Introduction
An inverter may shut down because the battery voltage is too low, restart several seconds or minutes later, and then shut down again as soon as the load is applied.
This cycle may repeat several times:
- Battery voltage falls under load.
- The inverter shuts down.
- The load is removed.
- Battery voltage recovers.
- The inverter reaches its restart threshold.
- The load reconnects.
- Battery voltage collapses again.
This behavior is sometimes called:
- Restart cycling
- Low-voltage restart loop
- Inverter hunting
- On-off cycling
- Repeated low-battery shutdown
Some inverter manufacturers program automatic low-voltage restart behavior. For example, Victron documents that certain inverter models restart after battery voltage rises above the configured restart threshold, but stop attempting after several rapid low-voltage shutdowns.
The restart function itself may be operating correctly. The real problem is that the battery or DC system cannot maintain sufficient voltage after the inverter reconnects the load.
Why Battery Voltage Recovers After Shutdown
When the inverter is operating, current flows from the battery.
The battery-terminal voltage under load is affected by:
- State of charge
- Internal resistance
- Discharge current
- Battery temperature
- Cable resistance
- Connection resistance
When the inverter shuts down, current drops sharply. The voltage loss caused by internal resistance and cable resistance disappears, so the measured battery voltage rises.
This voltage recovery does not mean the battery has recharged.
It only means the battery is no longer supporting the load.
If the recovered voltage exceeds the inverter’s restart setting, the inverter starts again. Once current returns, voltage falls and the shutdown repeats.
Reason 1: The Restart Voltage Is Too Close to the Shutdown Voltage
The inverter should have sufficient separation between:
- Low-voltage shutdown threshold
- Low-voltage restart threshold
This separation is called hysteresis.
If shutdown occurs at 44V and restart occurs at 44.5V, the battery may recover above the restart threshold almost immediately after the load is removed.
A larger separation gives the system time to receive meaningful charging before the inverter restarts.
Victron recommends setting restart voltage sufficiently above shutdown voltage to prevent rapid fluctuation between shutdown and startup.
The correct values depend on:
- Battery type
- Battery-bank voltage
- Load current
- Cable voltage drop
- Required depth of discharge
- Inverter manufacturer
- Battery manufacturer
Do not use arbitrary shutdown and restart values without considering loaded battery voltage.
Reason 2: The Battery Is Deeply Discharged
A deeply discharged battery may show partial voltage recovery after the load is removed but still contain very little usable energy.
When the inverter restarts:
- The battery current rises.
- Voltage falls immediately.
- The low-voltage threshold is reached.
- The inverter shuts down again.
The correct solution is usually to recharge the battery, not repeatedly reset the inverter.
Repeated low-voltage cycling can:
- Deepen the discharge
- Increase sulfation risk
- Stress contactors and relays
- Interrupt sensitive loads
- Create repeated motor startup surges
- Make troubleshooting more difficult
Reason 3: The Battery Has Lost Capacity
An aged battery may reach normal resting voltage but have much less usable capacity than its nameplate rating.
For example, a battery bank originally rated at 400Ah may now deliver only 220Ah under the required discharge conditions.
The inverter’s settings may have been acceptable when the batteries were new, but the aged battery bank now reaches low voltage much earlier.
Common signs include:
- Normal voltage after charging
- Short runtime
- Rapid voltage drop under load
- Fast voltage recovery after shutdown
- Frequent restart cycling
- One battery falling lower than the others
A capacity test should be performed before changing inverter settings.
Reason 4: One Battery in the Series String Is Weak
A 48V battery bank assembled from four 12V batteries may have one weak battery.
When the inverter is off, the total voltage may appear acceptable. Under load, the weak battery falls rapidly and causes the complete string voltage to reach the shutdown threshold.
After shutdown, the weak battery voltage partially recovers, allowing the inverter to restart.
Measure every battery:
- Before startup
- During inverter operation
- Immediately before shutdown
- Immediately after shutdown
The battery with the largest loaded voltage drop requires further testing.
Do not rely only on total bank voltage.
Reason 5: The Battery Bank Is Too Small for the Load
A battery bank may have sufficient nominal energy but insufficient power capability for the connected load.
The problem is especially common with:
- High-power inverters
- Water pumps
- Refrigerators
- Air conditioners
- Compressors
- Power tools
- Heating loads
- Transformers
The inverter starts, the load demands high current, and battery voltage falls below the shutdown threshold.
The approximate battery current is:
Battery Current = AC Load Power ÷ Battery Voltage ÷ Inverter Efficiency
For a 3,000W load on a 24V battery bank at 90% efficiency:
3,000 ÷ 24 ÷ 0.90 = approximately 139A
If a motor briefly requires twice its normal power, the battery current may rise much higher.
A small or aged battery bank may not maintain voltage at that current.
Reason 6: Motor or Compressor Loads Restart Automatically
After the inverter restores AC output, appliances may restart automatically.
