Introduction

A solar energy storage system may operate normally with lights, televisions and phone chargers but suddenly shut down when a refrigerator, water pump, air conditioner or power tool starts.

Customers often report:

“The inverter is rated at 5kW, but it switches off when I start a 1.5kW pump.”

This does not necessarily mean that the appliance consumes more than 5kW continuously.

Many appliances require a much higher short-duration current during start-up. At the same time, the LiFePO4 battery, BMS, cables and DC breaker must deliver the corresponding surge power to the inverter.

The system can shut down even when the inverter’s continuous AC rating appears sufficient.

The Difference Between Running Power and Starting Power

Electrical appliances can be divided into two general groups.

Resistive Loads

Examples include:

• Electric heaters
• Kettles
• Incandescent lamps
• Simple heating elements

Their starting power is usually close to their running power.

Motor and Compressor Loads

Examples include:

• Refrigerators
• Freezers
• Air conditioners
• Water pumps
• Air compressors
• Washing machines
• Power tools

These appliances may require a high current while the motor accelerates.

The start-up surge may be several times the normal running demand, although the exact ratio depends on:

• Motor type
• Compressor pressure
• Starting method
• Appliance condition
• Supply voltage
• Inverter waveform
• Ambient temperature

Therefore, appliance nameplate wattage does not always represent its maximum instantaneous demand.

The Inverter Rating Is Only One Part of the System

A complete battery-powered system contains several current limits:

1. Inverter continuous-power rating
2. Inverter surge-power rating
3. Battery continuous discharge rating
4. Battery BMS peak-current rating
5. Battery quantity
6. DC cable current capacity
7. Breaker and fuse rating
8. Busbar rating
9. Connector current capacity

The smallest limit determines what the system can actually deliver.

An inverter rated for a high surge cannot use that capability when the battery system cannot supply the required DC current.

Official inverter guidance emphasizes that the battery system must be capable of delivering both the required continuous and surge DC current.

How Much Battery Current Does an Inverter Need?

Battery current can be approximated using:

Battery Current = AC Load Power ÷ Battery Voltage ÷ Inverter Efficiency

Consider a 51.2V battery system powering a 5,000W AC load.

Assuming 93% inverter efficiency:

5,000 ÷ 51.2 ÷ 0.93 ≈ 105A

The battery must therefore provide approximately 105A under these assumed conditions.

If the load briefly reaches 8,000W during motor start-up:

8,000 ÷ 51.2 ÷ 0.93 ≈ 168A

A single battery with a 100A continuous-discharge BMS may therefore disconnect, even though the inverter itself is rated for the load.

The exact figures depend on actual battery voltage and inverter efficiency.

Example: 51.2V 100Ah Battery with a 5kW Inverter

Suppose the system contains:

• One 51.2V 100Ah battery
• 100A continuous-discharge BMS
• 5kW inverter
• Refrigerator
• Water pump
• Household lighting

At 5kW output, the inverter may require more than 100A from the battery.

Possible results include:

• BMS overcurrent alarm
• Battery contactor opening
• Inverter low-battery alarm
• Sudden AC power loss
• Automatic restart after several seconds
• Repeated on-off cycling

Adding battery capacity may be necessary not only to extend backup time but also to increase the available discharge-current capability.

However, parallel battery expansion must be approved by the battery manufacturer and installed with balanced branch cables and protection.

Cause 1: BMS Overcurrent Protection

The BMS monitors battery discharge current.

When current exceeds its programmed limit, it may:

• Issue a warning
• Reduce the permitted current through communication
• Open internal MOSFETs
• Open a contactor
• Disable discharge temporarily
• Require a manual reset

Check the BMS alarm history for terms such as:

• Discharge overcurrent
• Short-circuit protection
• Peak-current timeout
• Contactor open
• Discharge MOS off
• Pack overload

Do not automatically increase the BMS current setting. The limit may be based on:

• Cell capability
• Internal busbars
• MOSFET rating
• Contactor rating
• Terminal design
• Cable rating
• Product certification

Cause 2: Inverter Low-Voltage Shutdown

The battery may not trip first. Instead, the inverter may see its DC input voltage fall below the configured low-voltage limit.

Voltage at the inverter can fall because of:

• High load current
• Low battery SOC
• Battery internal resistance
• Undersized cables
• Long cable distance
• Loose terminals
• Damaged breaker
• Poor fuse-holder contact

The inverter then shuts down to protect itself and the battery.

