Inverter Not Switching To Mains (Solutions)

Some grid-tied inverter battery systems are programmed to charge the backup battery bank during off-peak hours using grid power and a rectifier circuit. The inverter battery system can supply alternating current (AC) to household appliances during peak grid demand.

An inverter will convert the direct current (DC) from the battery bank to alternating current (AC) to the connected AC-powered appliances until the battery has reached its minimum operational state of charge (SOC). The inverter will switch back to the rectifier circuit to recharge the batteries.

Something is wrong if the inverter does not automatically switch to “mains” power to recharge the battery bank. There are four possible reasons for this failure to switch to mains to charge the batteries:

  • Dead batteries
  • Burnt out rectifier
  • Burnt out fuse
  • Poor battery cable connections

Let’s look at the process to troubleshoot and resolve the issue with your Uninterrupted Power Supply (UPS) inverter not charging the batteries.

Fixing An Inverter Not Charging From Mains Power

Finding the cause for an inverter failing to charge the battery bank from grid (mains) power is a process of elimination. You will need a multimeter to measure the voltage of the batteries and the current continuity.

Using the multi-meter, establish the battery state of charge. The most common reason for the inverter not charging the battery is that the battery is dead. In this case, the only remedy is to replace the battery with a replacement.

The reason for the battery dying can be poor battery management. Lead-acid-type batteries have a limited lifespan and can only be recharged for a limited number of cycles. These batteries must never be discharged below 50% or deteriorate faster.

Li-ion batteries

Li-ion batteries typically have three to four times longer lifespans than lead-acid type batteries and can be discharged to almost 0% charge. They are more expensive but ultimately give the best investment return over their lifetime.

If the battery bank is functional, a burnt-out rectifier is most likely reason for the system not charging from mains power. A Rectifier steps down the grid AC and converts it to DC of a specified charge voltage and current to optimally charge the battery.

See also: Solar Inverter Problems and Solutions: A Comprehensive Guide to Troubleshooting Common Issues

rectifier circuit

The rectifier circuit in the inverter is kept cool by the cooling fan to prevent it from overheating and burning out. Inverter fans fail due to running for extended hours to provide cooling, resulting in a burnt-out rectifier.

The rectifier circuit will have to be repaired or replaced with the failed cooling fans. Once you have checked for current continuity on the inverter circuit, you can move on to the following most likely reason for failure.

Fuse Failure

Inverters are fitted with fuses to protect the inverter circuitry and connected appliances from damage due to current surges. The fuses are designed to fail if the current exceeds 125% of the maximum specified current flow.

Purchase fuses of equivalent Amp-ratings and replace the blown fuses. Identifying a blown a fuse can be done by visual inspection to see whether the fuse wire has been burnt off or by testing for current continuity with a multimeter.

Loose or badly corroded battery connections may be another reason the inverter charger fails to charge the battery. Inspect the battery cable terminals on both ends for signs of corrosion or that they are poorly tightened.


Clean up any corrosion on the battery cable terminals with sandpaper and properly reconnect them to the battery. Check that the crimped-on terminal connections to the inverter charger is well fastened and securely tightened to the inverter connectors.

The inverter should be charging the battery bank via the rectifier circuit. If the failure mode persists, take the inverter to a professional for fault diagnosis and repair. A dysfunctional inverter could cause a lot of damage if it damages the batteries in your system.

UPS Power connections

How Does A UPS Inverter Work?

Uninterrupted Power Supply (UPS) inverters are the most commonly used inverters in households, offices, and workshops. Electrical devices such as computer systems cannot sustain a sudden loss of power as they will lose their unsaved data.

The use of UPS to ensure power supply continuity in case of grid power outages or low voltage conditions in the grid has become quite common. A UPS consist of the following elements:

  • Rectifier circuit (Battery Charger)
  • Back up battery
  • Inverter circuit

The inverter/rectifier is connected to the battery via a common set of cable connections. The UPS is also connected to grid power, which will charge the battery via the rectifier circuit.

The UPS is connected to critical AC devices that need to be powered by a stable AC power source. The inverter must manage the battery’s charge state to prevent it from discharging below its operational charge.

Maintain Charge

The inverter must engage the rectifier to recharge and maintain the battery SOC above the point where the battery becomes damaged or dysfunctional. If the battery has become damaged or has reached the end of its lifecycle, the automatic charging from the rectifier will not engage.

Cooling fans are fitted to the inverter/rectifier housing to keep the wiring and the components cool during operation. These fans are prone to failure due to their extended running time, and when this happens, the rectifier circuit can overheat and burn out.

Voltage Surge

Voltage surges from the power grid can damage the rectifier which is thus protected by an over-current fuse. The fuse is rated at 125% of the maximum input current to the inverter/rectifier and will protect these circuits from damage by blowing in advance when the current surges beyond the maximum.

UPS inverters are often neglected, resulting in untimely failure. The cooling fans become blocked by dust or fibers drawn into the inverter housing. A buildup of dirt can result in short circuit conditions arising causing component failure.

The maintenance of the battery and the cables connecting the battery to the inverter and the battery bank are also vulnerable to poor maintenance. Cable terminals must be free of corrosion and securely connected to the battery terminals and the inverter connectors.


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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