Introduction

A customer may hear a repeated clicking sound from a LiFePO4 battery cabinet:

Click—off.
Click—on.
Click—off again.

The inverter display may repeatedly start and go dark. Battery voltage may appear and disappear. The BMS application may show alternating states such as:

• Contactor closed
• Contactor open
• Pre-charge active
• Discharge disabled
• Communication lost
• Battery online
• Battery offline

This condition is often described as:

• Contactor chattering
• Contactor cycling
• Repeated BMS connection
• Pre-charge loop
• Battery connect-disconnect loop

It should not be treated as normal operation.

Repeated contactor operation can cause:

• Contact wear
• Arcing
• Coil heating
• Connector stress
• Inverter restart cycles
• BMS alarm accumulation
• Loss of AC loads
• Premature contactor failure

The correct solution is to identify why the BMS repeatedly decides to connect and disconnect the battery.

What Does the Main Battery Contactor Do?

In larger LiFePO4 systems, the BMS may control one or more contactors between the battery cells and external DC bus.

The contactors allow the BMS to:

• Connect the battery to the inverter
• Disconnect loads during low-cell voltage
• Stop chargers during high-cell voltage
• Isolate the battery during a fault
• Control pre-charge
• Support emergency shutdown

Official BMS documentation describes load-disconnect signals that stop loads or inverters during low-cell-voltage events, and charge-disconnect signals that stop chargers during high-cell-voltage or temperature events.

The contactor should close only when the system is ready and remain closed during normal operation.

Why Pre-Charge Is Necessary

The inverter contains DC capacitors.

If the main contactor closes directly onto discharged capacitors, a high inrush current may:

• Weld the contactor
• Trigger overcurrent protection
• Damage the pre-charge resistor
• Trip the battery breaker
• Cause a voltage collapse

A pre-charge circuit connects the battery through a resistor or controlled path before the main contactor closes.

The objective is to raise the inverter DC-bus voltage close to the battery voltage.

After successful pre-charge:

1. Main contactor closes.
2. Pre-charge path is removed.
3. Normal high-current operation begins.

Professional BMS manuals provide adjustable pre-charge duration or voltage checks before the load contactor closes.

How a Pre-Charge Loop Develops

A common sequence is:

1. BMS begins pre-charge.
2. Inverter DC bus starts rising.
3. An active load consumes the pre-charge current.
4. Bus voltage fails to reach the required threshold.
5. BMS declares pre-charge failure.
6. BMS opens the circuit.
7. After a delay, it tries again.
8. The same condition remains.
9. Clicking repeats.

The problem may be caused by:

• Inverter switched on too early
• Large auxiliary DC load
• Short circuit
• Incorrect pre-charge resistor
• Damaged resistor
• Pre-charge contactor fault
• Main contactor fault
• Excessive inverter capacitance
• Incorrect timeout setting
• Low battery voltage
• Inverter internal fault

Symptom 1: Clicking Starts When the Inverter Is Switched On

Likely areas include:

• Capacitor inrush
• Pre-charge failure
• Inverter internal short or excessive load
• Incorrect start-up order
• Battery bank too small
• BMS peak-current protection

Prevention requires confirming the inverter’s DC capacitance and approved pre-charge architecture before installation.

Symptom 2: Clicking Starts Near Low SOC

Possible causes include:

• One cell reaching undervoltage
• Inverter shutdown voltage set below the BMS threshold
• Restart voltage too close to shutdown voltage
• Heavy automatic load restarting immediately
• Cable voltage drop
• Weak battery module
• Incorrect SOC calibration

Typical cycle:

1. Battery reaches low-cell protection.
2. BMS opens the contactor.
3. Load disappears.
4. Cell voltage recovers.
5. BMS closes the contactor.
6. Inverter and loads restart.
7. Voltage falls again.
8. BMS opens again.

Use adequate shutdown and restart hysteresis and require charging before full restart.

Symptom 3: Clicking Starts Near Full Charge

Possible causes include:

• Highest cell overvoltage
• Charger does not follow BMS current limit
• Charging voltage too high
• External MPPT continues charging
• Cell imbalance
• Charge-enable signal oscillation

The BMS may stop charging, cell voltage falls slightly and charging restarts immediately.

