Online Tool Store
Battery State of Charge Chart
Estimate battery state of charge from resting voltage for lead-acid, AGM, and lithium chemistries, with the resting time each needs.
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Math & Science
Battery State of Charge Chart
Frontend preview — no upload or external service.
State of charge
12.72 V on a rested flooded lead-acid battery is roughly 100% charged. The same reading after 20 minutes off charge means considerably less — surface charge inflates it.
How the Battery State of Charge Chart works
- Disconnect all loads and charging and let the battery rest — several hours for lead-acid.
- Measure at the terminals with a meter you trust, and note the temperature.
- Read the estimate for the correct chemistry; the voltage curves differ enormously between lead-acid and lithium.
The method
Resting voltage tracks state of charge, but the relationship is chemistry-specific and only valid once surface charge has dissipated.
lead-acid spans roughly 12.7 V full to 11.8 V empty; LiFePO4 is nearly flat from 20% to 80%
LiFePO4's flat curve is why voltage is a poor charge indicator for it in the middle of the range — a few hundredths of a volt covers half the capacity.
FAQ
Why does the battery need to rest?
Because surface charge left after charging inflates the reading, sometimes by several tenths of a volt. A lead-acid battery needs hours off both load and charger before the voltage means anything.
Why is voltage unreliable for LiFePO4?
Because its discharge curve is deliberately flat — that stability is a feature for the load and a problem for measurement. Coulomb counting with a shunt-based monitor is the practical alternative.
Does temperature matter?
Yes, particularly for lead-acid, where both voltage and available capacity shift with temperature. A cold battery reads lower and delivers less than the same battery warm.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Chemistry-specific curves | ✓ | One chart | Yes |
| Rest time guidance | ✓ | ✗ | Sometimes |
| No monitor hardware | ✓ | ✗ | ✗ |
| Coulomb counting accuracy | ✗ | ✗ | ✓ |
Battery State of Charge Chart insists on rest time and chemistry, because a reading taken straight off charge is inflated and a lithium curve is nearly flat.