Why UPS Not Working In AC Mode?

UPS backup batteries operate in one of two modes. The first is the battery mode when it discharges battery power during a blackout. However, the UPS operates in AC mode during normal times when you plug it into a wall socket. So, why would a UPS work in battery mode but not AC mode?

When your UPS works normally in battery mode but fails in AC mode, it’s likely because of an incoming power supply issue. The UPS will reject the incoming power supply if it’s unstable. Besides, the battery might be worn out, its connectors have come undone, or the internal charge and sensing circuits are faulty.

Keep reading to learn more about why your UPS isn’t working in AC mode and how you can get it working correctly again.

Why Is My UPS Not Working On AC Power?

When a UPS unit is in AC mode, it’s plugged into a wall socket and receives the standard alternating current (AC) electricity supply from it. Alternatively, the UPS goes into battery mode when there is no incoming power, and it’s discharging from its built-in batteries.

Batteries will eventually drain out, but the AC power supply should be constant. That’s why it can be pretty confusing when the UPS works fine in battery mode but not when plugged into AC power.

Here are likely causes you can troubleshoot if your UPS unit works on battery but not when in AC mode:

Power Supply Problems

What it is: A UPS unit’s ability to receive an incoming power supply is just as crucial as its ability to discharge battery power during a blackout.

That’s why the incoming AC power must be stable and continuous to ensure that the UPS can function correctly while recharging its internal batteries.

A stable power supply will be of the correct voltage and frequency with few or no fluctuations. When the UPS and its circuits sense that the incoming power supply is stable, it will accept that AC current flow into the unit.

What’s happened: The first reason your UPS works on battery mode but not with AC power is a problem with that AC power supply.

As mentioned earlier, the incoming power supply must be continuous and stable. 

Connect with an Appliance Repair Tech

Click here to use the chatbox to speak with one of our technicians.
No in-home service calls. No appointments.

Whenever it’s unstable, like with plenty of fluctuations or voltage drops, the UPS circuits will sense a problem. Those circuits are designed to prevent unstable currents from entering the unit.

Ultimately, that will prevent the UPS from working correctly in AC mode.

How to fix it: This problem has nothing to do with the UPS unit. Remember: your UPS doesn’t work on AC mode because it rejects the unstable power supply it receives from the wall socket.

Instead, it’s an issue with the electrical setup in your building or the municipal power supply you receive from outside.

So, you can contact your local power company to check if any issues affect the power supply’s stability. Besides that, you can also consult an electrician to diagnose the circuits in your building.

Read: How To Troubleshoot UPS Battery?

Disconnected Connectors

What it is: You’ll find several electrical connectors inside your UPS unit. They ensure that power can flow to where it needs to be. 

So, when correctly connected, the electrical current can flow into your battery to charge it and out of it to power your devices.

What’s happened: Another likely reason your UPS won’t work in AC mode is that one or more electrical connectors are detached inside.

Firstly, UPS units are shipped with some of their connectors undone. That’s for safety reasons to ensure that the units can be transported safely.

However, it’s also possible that some connectors have become loose or come undone after repairs.

When that happens, the UPS can’t receive and run on AC power, even though it might work perfectly fine in battery mode.

How to fix it: You can solve this problem by disconnecting the UPS from its power source and inspecting its internal components. Check that all the battery connectors are firmly attached as they should be.

You can refer to the user manual or any available technical sheets to ensure that you’ve inspected all the connectors in your UPS model.

Read: Why Ups (Battery Backup) Is Beeping Continuously?

Worn-Out Battery

What it is: The backup battery inside a UPS unit will wear out after an extended period. The typical lifespan of a UPS battery is 3-5 years. 

However, excessive wear can shorten that lifespan, while proper maintenance and use can maximize it.

Worn-out UPS batteries can cause several problems for users and, in some cases, can also prevent the UPS from working in AC mode.

What’s happened: Some UPS units can work in AC mode with or without a functioning battery. Unfortunately, many models require a battery that’s in excellent working condition to do so.

The battery inside is likely worn out when your UPS fails to work in AC mode. That might be confirmed by error messages or indicators that tell you to replace the battery as soon as possible.

How to fix it: Backup batteries are never repairable or serviceable. So, the only solution here is disconnecting your UPS and replacing the battery inside.

In most cases, UPS battery replacements are pretty straightforward as a DIY task. But if that’s not possible, let a qualified technician do it safely for you.

Read: How To Check UPS Battery Health?

Faulty Internal Circuits

What it is: As you read about above, your UPS has several built-in circuits that help it function correctly. These include the charging and sensing circuits that control how the unit behaves in AC or battery mode.

In simple terms, those circuits will sense when there’s an incoming AC power supply. Then, they’ll automatically switch the UPS to AC mode and allow the backup battery to recharge.

What’s happened: The last reason you should consider is that your UPS unit’s internal circuits have become faulty. When that happens, the UPS cannot correctly sense when to switch between AC or battery mode.

So, what happens is that the UPS won’t switch to AC mode even when you’ve plugged it into a wall socket that’s working perfectly fine.

How to fix it: Repairing your UPS’s internal circuitry is challenging and dangerous. You could cause a lot of damage to yourself and the area around you if you attempt to fix it without the necessary skills and knowledge.

So, it’s always best to refer to a qualified technician to complete the repairs.

The repairs they’ll perform include diagnosing and possibly replacing the faulty circuits inside your UPS unit. Assuming all other components function correctly, the repair is enough, and you won’t have to replace the whole unit.

Read: Can A UPS Last For 3 Hours?

Frequently Asked Questions (FAQs)

Here are a few frequently asked questions and answers to help you understand your UPS better:

What Does AC Mode Mean On UPS?

AC mode means your UPS runs on alternating current (AC) power from a wall socket. When you disconnect the UPS from the wall socket, it will automatically switch to battery mode and run on battery power instead.

Read: What Happens When UPS Battery Fails?

Can UPS Work Without Battery?

That depends on the UPS model. Some models will work without batteries when you plug them into an AC power source. However, many models won’t work if the battery inside is removed or faulty. Check your UPS user manual to know if your unit can work without a battery.


A UPS receives alternating current (AC) power from the wall socket to function and recharge its battery. However, the unit discharges direct current (DC) during a power outage when it goes into battery mode.

What Causes UPS Battery Failure?

A UPS battery can fail for several reasons. The most common causes are overheating and a faulty battery. Besides that, the UPS battery will fail with excessive wear, primarily if it has been used for many years.

How Do I Reset My UPS Battery?

You can reset your UPS battery by undoing its connectors inside the UPS unit. Then, press and hold the ON button for at least 5 seconds, depending on your UPS model. After that, you can reconnect the battery. That will complete the battery reset process.

DMCA.com Protection Status