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Analysis by Marcus & Team

SolarEdge Error 18x79 Internal Battery Fuse Blown

Diagnose and fix SolarEdge Error 18x79 (Internal Battery Fuse Blown) in residential hybrid inverter systems. Understand causes, solutions, and safety measures.

Frequently Asked Questions

•What are the common causes of a blown fuse in a solar inverter?

Overload, short circuit, surge events, and component failure are common causes.

•How can I prevent future fuse failures in my SolarEdge system?

Regular monitoring, firmware updates, proper ventilation, and professional maintenance can help prevent fuse failures.

•Is it safe for me to replace the fuse myself?

No. Working on electrical equipment with high voltages carries a risk of electric shock and arc flash. A qualified electrician should perform this task.

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Marcus leads the technical research department at Storage Faults. Together with a team of systems analysts, he specializes in translating complex solar storage error codes into actionable diagnostic steps for the US market.

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Technical Background:

The SolarEdge hybrid inverter integrates a DC/DC converter for battery charging/discharging, a DC/AC inverter for grid interaction, and a battery management system (BMS). The battery fuse protects the battery pack from overcurrent conditions during charging, discharging, or internal faults. The BMS monitors battery voltage, current, temperature, and SOC, and can trigger fuse blowing if safety limits are exceeded. AC coupling allows for existing grid-tie solar to charge the battery. Rapid shutdown (RSD) is critical for first responder safety.

Symptoms:

The SolarEdge hybrid inverter displays error code 18x79, indicating a blown internal battery fuse. The battery backup functionality is disabled. The system may still operate in grid-tie mode (solar power directly powering loads and exporting to the grid) if sufficient solar power is available.

Possible Causes:

Battery overcurrent: Excessive discharge current drawn from the battery due to high load demand exceeding inverter capacity.
Battery overcharge: Overcharging the battery beyond its maximum voltage limit due to BMS malfunction or incorrect charging parameters.
Internal battery fault: Short circuit within the battery pack due to cell failure or insulation breakdown.
BMS malfunction: Incorrect current/voltage readings by the BMS leading to false overcurrent/overvoltage detection.
Surge event: A transient voltage surge on the DC bus or battery terminals damaging the fuse.
Fuse degradation: Gradual weakening of the fuse element due to thermal cycling or aging.

Solution:

1

*Safety First:** Disconnect the AC power to the inverter using the main AC disconnect. Activate the Rapid Shutdown (RSD) system to de-energize the PV array DC conductors (NEC 690.12). Verify voltage is near 0V using a calibrated multimeter.
2

*DC Disconnect:** Open the DC disconnect switch located between the PV array and the inverter, and the DC disconnect between the battery and the inverter. Verify zero voltage on both sides of each disconnect using a calibrated multimeter.
3

*Visual inspection:** Carefully inspect the battery compartment (if accessible per manufacturer instructions) for any signs of physical damage, overheating, or electrolyte leakage. DO NOT open sealed battery compartments.
4

*Fuse Identification:** Consult the SolarEdge inverter's service manual to identify the exact location and specifications (voltage, current rating) of the internal battery fuse.
5

*Fuse Testing:** After verifying zero voltage, carefully access the fuse (qualified electrician only). Use a multimeter to test the fuse for continuity. If the fuse is blown, replace it with an exact replacement (same voltage and current rating) from SolarEdge or an authorized distributor.
6

*Troubleshooting (if fuse blows again):** If the replacement fuse blows immediately, a more serious underlying problem exists, such as an internal battery short or a faulty BMS. Further diagnostics are required.
7

*System Restart:** After replacing the fuse (or resolving the underlying issue), reconnect the DC disconnects, then the AC disconnect. Monitor the inverter's behavior and check for error codes. Verify battery charging and discharging functionality through the monitoring app.

Safety Warning:

SolarEdge hybrid inverters operate with high DC voltages (up to 600V or higher). Arc flash hazards exist. Never attempt to repair the inverter without proper training, personal protective equipment (PPE), and adherence to NEC guidelines. Always de-energize the system using the AC and DC disconnects and verify zero voltage before working on any components. Engage a qualified electrician for all repairs and maintenance.

Prevention:

• Regularly monitor the inverter's performance and battery status through the SolarEdge monitoring app.
• Ensure the battery's state of charge (SOC) remains within the recommended range (typically 20-80%) to prolong its lifespan.
• Schedule periodic system maintenance checks with a qualified electrician, including battery capacity testing and thermal monitoring.
• Keep the inverter's firmware updated to the latest version to ensure optimal performance and bug fixes. Review all BMS parameters annually.
• Ensure proper ventilation around the inverter to prevent overheating.

Expert Tip:

Access the SolarEdge service menu (refer to the service manual for the key combination – often requires pressing specific buttons in a sequence) to view detailed battery parameters, error logs, and BMS diagnostic data. This can provide valuable insights into the cause of the fuse failure. Note: Modifying settings in the service menu without proper knowledge can damage the system.