LiFePO4 SOC Chart

The Flat Curve Problem

Most battery chemistries have a relatively linear relationship between voltage and state of charge. Lead-acid batteries, for example, show a clear voltage drop as SOC decreases. LiFePO4 does not. The discharge curve of a LiFePO4 cell is nearly flat between 20% and 80% SOC, hovering around 3.2V per cell.

This flat plateau is actually an advantage for power delivery — the voltage stays stable across a wide SOC range, which means the device being powered sees consistent voltage. But it creates a significant challenge for SOC estimation. When the voltage at 30% SOC is essentially identical to the voltage at 70% SOC, you cannot meaningfully estimate SOC from voltage alone.

The voltage only becomes a useful SOC indicator at the extremes: above 80% SOC where the curve steepens toward the charge endpoint, and below 20% SOC where it drops toward the discharge cutoff. For the middle 60% of the charge curve, a different approach is needed.

Coulomb Counting

Coulomb counting is the method most BMS units use to track SOC. It works by measuring the current flowing in and out of the battery and integrating it over time. If you start at 100% SOC and draw 10A for 5 hours, the BMS calculates that 50Ah have been removed, reducing SOC by 50% on a 100Ah battery.

Coulomb counting is accurate within the flat plateau region where voltage provides no information. However, it has its own limitations. Small measurement errors accumulate over time, causing SOC drift. Temperature affects actual capacity, and the BMS may not track this perfectly. Periodic full charge-discharge cycles recalibrate the coulomb counter against the known voltage endpoints.

Resting vs Loaded Voltage

Resting voltage (open-circuit voltage, or OCV) is measured when no current is flowing. This is the truest representation of the cell's electrochemical state. Loaded voltage is measured while the battery is supplying current and is always lower due to internal resistance. The difference follows Ohm's law: V_loaded = V_OCV − (I × R_internal).

For a LiFePO4 cell with 20mΩ internal resistance under 50A load, the voltage drop is 1.0V — a significant deviation from the resting value. This means a battery reading 13.2V at rest might read 12.2V under a 50A load on a 12V pack. If you are trying to estimate SOC from a voltage reading taken under load, the result will be misleadingly low.

For reliable SOC estimation from voltage, always measure after the battery has been idle for at least 30 minutes. For systems that need SOC under load, rely on the BMS coulomb counter rather than a voltage reading.

SOC Reference Table

Values are approximate resting voltages at 25°C. Actual values vary by manufacturer and cell age.

Best Practices

For most applications, use a BMS with coulomb counting as the primary SOC method, and voltage as a cross-check at the charge endpoints. Calibrate the BMS periodically by fully charging to 100% (the BMS resets SOC to 100% when it detects the charge endpoint voltage).

If you are building a custom system without a sophisticated BMS, you can estimate SOC from voltage — but only at the extremes. Above 80% or below 20% SOC, voltage provides a reasonable estimate. In the middle range, you must track amp-hours manually or accept that SOC is unknown.

For critical applications (medical, telecom, BESS), use both methods in parallel. The BMS coulomb counter provides continuous SOC, while periodic voltage checks at rest confirm the coulomb counter has not drifted.

Common Mistakes