Introduction

One of the most confusing questions from solar energy storage users is:

“Why does my inverter show 20% battery while the battery screen shows 45%?”

Other common reports include:

• Battery display shows 100%, but the inverter continues charging.
• Inverter shows 100%, but the battery BMS shows 85%.
• SOC remains at 100% for several hours and then drops suddenly.
• Battery shuts down while the inverter still shows 30%.
• Different batteries in the same parallel bank display different SOC values.
• SOC becomes inaccurate after a power cut or battery reset.

These readings do not necessarily mean that the battery has lost capacity.

The inverter and battery may be calculating, receiving or displaying SOC in different ways.

To diagnose the issue, the user must first identify where each SOC value comes from.

What Is State of Charge?

State of charge, or SOC, represents the estimated remaining battery capacity.

It is normally shown as a percentage:

• 100% means fully charged.
• 50% means approximately half the usable capacity remains.
• 0% means the system has reached its configured empty point.

SOC is an estimate rather than a direct physical measurement.

A battery management system cannot look inside a cell and directly count the remaining lithium ions. It estimates SOC using data such as:

• Current entering the battery
• Current leaving the battery
• Battery voltage
• Cell voltage
• Temperature
• Nominal battery capacity
• Charging efficiency
• Previous synchronization point
• Internal algorithms

Because different devices may use different data and algorithms, their percentages may differ.

Where Can the SOC Value Come From?

A solar system may contain several separate SOC values.

Battery BMS SOC

Calculated by the battery’s internal BMS.

Inverter SOC

Received from the battery through CAN or RS485, or estimated independently by the inverter.

External Battery Monitor SOC

Calculated using a shunt that measures current entering and leaving the battery bank.

Mobile Application SOC

May come from:

• Battery Bluetooth
• Inverter cloud platform
• Energy management system
• External monitoring gateway

These values may update at different speeds and may not share the same calculation method.

Closed-Loop Communication Mode

In a properly integrated closed-loop system, the battery BMS sends data to the inverter through CAN or RS485.

The transmitted data may include:

• SOC
• State of health
• Pack voltage
• Charge-current limit
• Discharge-current limit
• Maximum charging voltage
• Battery temperature
• Warning and alarm status

In this arrangement, the inverter should normally display the SOC received from the battery master BMS.

Official inverter support documentation for compatible lithium batteries requires BMS communication in supported configurations.

However, an SOC difference can still occur because of:

• Communication delay
• Incorrect protocol
• Wrong battery address
• Firmware incompatibility
• Data scaling error
• Inverter displaying cached information
• Parallel-bank capacity not configured correctly

Open-Loop or User-Defined Battery Mode

When there is no communication, the inverter may estimate SOC from battery voltage.

This is less accurate for LiFePO4 batteries because their voltage remains relatively stable through a large portion of the discharge cycle.

The inverter may therefore show:

• 70% for a long time
• A rapid fall near empty
• 100% before the battery is fully balanced
• A large change when a heavy load starts
• A sudden recovery after the load stops

Voltage-based SOC should not be expected to match a BMS using current integration.

Why LiFePO4 Voltage Is Difficult to Use for SOC

LiFePO4 cells have a relatively flat operating-voltage region.

During moderate discharge, the voltage difference between a relatively high and relatively low SOC can be small.

Battery voltage is also affected by:

• Charge or discharge current
• Temperature
• Cell resistance
• Time since charging
• Cable voltage drop
• Surface charge
• Cell imbalance

For example, voltage may fall when a pump starts and recover when it stops. An inverter using voltage-based SOC may interpret this as a sudden loss and recovery of battery capacity.

The BMS may show a more stable percentage because it is integrating current over time.

What Is Coulomb Counting?

Coulomb counting estimates SOC by measuring current over time.

In simplified terms:

• Charging current adds to SOC.
• Discharging current subtracts from SOC.

For example, if a fully charged 100Ah battery supplies 10A for five hours, approximately 50Ah has been removed before accounting for system efficiency and other factors.

The method can be accurate, but small measurement errors accumulate over time.

The SOC calculation therefore needs periodic synchronization at a known full-charge point.

Why SOC Drifts Over Time

SOC drift may occur because of:

• Current-sensor offset
• Incorrect battery-capacity setting
• Small unmeasured loads
• Chargers bypassing the current sensor
• Battery not reaching full charge
• Battery reset
• BMS firmware change
• Temperature variation
• Battery ageing
• Incorrect charge-efficiency setting

A one-percent calculation error per cycle can accumulate into a significant display difference.

Why a Full Charge Helps Recalibrate SOC

Many battery monitors synchronize SOC when defined full-charge conditions are met.

These conditions may include:

• Battery voltage above a configured charged-voltage threshold
• Charging current below a tail-current threshold
• Conditions maintained for a specified time

Victron battery-monitor documentation, for example, explains that the monitor can set SOC to 100% after it detects the configured charged voltage and sufficiently low tail current for the required period.

If the battery never reaches these conditions, SOC may never synchronize correctly.

