/cell (about -18 mV/°C for 12V). Hot battery needs lower voltage; cold needs higher. Modern chargers have temperature sensor at the battery.

Common Use Cases

Solar Off-Grid

3-stage charge controller: bulk → absorb at 14.4V → float at 13.5V (AGM 12V pack).

UPS Bank

Float charge at 2.25 V/cell holds battery at 100% SOC indefinitely.

Automotive

Modern smart alternators target 13.8V float, 14.4V absorb.

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