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UPS won't switch from battery to generator.

APC UPS for Home and Office Forum

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zuhnc
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Posted: ‎2025-09-25 08:09 PM . Last Modified: ‎2025-09-25 08:24 PM

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Posted: ‎2025-09-25 08:09 PM . Last Modified: ‎2025-09-25 08:24 PM

UPS won't switch from battery to generator.

I have multiple APC UPS's. All of them except the BackUPS 1000 will run on my generator. Onan 15KW gaseous generator, electronic governor for 60Hz (+/- 1/2 Hz), and electronic voltage regulator to keep voltage +/- 5V of 120/240 under any load.  Normal load on generator is less than 3 KW.

Parameters on UPS set at widest limits.  Unit shuts down after two minutes under any condition.  UPS configured to not initiate any shutdown.

 

Edit:  Battery is less than 4 months old.  Replaced due to UPS overcharging the first battery, which was only 9 months old.  UPS passes "self-test", according to the logs.

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Teken
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Posted: ‎2025-09-26 02:50 AM

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Posted: ‎2025-09-26 02:50 AM

The APC Back-UPS Series is well known to react poorly when on generator power if the output isn’t clean when compared to the POCO.

 

You may refer to this APC Resource Link:

 

https://www.se.com/us/en/faqs/FA158842/

 

A basic validation is to measure the output while on generator power at different locations using a calibrated and certified oscilloscope / power quality analyzer.

 

In the ideal situation nothing would be on and powered. At this point you would connect the Back-UPS to the outlet and monitor its behaviour.

 

If the Back-UPS operates as intended than add more loads to the building. You’ll quickly see the Back-UPS will switch over to battery only mode.

 

Ask yourself why??

 

NOTE: You can not simply rely on measuring the voltage / frequency as the final determination. 

Questions Ask . . . 👍 

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zuhnc
zuhnc
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Posted: ‎2025-09-26 06:13 AM

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Posted: ‎2025-09-26 06:13 AM

What does this mean - " if the output isn’t clean when compared to the POCO."?  The load on the generator is less than 2KW, and the UPS load is 30%.  The waveform of the generator looks identical to that of my POCO.

 

Can a firmware upgrade remedy this?  Is one even available?

 

The issue is the UPS doesn't even TRY to go back to supplied power, and instead ALWAYS shuts everything down after 2 minutes, regardless of settings.  Is this normal?

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Teken
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Posted: ‎2025-09-29 04:48 AM

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Posted: ‎2025-09-29 04:48 AM

There are no user available firmware on the Back-UPS Series. As it relates to the power output from your generator it’s obviously not clean.

 

Meaning it’s generating dirty power which the Back-UPS doesn’t like. That can span from voltage, frequency, sine wave, THD, etc.

 

The litmus test is the fact the Back-UPS operates fine while AC power is provided by the POCO. It obviously doesn’t operate fine while on generator as such the output is not 1:1 in terms of power quality.

 

In the vast majority of times when it’s not voltage / frequency the primary reason is 3rd harmonic THD.

 

If you have any high quality filters available you can affirm & test this theory out. 

Using a scope would also show how much and the range of frequencies being generated by your Onan.

 

Lastly, if the system is connected to a computer running APC Power Chute. Provide the following logs for review: Event, Data, UPS during the time the system is on generator power.

 

Questions Ask . . . 👍

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