After the load disappears, battery voltage recovers. This can make the customer believe that the battery still has plenty of energy.

Why Cable Voltage Drop Becomes Worse During Starting Surge

Voltage drop follows the relationship:

Voltage Drop = Current × Resistance

As current increases, voltage drop increases.

For example, a connection with 0.01Ω total resistance produces:

• 0.5V drop at 50A
• 1.0V drop at 100A
• 2.0V drop at 200A

Resistance comes from the complete current path, including:

• Positive cable
• Negative cable
• Cable lugs
• Battery terminals
• Breakers
• Fuses
• Busbars
• Connectors

Official DC-wiring guidance notes that cable voltage drop becomes especially important during high-current events and can cause the inverter to receive a lower voltage than is present at the battery.

How to Test for Cable Voltage Drop

Use a correctly rated multimeter.

During a controlled load test, measure:

1. Voltage directly at the battery terminals.
2. Voltage at the inverter DC terminals.
3. Current from the battery.
4. Voltage while the motor starts.

Example:

• Battery voltage during start: 50.8V
• Inverter terminal voltage during start: 46.5V
• Difference: 4.3V

This indicates excessive resistance between the battery and inverter.

Possible corrective actions include:

• Increase cable cross-sectional area.
• Reduce cable length.
• Replace damaged lugs.
• Recrimp poor connections.
• Replace overheated breakers.
• Clean busbar contact surfaces.
• Tighten terminals to the specified torque.

Cause 3: The Battery SOC Is Too Low

A battery at 20% SOC may still operate light loads normally.

However, when a motor starts:

• Voltage may fall more sharply.
• Available BMS peak current may be reduced.
• One weak cell may reach its undervoltage limit.
• The inverter may reach its cut-off voltage.

Therefore, a system that starts a pump successfully at 90% SOC may fail at 20% SOC.

The test should be repeated with a properly charged battery before concluding that the equipment is defective.

Cause 4: One Cell Reaches Low Voltage First

The inverter sees total pack voltage, but the BMS monitors individual cells.

During a high-current event, one cell may fall below the BMS low-voltage threshold before the overall pack appears empty.

Possible causes include:

• Cell imbalance
• Weak cell
• Poor internal connection
• Low temperature
• High discharge rate
• Incomplete previous charging

Check BMS data for:

• Minimum cell voltage
• Maximum cell voltage
• Cell-voltage difference
• Which cell triggered protection

If the same cell repeatedly reaches the limit first, the battery requires further analysis.

Cause 5: The DC Breaker Is Undersized

The inverter may require high current for several seconds while a motor starts.

A breaker selected too close to normal operating current may trip during this period.

Check:

• Breaker continuous current
• Trip curve
• DC voltage rating
• Installation temperature
• Cable ampacity
• Inverter surge current
• Manufacturer recommendation

Do not install a larger breaker without checking cable size. The purpose of the breaker is to protect the wiring.

Cause 6: The Inverter Surge Rating Is Insufficient

Two inverters with the same continuous rating may have different surge capabilities.

Compare:

• Maximum surge power
• Permitted surge duration
• Load type
• Motor-starting capability
• Low-voltage behaviour
• Power-factor limits

An inverter advertised as 5kW may support:

• A limited overload for a few seconds
• A higher overload for milliseconds
• No meaningful motor-starting overload

The inverter manual must be checked for the specific model.

Cause 7: Several Appliances Start at the Same Time

A system may fail because multiple loads overlap.

For example:

• Refrigerator compressor starts.
• Water pump starts.
• Air conditioner restarts.
• Inverter begins charging another appliance.
• Electric kettle is already operating.

The combined instantaneous load can exceed the system limit.

Use inverter monitoring data or a power logger to identify peak demand.

Solutions may include:

• Start heavy loads at different times.
• Use programmable load control.
• Install soft starters where appropriate.
• Use variable-frequency drives for suitable motors.
• Increase battery quantity.
• Upgrade the inverter.
• Separate critical and non-critical loads.

Cause 8: Battery and Inverter Current Limits Are Not Coordinated

In closed-loop systems, the battery communicates its permitted discharge current to the inverter.

The inverter should respect this value.

Problems may occur when:

• The wrong battery protocol is selected.
• Communication is lost.
• The inverter ignores the battery limit.
• Battery quantity is entered incorrectly.
• Master-slave battery configuration is incorrect.
• Firmware versions are incompatible.

