Calculator inputs
Use rested voltage whenever possible. This tool is designed for 12 volt lead-acid batteries and battery banks wired in parallel.
Example data table
| Battery type | Measured V | Temp °C | Mode | Specific gravity | Approx. state of charge |
|---|---|---|---|---|---|
| Flooded | 12.42 | 25 | Rested | 1.225 | About 67% |
| AGM | 12.67 | 20 | Rested | Not used | About 81% |
| Gel | 12.18 | 30 | Under load | Not used | About 43% |
| Flooded | 12.05 | 10 | Fresh off charge | 1.190 | About 34% |
Formula used
This calculator estimates state of charge for 12 volt lead-acid chemistry by combining corrected voltage and optional hydrometer information.
- Temperature-corrected voltage: Equivalent rested voltage = measured voltage + temperature correction + mode correction.
- Temperature correction: Temperature correction = (25 - temperature in °C) × 0.0018.
- Mode correction: A small correction offsets discharge sag, fresh charge, or charging elevation.
- Specific gravity correction: Corrected gravity = measured gravity + (temperature in °C - 25) × 0.0007.
- Gravity voltage estimate: Approximate open-circuit voltage = 6 × (corrected gravity + 0.84).
- Interpolation: The tool interpolates between chemistry-specific charge points to estimate state of charge.
- Remaining charge: Remaining amp-hours = effective capacity × state of charge ÷ 100.
- Remaining energy: Remaining watt-hours = remaining amp-hours × 12.
- Usable runtime: Runtime above reserve = usable amp-hours above reserve ÷ load current.
Flooded batteries benefit most from specific gravity input because electrolyte density directly tracks sulfuric acid concentration.
How to use this calculator
- Select the battery type that matches the battery construction.
- Enter the rated amp-hour capacity of one battery.
- Enter the number of batteries connected in parallel.
- Measure terminal voltage with a digital meter.
- Add the temperature near the battery or electrolyte.
- Choose whether the reading was rested, loaded, charging, or freshly charged.
- Enter load or charge current when it is known.
- For flooded batteries, enter average specific gravity if you used a hydrometer.
- Add battery health to reduce effective capacity for older batteries.
- Set a reserve state of charge and optional accessory load.
- Press the button to see the result above the form.
- Use the chart and exports for reporting or maintenance logs.
Frequently asked questions
1. Is voltage alone enough to estimate charge?
Voltage is useful, but it is best after the battery rests. Temperature, recent charging, and active loads change terminal voltage and can distort the estimate.
2. Why does the calculator ask for battery type?
Flooded, AGM, and gel batteries follow slightly different open-circuit voltage curves. Using the correct chemistry improves the interpolation and gives a better state of charge estimate.
3. When should I use specific gravity?
Use it when you can safely measure electrolyte with a hydrometer on flooded batteries. It directly reflects acid density and often improves confidence.
4. Why is my reading lower in cold weather?
Cold electrolyte reduces voltage and available power. The calculator corrects the reading, but a cold battery may still deliver less practical performance than a warm one.
5. What does reserve state of charge mean?
It is the lowest charge level you want to keep for battery protection. Runtime above reserve helps you plan usage without draining too deeply.
6. Why can charging readings look too high?
Charging current and surface charge temporarily raise terminal voltage. That makes the battery look fuller than it really is until the excess charge dissipates.
7. Does battery health affect state of charge?
State of charge and state of health are different. A weak battery can be fully charged yet store less energy, so the calculator reduces effective capacity using the health input.
8. Can I use this for lithium batteries?
No. Lithium batteries use different chemistry, voltage profiles, and management systems. This calculator is intended for 12 volt lead-acid battery families only.