iMac shuts off when UPS switches to battery power

[Anonymous user]

Hi,

I have a BX-1400U-MS that has worked great for the last couple of years.

Recently, during power interruptions, my iMac has just shut off when the UPS has switched to battery output. To power up the iMac again, I must physically unplug the iMac's power supply cable and plug it back in.

Today, I had the internal batteries replaced, and when I got the UPS home and tested it out, it has the same symptoms with my iMac shutting off as soon as I pull the plug on the UPS.

I swapped it out with cheap unbranded $50 UPS and confirmed that my iMac continues to operate when I disconnect the power from the wall.

I have VM Fusion installed with Windows 10, so I downloaded PowerChute and ran a self test, however I can't see the results because the iMac shuts down during the test.

Is this an issue with my UPS, or is there an issue with my iMac power supply?

Thanks,

Carl

[Anonymous user]

I just ran another test:

Removed input power to the UPS - iMac immediately shuts off.

While still on battery output, I unplugged the iMac and plugged it back in.

The iMac will power up and operate under battery output.

This confirms that the problem only occurs during the change over from mains to battery power.

[Anonymous user]

When the UPS switches over to battery, there's a very small "window" during which the incoming power disappears. This takes only milliseconds and most power supplies have enough of a "buffer" to ride through the brief interruption.

Exactly where the change has taken place is hard to say, especially since a different UPS worked fine. I'd be more inclined to think it's something within the iMac's power supply having drifted a bit out of tolerance. The other UPS may switch to battery power a little bit more smoothly or quickly.

I'm not sure it'll help, but you might try adjusting the sensitivity of your APC UPS, if it supports doing so.

[Anonymous user]

Hi William,

Thanks for your reply.

I have a smaller BX-700U-MS that is used for my router and is the same age.

I hooked it up to my iMac and then pulled the inlet cord from the wall. My iMac stayed on during the switch to battery power.

This tells me that my iMac is fine and the problem lies with the UPS switching window. Do you concur?

Cheers,

Carl

[Anonymous user · Accepted solution]

I can't really say with any certainty exactly where the trouble lies, only that something has definitely changed over time. It could be that both the UPS and the Mac have experienced changes in the tolerance of their components, and only between the two is a troublesome combination created.