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Battery State of Charge Calculator

Estimate battery state of charge from resting voltage.

About Battery State of Charge Calculator

Voltage-based estimation only works at rest, because load or charging shifts terminal voltage substantially. Lithium chemistries have a very flat curve, making voltage a poor state-of-charge indicator for them. Coulomb counting is far more accurate than voltage for lithium, which is why battery management systems track charge in and out rather than relying on voltage.

Formula

Battery State of Charge

SoC % = (measured − empty voltage) ÷ (full − empty voltage) × 100

resting voltage
Measured after at least an hour with no load or charge

Worked example

With the default values below, Battery State of Charge Calculator returns State of Charge of 43.75%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Measured resting voltageminimum 0.1 V
12.7 V
Voltage at full chargeminimum 0.1 V
13.6 V
Voltage at the discharge limitminimum 0.1 V
12 V
Battery capacityminimum 0 Ah
100 Ah

Results

State of Charge
43.75%
Charge Remaining
43.75 Ah
Charge Needed to Refill
56.25 Ah
Depth of Discharge Used
56.25%
Status
Low — recharge soon

Step by step

  1. 1Values used
  2. 2Battery State of Charge
  3. 3State of Charge= 43.75
  4. 4Charge Remaining= 43.75 Ah
  5. 5Charge Needed to Refill= 56.25 Ah
  6. 6Depth of Discharge Used= 56.25
  7. 7Status= Low — recharge soon

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Battery State of Charge calculated?

SoC % = (measured − empty voltage) ÷ (full − empty voltage) × 100 Voltage-based estimation only works at rest, because load or charging shifts terminal voltage substantially. Lithium chemistries have a very flat curve, making voltage a poor state-of-charge indicator for them.

Why does Battery State of Charge matter?

Coulomb counting is far more accurate than voltage for lithium, which is why battery management systems track charge in and out rather than relying on voltage.

What values do I need to enter?

This calculator takes 4 inputs: Measured resting voltage, Voltage at full charge, Voltage at the discharge limit, Battery capacity. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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