Examples include:
- Refrigerators
- Freezers
- Water pumps
- Air compressors
- Air conditioners
- Pressure pumps
Their startup current may be several times greater than their running current.
The sequence becomes:
- Inverter reaches restart voltage.
- AC output returns.
- Compressor or motor attempts to start.
- Battery current surges.
- Battery voltage collapses.
- Inverter shuts down.
- Motor stops.
- Voltage recovers.
- Cycle repeats.
Victron notes that motor and pump startup current can exceed an inverter’s overcurrent threshold and trigger shutdown followed by automatic restart attempts.
Reason 7: Cable Voltage Drop Is Triggering Shutdown
The inverter responds to voltage at its DC terminals.
If cables or connections have excessive resistance, inverter voltage may be much lower than battery-terminal voltage.
For example:
- Battery voltage under load: 47.5V
- Inverter DC input: 44.5V
- Inverter shutdown setting: 45V
The battery itself may remain above the threshold, but cable loss causes the inverter to shut down.
Possible causes include:
- Undersized battery cables
- Excessively long cables
- Loose terminals
- Corroded connectors
- Poor cable-lug crimps
- High-resistance fuse holders
- Damaged breakers
- Overloaded busbars
Measure voltage simultaneously at the battery and inverter during startup and immediately before shutdown.
Reason 8: A Loose Connection Changes Resistance as It Heats
A poor connection may operate briefly when cool but develop more voltage drop as current and temperature rise.
The inverter may therefore start normally, run for a short period, and then shut down.
After shutdown:
- Current stops.
- The connection cools.
- Voltage recovers.
- The inverter restarts.
- The connection heats again.
Inspect:
- Battery terminals
- Inter-battery connectors
- Main cable lugs
- Fuse holders
- Breaker terminals
- Busbar bolts
- Inverter DC terminals
Use a thermal camera or non-contact thermometer to compare connection temperatures.
Reason 9: Parallel Battery Strings Are Not Sharing Current Equally
In a multi-string battery bank, one string may carry most of the inverter current.
The heavily loaded string experiences:
- Greater voltage drop
- Faster discharge
- More heating
- Earlier protection operation
The complete bank may have adequate nominal capacity, but poor current sharing prevents it from supplying the load correctly.
Measure current in each parallel string during inverter startup and steady operation.
Check for:
- Unequal cable lengths
- Different cable sizes
- Mixed battery ages
- Different battery models
- Loose connections
- Incorrect busbar arrangement
- Blown string fuses
Reason 10: The Charger Raises Voltage but Supplies Too Little Current
A small solar panel or charger can raise battery voltage above the inverter’s restart threshold without providing enough energy to support the load.
For example:
- Charger output: 5A
- Inverter idle current: 2A
- Connected load demand: 50A
The battery voltage slowly rises while the inverter is off. Once the inverter restarts and the load reconnects, demand greatly exceeds the charging current, causing another shutdown.
This commonly occurs:
- Early in the morning
- During cloudy weather
- With a small maintenance charger
- When the generator charger is current-limited
- When solar panels are shaded
The restart condition should indicate meaningful battery recovery, not only a temporary voltage rise.
Reason 11: Restart Is Based Only on Voltage
Voltage-based restart logic cannot always distinguish between:
- A battery that has been genuinely recharged
- A weak battery recovering after load removal
- Surface charge
- A small charger raising terminal voltage
- High-resistance battery behavior
More advanced systems may allow restart based on:
- Battery SOC
- Charge-detect voltage
- Minimum charging time
- External battery monitor
- Generator availability
- Remote control signal
Victron documents separate low-battery restart and charge-detect settings on some inverter models.
Where supported, using charge detection or a reliable shunt-based SOC signal can reduce unnecessary restart cycling.
Reason 12: Inverter Search Mode Is Being Confused with Restart Cycling
Some inverters use a power-saving or search mode.
The inverter periodically checks whether an AC load is present. This can cause the output to pulse or switch between low-power and active operation.
This is different from low-voltage restart cycling.
Check the inverter event log or alarm indicators for:
- Low battery
- Overload
- High temperature
- Search mode
- AC short circuit
- Communication fault
Do not assume every on-off pattern is caused by the battery.
Reason 13: The Inverter Is Overheating
An inverter may shut down because of high temperature and restart after cooling.
This can look similar to a battery-related restart loop.
Check:
- Inverter ventilation
- Cooling fans
- Dust accumulation
- Ambient temperature
- Installation clearance
- Continuous load
- Direct sunlight
Compare the alarm code with battery voltage during shutdown.
If battery voltage remains stable while inverter temperature is high, the cause may be thermal rather than low voltage.
Reason 14: High DC Ripple or Unstable Charging Voltage
An unstable charger, generator, or poorly configured hybrid system can cause fluctuating DC voltage.
The inverter may alternate between:
- Low-voltage shutdown
- Restart
- High-voltage shutdown
- Charge detection
Check whether the problem occurs only when:
- The generator is running
- The grid charger is active
- A particular solar controller is connected
- Several chargers are operating together
Record minimum and maximum battery voltage rather than relying only on an average reading.