A suitable delay and lower current near full SOC can prevent rapid cycling.

Symptom 4: Clicking Occurs During Communication Failure

A battery may disconnect when:

• CAN communication is lost.
• Master BMS goes offline.
• Inverter repeatedly changes between lithium and fallback mode.
• Battery addresses conflict.
• Termination is unstable.
• Communication power supply resets.

The system should have a defined communication-loss response.

It should not alternate rapidly between:

• Full inverter power
• Zero power
• Fixed fallback current
• BMS-controlled current

Symptom 5: Contactor Coil Voltage Is Unstable

The contactor coil or BMS controller may use:

• Internal battery supply
• 12V auxiliary supply
• 24V control supply
• DC-DC converter
• External power supply

If control voltage falls below the contactor holding requirement:

1. Contactor closes.
2. Load current increases.
3. Control voltage falls.
4. Contactor releases.
5. Load disappears.
6. Voltage recovers.
7. Contactor closes again.

Check:

• Auxiliary power supply rating
• DC-DC converter condition
• Coil voltage during closing
• Ground connection
• Fuse and relay condition
• Control cable resistance
• BMS power supply

Prevention Step 1: Use an Approved Pre-Charge Design

The design should define:

• Battery voltage
• Inverter capacitance
• Pre-charge resistance
• Resistor pulse-energy rating
• Maximum resistor temperature
• Pre-charge duration
• Required final bus voltage
• Main contactor closing condition
• Failure timeout
• Retry limit

Do not select a resistor using resistance alone.

It must survive the energy required to charge the inverter capacitors.

Integrated BMS products may include a built-in pre-charge circuit specifically intended to prevent high inrush current before the main contactor closes.

Prevention Step 2: Keep Inverter Loads Off During Pre-Charge

Before the battery contactor closes:

• Turn off major AC loads.
• Disable inverter output if required.
• Stop DC loads connected to the inverter bus.
• Disable heaters or accessories.
• Stop external devices that draw from the pre-charge path.

The pre-charge circuit is intended to charge capacitors—not operate the inverter or household loads.

Prevention Step 3: Verify the Pre-Charge Voltage Rise

During commissioning, measure:

• Battery voltage
• Inverter DC-bus voltage
• Time required for bus voltage to rise
• Final voltage before main contactor closure
• Pre-charge resistor temperature
• BMS pre-charge status

A healthy sequence normally shows the inverter bus rising smoothly toward battery voltage.

Possible abnormal patterns include:

Bus Voltage Does Not Rise

Potential short circuit, open pre-charge circuit or excessive load.

Bus Voltage Rises Slowly and Stops

Possible continuous load or incorrect resistor.

Bus Voltage Reaches Target but Main Contactor Does Not Close

Possible control, contactor-coil or feedback problem.

Main Contactor Closes and Immediately Opens

Possible overcurrent, communication or inverter fault.

Prevention Step 4: Use a Retry Limit

The BMS should not attempt pre-charge indefinitely.

A safe control strategy may:

• Attempt pre-charge
• Stop after a defined failure
• Generate an alarm
• Require fault investigation
• Limit automatic retries
• Require manual or remote authorized reset

Repeated clicking should be treated as a failed start—not as a normal automatic recovery.

Prevention Step 5: Coordinate the Inverter Shutdown Before the BMS Opens

Where supported, the BMS should first request that the inverter or charger stop current.

For example:

• Low-cell condition → load-disconnect command
• High-cell condition → charge-disconnect command
• Temperature condition → reduced or zero current
• Pre-alarm → load shedding

Then the main contactor can open under lower current.

Official BMS architectures use load- and charge-disconnect control for this purpose.

Opening a contactor under full battery current should be an emergency action rather than the normal daily control method.

Prevention Step 6: Add Hysteresis to Voltage and SOC Thresholds

Do not use nearly identical values for:

• Shutdown
• Recovery
• Contactor reconnect
• Inverter restart
• Generator start
• Generator stop

Example of poor control:

• Shutdown at 47.0V
• Reconnect at 47.1V

After the load disappears, battery voltage can recover enough to reconnect without receiving meaningful charging energy.

