Calculator KB-29
Battery charge level calculator by voltage
Measure the voltage of a battery that has been resting without a load for at least an hour.
Result
In brief: Estimate the state of charge from voltage for Li-ion, LiFePO4, and lead-acid batteries of any configuration: charge percentage, voltage per cell, and limits. Example: Charge level: 50 %.
Calculation example
| Battery type | Li-ion / Li-pol |
|---|---|
| Cells in series | 1 pcs |
| Voltage | 3.75 V |
| Charge level | 50 % |
|---|---|
| Per cell | 3.75 V |
| Fully charged | 4.2 V |
| Fully discharged | 3 V |
How it is calculated
Each battery type has its own relationship between resting voltage and charge. These are typical values: actual values depend on the manufacturer, temperature, and wear, so treat the result as an estimate.
What to look for
- Under load and immediately after charging, voltage does not indicate the actual charge level: let the battery rest.
- For LiFePO4, with its flat discharge curve, a monitor that counts amp-hours is more reliable.
Frequently asked questions
What state of charge corresponds to a Li-ion voltage?
At rest, 3.75 V is about 50%, 4.2 V means fully charged, and 3.0 V is practically fully discharged. These are typical values and depend on the cell model.
What is the voltage of a fully charged 12 V lead-acid battery?
About 12.8 V at rest. 12.4 V corresponds to about 60%, while 11.8 V is generally considered fully discharged.
Why is it hard to determine the state of charge of a LiFePO4 battery from its voltage?
Its voltage curve is almost flat: between 20 and 80% charge, the cell voltage changes by about 0.1 V. It's more reliable to track amp-hours with a battery monitor.