Why the Battery Shows 100% but Charging Continues

Possible explanations include:

SOC Reached 100% Before Cell Balancing Was Complete

The BMS may show 100% while allowing a small amount of charging current so that cell balancing can continue.

Display Rounding

The actual SOC may be 99.5% but displayed as 100%.

Charger Is Supplying the Loads

The inverter may show charging power even when much of the solar or grid energy is directly supporting household loads.

Different Full-Charge Criteria

The battery and inverter may use different voltage, current or timing criteria.

Communication Delay

The inverter display or cloud application may update more slowly than the battery screen.

A small charging current near full SOC is not automatically a fault.

Why the Inverter Shows 100% but the Battery Shows Less

Possible causes include:

• Inverter uses voltage-based estimation.
• Incorrect lithium-battery profile is selected.
• Inverter capacity setting is wrong.
• CAN communication has failed.
• Inverter is displaying an old value.
• Battery BMS has been reset.
• Battery was not fully charged after installation.

Confirm whether the inverter displays:

• BMS connected
• Lithium battery normal
• CAN connected
• Battery online

If the inverter is in “user-defined,” “lead-acid” or “voltage” mode, its percentage may not come from the battery BMS.

Why the Battery Shuts Down While the Inverter Shows 20% or 30%

This is a more serious mismatch.

Possible causes include:

SOC Calibration Error

The BMS believed more capacity remained than was actually available, or the inverter’s estimate was incorrect.

One Cell Reached Undervoltage

The total battery SOC may show energy remaining, but one weak or imbalanced cell can trigger BMS protection.

High-Current Voltage Sag

A heavy load can cause one or more cells to reach the low-voltage threshold temporarily.

Inverter Cut-Off and Battery Empty Points Differ

The inverter may define 0% differently from the BMS.

Battery Capacity Setting Is Incorrect

A 100Ah battery programmed as 200Ah will discharge faster than the SOC algorithm expects.

Check:

• Minimum cell voltage
• Pack voltage
• Battery current
• BMS alarm history
• Cell-voltage difference
• Programmed battery capacity

Parallel Batteries with Different SOC Readings

In a parallel bank, individual battery displays may show different SOC values.

Possible reasons include:

• Batteries were connected at different SOC levels.
• Current sharing is unequal.
• One battery has higher internal resistance.
• Branch cables have different resistance.
• Batteries have different capacity settings.
• Firmware versions differ.
• One battery has recently been reset.
• One battery did not reach full-charge synchronization.

The master battery may report a combined or averaged SOC to the inverter.

A difference of a few percentage points may not require immediate action. A persistent large difference should be investigated.

The Importance of Correct Master-Slave Configuration

In multi-battery systems, one battery may act as the master.

The master collects data from slave batteries and communicates with the inverter.

Incorrect configuration may cause the inverter to see:

• Only one battery
• Wrong total capacity
• Inaccurate combined SOC
• Incorrect charge-current limit
• Incorrect discharge-current limit

Check:

• DIP switch positions
• Battery addresses
• Master communication port
• Battery-to-battery cables
• Termination settings
• Maximum supported battery quantity
• Firmware compatibility

External Shunt Wiring Problems

An external battery monitor uses a shunt installed in the main negative path.

For accurate SOC calculation, all load and charging current must pass through the shunt.

Incorrect arrangement:

• Inverter connected through shunt
• Solar controller connected directly to battery negative
• DC load connected directly to battery negative

The monitor cannot measure the bypassing current and its SOC calculation becomes inaccurate.

Official battery-monitor troubleshooting guidance states that loads or charge sources connected directly to the battery side of the shunt will not be included in current and SOC calculations.

The correct general arrangement is:

Battery negative
→ Battery side of shunt
→ System side of shunt
→ Negative busbar
→ Inverter, chargers and DC loads

Battery Capacity Settings

Verify the programmed battery capacity in:

• Battery BMS
• Inverter
• External battery monitor
• Energy management system

Example:

Actual battery bank:

• Four 51.2V 100Ah modules in parallel
• Total capacity: 400Ah

If the monitor is configured for only 100Ah, its SOC may change four times faster than expected.

If configured for 800Ah, it may change too slowly.

When expanding the battery bank, update all relevant capacity settings.

Charging Current Can Affect SOC Accuracy

If the battery reaches the charge-voltage threshold but charging current remains high, the monitor may not recognize the battery as fully charged.

Possible causes include:

• Active household loads
• Incorrect tail-current setting
• Charging current too high
• Cell balancing
• Parallel battery imbalance
• Voltage drop between charger and battery
• Charger ending the cycle too early

Measure voltage at both:

• Battery terminals
• Inverter or charger terminals

A cable voltage difference can cause the charger to believe the battery has reached its target voltage before the battery itself has done so.

Recommended Troubleshooting Procedure

Step 1: Record All SOC Readings

Record:

• Battery display
• Battery Bluetooth application
• Inverter display
• Inverter application
• External battery monitor
• Individual parallel modules

Note the time because cloud applications may update slowly.

