Understanding the relationship between voltage and state of charge (SOC) is essential for proper battery management, sizing, and maintenance. This guide provides accurate voltage-to-SOC data for lead-acid batteries.
Resting Voltage vs Loaded Voltage
Resting voltage: Measured after battery has been disconnected for at least 1 hour. Used for accurate SOC determination.
Loaded voltage: Measured while battery is under load. Will always read lower due to voltage sag. Not accurate for SOC measurement.
12V Battery: Voltage vs State of Charge
- 100% SOC (full): 12.7–12.9V resting
- 90% SOC: 12.5–12.6V resting
- 80% SOC: 12.3–12.4V resting
- 70% SOC: 12.1–12.2V resting
- 60% SOC: 11.9–12.0V resting
- 50% SOC: 11.7–11.9V resting
- 40% SOC: 11.5–11.6V resting
- 30% SOC: 11.3–11.4V resting
- 20% SOC: 11.0–11.1V resting
- 10% SOC: 10.5–10.7V resting (LOW BATTERY WARNING)
- 0% SOC: 9.5–10.0V resting (DISCHARGE CUTOFF)
2V Cell Voltage vs SOC
- 100% SOC: 2.10–2.15V resting
- 80% SOC: 2.02–2.05V resting
- 60% SOC: 1.95–1.98V resting
- 50% SOC: 1.92–1.95V resting
- 40% SOC: 1.88–1.91V resting
- 20% SOC: 1.80–1.83V resting
- 10% SOC: 1.75V resting (CUTOFF)
Using Voltage for Battery Management
For systems without a battery monitor (BMV), a quality voltmeter can provide reasonable SOC estimation. However, for accurate management, invest in a proper battery monitor that tracks amphours in and out (coulomb counting).
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