A better control method may require:

• Higher restart voltage
• Higher restart SOC
• Charging current detected
• Stable condition for a defined time
• No active BMS alarm

The exact values must follow the battery specification.

Prevention Step 7: Prevent All Heavy Loads from Restarting Together

When the battery reconnects, the following may start simultaneously:

• Inverter capacitors
• Refrigerator
• Freezer
• Water pump
• Air conditioner
• DC converter
• Network equipment
• External contactors

This can recreate the original overload immediately.

Use:

• Delayed relays
• Load-sequencing controller
• Priority contactors
• Soft starters
• SOC-based load permission
• Manual restart for heavy equipment

The battery and inverter should stabilize before large loads reconnect.

Prevention Step 8: Coordinate Every Charger

The BMS charge-disconnect request must control:

• Hybrid inverter charger
• External MPPT
• Grid charger
• Generator charger
• Wind controller
• Separate AC charger

If one charger continues operating after the BMS opens the contactor, the battery bus or inverter bus may behave unpredictably.

Independent chargers may require:

• Remote-enable input
• BMS relay
• EMS command
• External contactor
• Conservative voltage control

Prevention Step 9: Stabilize CAN and RS485 Communication

Inspect:

• Correct protocol
• Correct cable pinout
• Twisted-pair cable
• Shielding method
• Termination resistance
• Unique battery addresses
• Master battery
• Firmware compatibility
• Cable separation from high-current conductors

Intermittent communication can repeatedly change inverter permissions and BMS states.

Communication cables should not be routed tightly alongside:

• Inverter AC output cables
• High-current battery cables
• Contactors
• Variable-frequency drives
• Generator ignition wiring

Prevention Step 10: Check Contactor Feedback

Larger BMS systems may monitor whether the contactor is actually open or closed.

Possible faults include:

• Main contactor welded closed
• Contactor fails to close
• Auxiliary feedback contact incorrect
• Pre-charge contactor stuck
• Coil damaged
• Mechanical obstruction

The BMS should compare commanded state with actual feedback.

Do not bypass contactor feedback merely to clear an alarm.

Prevention Step 11: Check the Main DC Breaker and Fuse

A high-resistance breaker or fuse can cause voltage collapse when the contactor closes.

Inspect:

• Breaker DC rating
• Breaker current rating
• Fuse condition
• Terminal torque
• Heat damage
• Voltage drop
• Interrupting capacity
• Cable contact

The contactor may appear to be the problem while the actual resistance is elsewhere in the DC path.

Prevention Step 12: Monitor the Lowest Cell During Connection

Pack voltage can look normal even when one cell falls sharply as the inverter connects.

Record:

• Lowest cell voltage
• Highest cell voltage
• Cell-voltage difference
• Pack voltage
• Current
• Battery temperature

If the same cell causes every disconnect, the battery requires cell-level investigation.

Do not raise the undervoltage threshold or disable protection to hide the problem.

Diagnostic Sequence

Step 1: Stop Repeated Restart Attempts

Switch off the inverter and chargers using the approved procedure.

Step 2: Read BMS Event History

Look for:

• Pre-charge timeout
• Discharge overcurrent
• Short circuit
• Low cell voltage
• High cell voltage
• Contactor feedback fault
• Communication loss
• Auxiliary supply undervoltage

Step 3: Determine When Clicking Occurs

Does it happen:

• During initial start-up?
• At low SOC?
• Near full charge?
• When a motor starts?
• When CAN communication fails?
• During grid transfer?
• When one battery branch closes?

Step 4: Measure Battery and DC-Bus Voltage

Observe the pre-charge sequence.

Step 5: Remove External Loads

Confirm whether pre-charge succeeds with the inverter and accessories in the approved standby state.

Step 6: Test Control Power

Measure contactor-coil or BMS auxiliary voltage during the event.

Step 7: Inspect Communication

Check protocol, addresses and termination.

Step 8: Test at Moderate SOC

Avoid testing near low- or high-voltage protection thresholds.

