Why does my SOC% not read correctly?

Why Does My Battery SOC% Not Read Correctly?

If you rely on a 12V or 24V battery system in your campervan, motorhome, boat or off-grid setup, your State of Charge (SOC%) is one of the most important numbers you look at. But what happens when that percentage doesn’t match reality? Maybe your monitor says 100% but the battery clearly isn’t full — or it crashes to 0% even though everything is still running.

Incorrect SOC readings are extremely common, especially with lithium batteries. In this guide, we explain why SOC becomes inaccurate and what you can do to fix it.

What Is SOC%?

SOC (State of Charge) is an estimate of how much usable energy remains in your battery. Most systems display it as a percentage — for example, 78% full.

However, SOC is not directly measured. It is calculated using:

Voltage readings
Current flow in and out of the battery (via a shunt)
Battery capacity settings
Historical usage data

Because SOC is a calculation, several things can cause it to drift, reset or become inaccurate.

Common Reasons Your SOC% Is Incorrect

1. Your Battery Monitor Isn’t Calibrated

A shunt-based battery monitor must be set up correctly to track the battery’s true capacity. If the monitor thinks your battery is 100Ah but it’s actually 120Ah, SOC errors will build up quickly.

Fix:

Check and update the battery capacity setting.
Set the correct tail current and absorption voltage for lithium or AGM.
Perform a full charge calibration cycle.

2. Voltage-Based Monitors Are Misleading

Many cheaper systems calculate SOC using voltage only. This method is highly inaccurate, especially with lithium batteries, which hold a flat voltage until nearly empty.

This can cause:

Stuck SOC readings
SOC jumping from 30% to 0% suddenly
Batteries appearing “full” when they aren’t

Fix: Use a shunt-based monitor for accurate SOC readings.

3. The Shunt Isn’t Installed Correctly

A shunt must read all the current going into and out of the battery. If even one device is connected directly to the battery negative instead of the shunt, SOC accuracy is impossible.

Fix:

Ensure every negative cable except the battery-to-shunt connection goes on the system side of the shunt.
Recheck inverter, solar, charger and auxiliary loads.

4. SOC Drift Over Time

Even with a perfect setup, battery monitors drift slightly over time. Small measurement errors in charging and discharging add up.

This is normal and expected — especially if:

Your battery rarely reaches 100%
The system never rests at full charge
You have inconsistent charging sources (solar + alternator + mains)

Fix: Perform a “full charge reset” where the battery reaches absorption voltage and current tapers off.

5. Your Battery Never Reaches a True 100% Charge

Many systems assume the battery is “full” once it hits a certain voltage, but lithium batteries need to sit at their absorption voltage long enough for the current to drop to a specific tail current.

If this doesn’t happen:

SOC jumps to 100% too early
The monitor resets but the battery isn’t full
Drift becomes worse over time

Fix: Allow the battery to complete a full charge cycle from time to time.

6. Incorrect Charge Efficiency Settings

Lead-acid and lithium batteries have different charge efficiencies. If your monitor uses the wrong efficiency values, SOC will be miscalculated.

Fix: Set the correct battery chemistry in your monitor or shunt settings.

7. Battery Capacity Loss Over Time

Older batteries no longer have their full rated capacity. A 100Ah battery that has degraded to 80Ah will behave strangely if the monitor still thinks it’s 100Ah.

Fix: Lower the capacity setting to match the battery's real-world performance.

8. Temperature Effects

Battery voltage can be affected by temperature, which can confuse voltage-based SOC systems.

Fix: Use a shunt-based monitor or ensure your system has temperature compensation.

How to Fix Incorrect SOC Readings

1. Fully Charge the Battery

Charge to absorption voltage
Hold until current tapers to tail current (often around 2–5%)
Let your monitor reset automatically

2. Recalibrate the Monitor

Update the rated battery capacity
Set the correct charge profile
Reset SOC at full charge

3. Recheck Shunt Wiring

Make sure every negative cable passes through the shunt
Ensure no chargers or loads bypass the shunt

4. Use the Right Monitoring Technology

If your SOC is always wrong and your system uses voltage-only monitoring, upgrading to a shunt-based system is the best solution.

When SOC Inaccuracy Is Normal

Some SOC variation is normal — especially when:

You run high loads (voltage dips temporarily)
You charge from multiple sources
Your battery has just come off charge (surface charge)

This doesn’t mean your system is faulty — it’s simply how batteries behave.

Conclusion

SOC% is one of the most useful pieces of information in any off-grid or leisure battery system, but it’s also one of the easiest numbers to misread or misunderstand.

Incorrect SOC usually comes down to calibration, wiring, charge settings or the limitations of voltage-based monitoring. With the right setup — especially a properly installed shunt — you can get accurate, reliable SOC readings every day.

If your SOC keeps drifting or behaving strangely, checking your shunt installation and performing a full reset cycle often solves the issue instantly.