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.
How to Use
- Pick battery type (flooded, AGM, gel) and number of cells in series.
- Enter ambient temperature for correction.
- Tool returns float, absorb, and equalize voltages.
Show Work
Reference (per cell @ 25°C)
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.
About the Lead-Acid Float / Absorb / Equalize Calculator
The Lead-Acid Float / Absorb / Equalize Calculator is a simple, free helper for electronics and circuit design that runs entirely on your own device. 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.
How it works
Enter your figures and the result appears instantly, updating the moment you change anything. There is no submit button and nothing to wait for, so it is easy to try a few what-if numbers and compare the results. Just check each box holds the kind of value it expects.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
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.
How do I use the Lead-Acid Float / Absorb / Equalize Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
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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