Example: Contactor Clicks Every Five Seconds

Possible event:

• Battery voltage: 51.5V
• Inverter bus begins at 0V
• Pre-charge starts
• Bus reaches only 20V
• BMS timeout occurs
• Pre-charge opens
• Bus falls to 0V
• BMS retries

Likely areas to investigate:

• Inverter not switched off
• Active DC load
• Inverter internal fault
• Open or incorrect pre-charge resistor
• Pre-charge relay fault
• Incorrect timing
• Excessive capacitance for the design

Do not repeatedly close the main contactor manually.

Commissioning Acceptance Test

Before handover, confirm:

• Battery starts without repeated clicking.
• Pre-charge voltage rises smoothly.
• Main contactor closes once.
• Inverter starts normally.
• No contactor retry alarm occurs.
• Low-SOC shutdown is controlled.
• Restart requires adequate recovery.
• High-SOC charging does not cycle rapidly.
• Communication loss creates a defined safe response.
• Heavy loads reconnect in stages.
• All chargers follow the BMS command.
• Contactor temperature remains normal.

Customer Handover Instructions

Tell the customer:

• One or two normal clicks during controlled start-up may be equipment-specific.
• Continuous repeated clicking is not normal.
• Do not repeatedly reset the battery.
• Do not force the breaker closed.
• Record battery and inverter alarms.
• Switch off large loads.
• Contact technical support.
• Do not open the battery cabinet.

Provide instructions for a safe controlled shutdown.

Common Prevention Mistakes

Increasing the Pre-Charge Time Without Finding the Cause

A short circuit or active load may remain.

Replacing the Main Contactor First

The real fault may be the resistor, auxiliary supply or inverter.

Allowing Unlimited Automatic Retries

Contacts and resistors can overheat.

Restarting All Loads Immediately

The battery trips again.

Using BMS Contactors as Daily Load Switches

The inverter should normally respond to controlled current commands first.

Ignoring External Chargers

One uncontrolled charger can keep the system cycling.

Raising Cell-Voltage Limits

This hides protection events rather than solving them.

Assuming Every Click Is a Failed Contactor

The contactor may be responding correctly to another fault.

Frequently Asked Questions

Is one click when the battery starts normal?

Many contactor-based batteries make a limited number of clicks during pre-charge and main-contactor closing. Continuous repeated clicking requires investigation.

Why does the battery click only when connected to the inverter?

The inverter’s DC capacitors, start-up load or an internal fault may be preventing successful pre-charge.

Can a larger battery solve the problem?

It may help when insufficient current capability is the cause, but it will not correct a failed pre-charge circuit or short circuit.

Can I bypass the pre-charge resistor?

No. Directly closing the main contactor can create damaging inrush current.

Why does clicking happen only at low SOC?

Voltage sag or one low cell may cause BMS disconnect and rapid voltage recovery.

Why does it happen near 100% SOC?

One high cell or an uncontrolled charger may repeatedly trigger charge disconnection.

Should I replace the contactor after repeated chattering?

The contactor should be inspected because repeated operation can damage its contacts, but the original control problem must also be corrected.

Conclusion

BMS contactor chattering is a symptom of unstable system control, not a normal operating mode.

Preventing it requires:

• Correctly engineered pre-charge
• No active loads during pre-charge
• Stable auxiliary control power
• Limited automatic retries
• Coordinated BMS and inverter shutdown
• Adequate voltage and SOC hysteresis
• Staged load reconnection
• Control of every charging source
• Reliable CAN or RS485 communication
• Contactor feedback monitoring
• Cell-level voltage analysis
• Full start-up and recovery testing

For HIZN Lithium technical analysis, provide:

• Battery model and voltage
• Battery quantity
• BMS model
• Inverter brand and model
• Time between contactor clicks
• SOC when the event occurs
• Highest and lowest cell voltage
• BMS event log
• Inverter error code
• Video of the start-up process
• Pre-charge settings
• DC-bus voltage measurements

This information helps determine whether the repeated clicking is caused by pre-charge, inverter load, cell protection, communication or contactor control.