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Analysis by Marcus & Team

Inverter: Battery Discharging During Solar Production

Troubleshoot why your solar battery discharges during daylight hours when it should be charging. Potential causes and solutions for residential hybrid inverters.

Frequently Asked Questions

•What is the typical lifespan of a solar battery in a residential system?

The lifespan varies depending on the battery type (e.g., lithium-ion, lead-acid) and usage patterns. Lithium-ion batteries typically last 10-15 years with proper maintenance. Monitor the battery's SOH regularly.

•How often should I have my solar and battery system inspected by a professional?

Annual inspections by a qualified electrician are recommended to ensure system safety and optimal performance. More frequent inspections may be necessary in harsh environments.

•What are the signs of a failing solar battery?

Reduced capacity, faster discharge rates, inconsistent SOC readings, overheating, and error messages from the inverter or BMS are all signs of a failing solar battery.

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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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Safety Warning:

This diagnostic procedure involves working with high voltage DC circuits. Improper handling can result in severe electrical shock, arc flash, or thermal burns. ALWAYS de-energize the system by disconnecting the DC disconnect switch and following the manufacturer's safety procedures. Only qualified electricians with appropriate PPE (personal protective equipment) should perform these tasks. Compliance with NEC Article 690 (Solar Photovoltaic Systems) and local electrical codes is mandatory. Rapid shutdown systems MUST be verified to be functional before any work begins.

Recommended Action:

1

Using a clamp meter, measure the AC current draw of the house loads and compare it to the solar production current (DC converted to equivalent AC). Verify if load exceeds production.
2

Access the inverter's configuration settings (requires qualified electrician). Check the battery charging parameters (voltage, current limits) and ensure they are within the manufacturer's specifications.
3

Review the inverter's operational mode. Confirm that the system is configured to prioritize battery charging when solar power is available. Disable any grid export settings temporarily for diagnostic purposes.
4

Inspect the battery connections for corrosion or loose wiring (DC disconnect required first per NEC 690). Measure the battery voltage at the terminals and compare it to the BMS reading. Discrepancies indicate a wiring or sensor fault.
5

Conduct a battery capacity test (requires specialized equipment and a qualified technician). Measure the actual discharge capacity and compare it to the rated capacity. Significant deviation indicates battery degradation.
6

Using an IR thermometer, check for hotspots on the battery terminals, inverter components (especially DC-DC converter), and wiring. Elevated temperatures indicate excessive resistance or component failure. Refer to manufacturer specifications for safe operating temperatures.
7

Consult the inverter's error logs for any fault codes related to battery charging, BMS communication, or current sensor errors. Contact the manufacturer's technical support for assistance.

Possible Causes:

High AC load consumption exceeding solar production, causing the inverter to draw power from the battery to supplement the solar input.
Incorrect inverter settings prioritizing grid export over battery charging.
Faulty current sensor in the inverter or BMS, misreporting battery charge/discharge current.
Parasitic loads within the battery or inverter consuming power even when the system is idle.
Degraded battery cells with reduced capacity and increased self-discharge rate. Lowered SOH triggers premature discharge.
Communication errors between the inverter and BMS, leading to incorrect charging commands.

Observed Symptoms:

The battery state of charge (SOC) decreases during daylight hours while the solar array is actively producing power, as indicated by the inverter's monitoring app or display.

Technical Background:

Hybrid inverters manage power flow between the solar array, battery, grid, and house loads. DC power from the solar array is converted to AC for household use or grid export. Excess DC power charges the battery via a DC-DC converter controlled by the Battery Management System (BMS). The BMS monitors cell voltages, current, and temperature to optimize charging and discharging while preventing overcharge/over-discharge. Unexpected discharge indicates a fault in this energy management process. Anti-islanding protection is crucial for grid safety; the inverter must cease operation if grid voltage or frequency deviates from acceptable limits. Rapid shutdown functionality, compliant with NEC 2017/2020, is essential for first responder safety.

Pro Tip:

Many inverters have a "forced charge" or "equalization charge" mode accessible through the service menu (refer to the manufacturer's service manual for key combinations). Activating this mode can temporarily override the normal charging parameters and help diagnose battery charging issues. Monitor the battery voltage and current closely during this process to avoid overcharging.

Maintenance Tips:

Regularly update the inverter's firmware to ensure optimal battery management and system performance.
Schedule annual system inspections by a qualified electrician to check for wiring issues, corrosion, and component degradation.
Monitor the battery's state of health (SOH) using the inverter's monitoring app or a dedicated BMS monitoring tool. Replace the battery when the SOH falls below a predefined threshold (e.g., 80%).
Periodically perform a full charge/discharge cycle to calibrate the BMS and maintain accurate SOC readings.
Clean the solar panels regularly to maximize solar production and reduce the reliance on battery power.