Check whether the inverter displays:

• Battery discharge-current limit
• Battery communication status
• Number of battery modules
• Battery alarm state

A Step-by-Step Troubleshooting Procedure

Step 1: Identify the Exact Load

Record:

• Appliance type
• Running wattage
• Starting wattage if available
• Whether it contains a motor or compressor
• Whether other loads are operating

Step 2: Read the Error Codes

Check both:

• Inverter error history
• Battery BMS alarm history

The inverter may show low voltage while the battery records overcurrent. Both records are useful.

Step 3: Charge the Battery

Repeat the test at a high SOC under normal temperature conditions.

Step 4: Test with a Smaller Load

Run:

• Lighting only
• One resistive load
• The problem appliance alone
• Several loads together

This helps determine whether the failure is power-related.

Step 5: Measure DC Voltage

Measure battery and inverter-terminal voltage during the load start.

Step 6: Measure Current

Use a DC clamp meter capable of capturing peak current where possible.

Step 7: Inspect the DC Path

Check cables, lugs, busbars, fuse holders and breakers for heat or discolouration.

Step 8: Compare System Ratings

Compare:

• Inverter continuous power
• Inverter surge power
• Battery BMS continuous current
• Battery BMS peak current
• Breaker rating
• Cable ampacity
• Battery quantity

Step 9: Review Communication

Confirm that the inverter is reading the correct battery discharge limit.

Should You Add Another Battery?

Adding a compatible battery in parallel may help when the existing bank is limited by:

• Continuous discharge current
• Peak discharge current
• Excessive current per module
• Insufficient energy capacity
• Voltage sag

For example, two identical batteries can share the load, reducing the current drawn from each unit.

However, expansion is appropriate only when:

• The battery model supports parallel operation.
• Battery voltages are matched.
• BMS firmware is compatible.
• Cables are balanced.
• Each branch is protected.
• Busbars and main cables are correctly rated.

Adding another battery will not solve:

• A defective inverter
• Incorrect AC wiring
• Reverse polarity
• A damaged breaker
• Severe cable voltage drop
• An incompatible battery protocol

Practical Customer Example

System:

• 25.6V 150Ah LiFePO4 battery
• 100A BMS
• 1,500W inverter
• 600W hair dryer

Approximate inverter DC current at 600W may be moderate, so the system should normally support the running load if wiring and battery condition are correct.

However, if the system fails only during inverter start-up and not when the hair dryer is running, the likely issue may be:

• Inverter capacitor inrush
• BMS short-circuit sensitivity
• Incorrect start-up sequence
• Inadequate pre-charge

This is different from a true continuous-load overload.

The timing of the shutdown is therefore important:

• Trips when inverter is switched on: investigate inrush and pre-charge.
• Trips when appliance starts: investigate surge power and voltage drop.
• Trips after several minutes: investigate continuous current, heat and battery capacity.

Frequently Asked Questions

Why can the inverter run lights but not a refrigerator?

Lighting has relatively low and predictable demand. A refrigerator compressor requires higher short-duration starting current.

Why does the inverter show low battery when the battery display shows 60%?

High current and cable resistance can make inverter-terminal voltage fall below the cut-off threshold even when BMS SOC remains above zero.

Can I reduce the inverter low-voltage cut-off?

Only within the battery manufacturer’s approved range. Setting it too low can cause repeated BMS undervoltage protection.

Will a larger cable solve the problem?

It will help only when cable resistance is a significant cause. Battery BMS and inverter limits must also be checked.

Can a soft starter help?

For suitable motors, a correctly selected soft starter or variable-frequency drive may reduce starting demand. It is not suitable for every appliance.

Why does the battery restart automatically?

Some BMS units automatically recover after the overload disappears. Repeated automatic recovery does not mean that the overload should be ignored.

Conclusion

When an inverter shuts down as a refrigerator, pump or air conditioner starts, check the complete power path—not only the inverter’s wattage rating.

The main causes include:

• Appliance starting surge
• Insufficient inverter surge capability
• BMS overcurrent protection
• Battery bank too small
• Low battery SOC
• Cell undervoltage
• Cable voltage drop
• Undersized breaker
• Loose connections
• Incorrect communication limits

For correct battery selection, provide HIZN Lithium with:

• Inverter model
• Continuous and surge power
• Appliance list
• Largest motor or compressor
• Battery voltage
• Required backup time
• Battery cable length
• Installation temperature

This allows the battery capacity, BMS current and inverter requirements to be matched before installation.