Step 2: Confirm Communication Mode

Check whether the inverter is using:

• CAN
• RS485
• User-defined lithium mode
• Lead-acid mode
• Voltage-control mode

Step 3: Read Actual Electrical Data

Record:

• Pack voltage
• Minimum cell voltage
• Maximum cell voltage
• Charge or discharge current
• Battery temperature
• BMS alarms

Step 4: Verify Capacity Settings

Confirm the total Ah or kWh capacity in all devices.

Step 5: Inspect Shunt Wiring

Make sure every charger and load passes through the shunt.

Step 6: Perform a Controlled Full Charge

Use the battery manufacturer’s approved charging settings.

Allow the battery to:

• Reach the required charge voltage
• Reduce charging current
• Complete balancing where required
• Reach the synchronization condition

Step 7: Check SOC After Resting

Allow the battery to rest with minimal load and compare the readings again.

Step 8: Perform a Controlled Discharge Test

Use a known load and record:

• Starting SOC
• Energy delivered
• Ending SOC
• Minimum cell voltage
• BMS alarms

This helps distinguish display error from actual capacity loss.

Quick Symptom Analysis

Customer Complaint	Possible Cause
Inverter shows 100%, battery shows 85%	Inverter voltage estimate or communication failure
Battery shows 100%, small charging current continues	Balancing, rounding or load support
SOC drops suddenly under heavy load	Voltage-based estimate or voltage sag
SOC increases after load is removed	Battery-voltage recovery
Battery shuts down at inverter 30%	Incorrect SOC calibration or weak cell
Parallel batteries differ by 10–20%	Unequal current sharing or unsynchronized BMS units
SOC became wrong after reset	Lost coulomb-counting history
External monitor SOC drifts daily	Shunt bypass or incorrect capacity setting

Should the Inverter Display or Battery Display Be Trusted?

In a properly configured closed-loop system, the battery BMS is usually the primary source of battery limits and SOC information.

However, no display should be trusted blindly.

When the readings conflict, compare:

1. BMS cell voltages
2. Battery current
3. Pack voltage
4. Alarm history
5. Actual energy delivered
6. Communication status
7. External calibrated measurement

The battery display may also be inaccurate if its SOC has not been synchronized.

When Is an SOC Difference Acceptable?

A small temporary difference may occur because of:

• Display rounding
• Different update intervals
• Communication delay
• Current measurement tolerances

Further investigation is appropriate when:

• Difference remains above approximately 10% for long periods.
• Battery shuts down while inverter SOC is high.
• Inverter stops charging while battery SOC is low.
• Individual batteries show large differences.
• SOC jumps suddenly without substantial charging.
• SOC remains fixed for an unusually long period.

The acceptable difference and troubleshooting thresholds should follow the battery and inverter manufacturers’ instructions.

Information to Provide to the Supplier

Send:

• Battery model and serial number
• Inverter model
• Battery quantity
• Communication protocol
• DIP switch settings
• Battery firmware
• Inverter firmware
• BMS screenshot
• Inverter battery-data screenshot
• Individual cell voltages
• Pack voltage
• Charging or discharge current
• Alarm history
• Battery capacity settings
• Wiring photographs

This information helps determine whether the problem is caused by communication, calibration, configuration or actual battery condition.

Frequently Asked Questions

Can SOC be calculated accurately from voltage alone?

Voltage provides useful information near the upper and lower ends of the operating range, but it is less precise through the flat middle region of a LiFePO4 discharge curve.

How often should SOC be calibrated?

There is no universal interval. Calibration may be required when readings drift, after a BMS reset, after battery expansion or when the battery has not reached full charge for a long period.

Will discharging to zero recalibrate the battery?

Do not intentionally force the battery into low-voltage protection unless the manufacturer specifically requires such a procedure.

Why do individual parallel batteries show different SOC?

They may have different current flow, initial SOC, capacity, calibration or internal resistance.

Can CAN communication fix all SOC problems?

CAN communication helps the inverter receive BMS data, but the BMS itself must still be correctly configured and synchronized.

Why does SOC remain at 100% overnight?

Possible reasons include low overnight consumption, incorrect current measurement, capacity setting errors or a load bypassing the BMS or shunt measurement path.

Conclusion

A difference between inverter SOC and LiFePO4 battery SOC does not automatically indicate defective cells.

The most common causes are:

• Inverter using voltage-based SOC
• CAN or RS485 communication failure
• Different full-charge definitions
• Unsynchronized coulomb counting
• Incorrect battery-capacity settings
• Shunt wiring errors
• Parallel-battery configuration errors
• Cell imbalance
• High-current voltage sag
• Delayed application updates

A reliable diagnosis requires more than comparing two percentages.

The installer should review:

• Communication mode
• Actual battery voltage
• Individual cell voltage
• Charge and discharge current
• Capacity settings
• Shunt wiring
• BMS alarm history
• Full-charge synchronization

For HIZN Lithium technical support, provide the inverter model, battery model, battery quantity, communication protocol and screenshots of both battery and inverter data.