A Typical Restart-Loop Example
Assume a 48V lead-acid system has:
- Low-voltage shutdown: 44V
- Restart voltage: 46V
- Battery voltage under load: 43.8V
- Battery voltage after shutdown: 46.3V
Sequence:
- Inverter shuts down at 44V.
- Load current falls to nearly zero.
- Battery voltage recovers to 46.3V.
- Inverter restarts at 46V.
- Load current returns.
- Battery voltage falls to 43.8V.
- Inverter shuts down again.
The recovered voltage exceeds the restart setting, but the battery has not received additional charge.
Possible corrections include:
- Recharge the battery
- Increase the restart threshold appropriately
- Require charge detection before restart
- Reduce the load
- Replace weak batteries
- Correct cable voltage drop
- Increase battery-bank capacity
The correct solution depends on the measured cause.
Practical Troubleshooting Procedure
Step 1: Read the Inverter Alarm History
Determine whether shutdown was caused by:
- Low battery
- Overload
- High temperature
- Short circuit
- High battery voltage
- Communication fault
Step 2: Disconnect Major AC Loads
Start the inverter with no external loads.
If it remains operating, reconnect loads one at a time.
Step 3: Measure Battery Voltage During the Entire Cycle
Record:
- Voltage before startup
- Minimum voltage during startup
- Voltage immediately before shutdown
- Recovery voltage after shutdown
- Restart voltage
Step 4: Measure at Two Locations
Measure voltage at:
- Battery-bank terminals
- Inverter DC terminals
A large difference indicates cable or connection resistance.
Step 5: Measure Individual Batteries
Identify one battery that falls more quickly than the others.
Step 6: Measure DC Current
Compare startup and continuous current with:
- Battery discharge capability
- Cable rating
- Breaker rating
- Inverter specification
Step 7: Check Restart Settings
Review:
- Low-voltage alarm
- Shutdown voltage
- Restart voltage
- Charge-detect voltage
- Restart delay
- Dynamic cutoff
- SOC-based shutdown
Step 8: Check Charging Current
Confirm that solar, grid, or generator charging supplies enough net current to recover the battery before restart.
Step 9: Perform a Capacity Test
A weak battery bank may require replacement rather than setting adjustments.
How to Stop the Restart Loop Safely
Depending on the cause, appropriate actions may include:
- Switch off the inverter until the battery is recharged.
- Disconnect non-essential loads.
- Increase restart hysteresis within approved limits.
- Use charge-detect logic.
- Delay automatic load reconnection.
- Install sequential motor-start controls.
- Correct cable and terminal problems.
- Replace a weak battery string.
- Increase battery capacity.
- Increase charging capacity.
- Adjust low-voltage settings according to battery data.
Do not simply lower the shutdown voltage. This may increase usable time temporarily but can cause damaging deep discharge.
Common Mistakes
Repeatedly Resetting the Inverter
This does not restore battery capacity and may deepen the discharge.
Lowering the Shutdown Voltage
The battery may be damaged if operated below its recommended discharge limit.
Increasing the Breaker Size
A restart loop is not usually corrected by a larger breaker.
Assuming Voltage Recovery Means Recharging
Voltage can recover when load current is removed without any energy entering the battery.
Ignoring Automatic Motor Restart
A compressor or pump may be causing every repeated shutdown.
Replacing the Inverter Before Testing the Battery
The inverter may be responding correctly to unstable DC voltage.
Frequently Asked Questions
Why does the inverter restart when the battery has not been charged?
Battery voltage rises after the load is removed. If it exceeds the restart threshold, the inverter may restart automatically.
Can I disable automatic restart?
Some inverters allow restart settings or remote-control behavior to be changed. Follow the inverter manufacturer’s instructions.
Why does the inverter run with no load but shut down with appliances?
The battery or DC circuit may maintain voltage at low current but collapse when the load current increases.
Can a weak battery show normal voltage?
Yes. A weak battery may show normal resting voltage but experience severe voltage drop under load.
Should restart voltage be higher?
It may need greater separation from shutdown voltage, but it must remain compatible with the battery and inverter manufacturer’s recommendations.
Can a generator stop the restart loop?
A properly sized generator and charger may restore the battery, but a small charger may only raise voltage temporarily without supplying sufficient net current.
Conclusion
An inverter that repeatedly restarts after a low-battery shutdown is usually responding to battery voltage recovery rather than true battery recharging.
The most common causes are:
- Restart voltage too close to shutdown voltage
- Deeply discharged batteries
- Reduced battery capacity
- A weak battery in the series string
- Excessive startup load
- Cable voltage drop
- Loose connections
- Unequal parallel-string current
- Insufficient charging current
- Voltage-only restart logic
For a project-specific diagnosis, provide the inverter model, battery type, bank voltage and capacity, shutdown and restart settings, connected loads, cable size and length, minimum startup voltage, individual battery voltages, and charging current.
