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Summary:
Time Machine backups in El Capitan 10.11.1 will silently stop running if a UPS battery backup device is connected with a USB cable.

Steps to Reproduce:
1. Get a UPS battery backup that connects to your Mac with a USB cable. For example, I have a CyberPower CP 1500C.
2. Connect your UPS battery backup to iMac with a USB cable
3. Go to System Preferences and configure Energy Saver preferences as shown in the two attached screenshots.
4. Setup and configure Time Machine backups


Expected Results:
1. I expect Time Machine backups to run when a UPS is connected and the computer is not running on battery power

2. I expect Time Machine to notify the user if backups have not run within a certain amount of time as a result of running on UPS battery backups.

Actual Results:
1. Time Machine backups silently fail and simply do not run even though the computer is NOT running on the battery backup.

2. Time Machine silently fails and does not run. TM also does NOT notify the user that backups have stopped running for an extended period of time. For example, I discovered this after having gone more than 1 week (7 days) without a backup having run and without any kind of warning message.

Version:
10.11.1 (15B42)

Notes:
Temporary workaround discovered out on the Internet no thanks to Apple support is to change the defaults with this command:

sudo defaults write /Library/Preferences/com.apple.TimeMachine RequiresACPower -bool FALSE

This bug was introduced in either 10.11 or 10.11.1. This issue did not exist on 10.10 and prior OS X versions with the same UPS device and Time Machine configuration

Configuration:
iMac (27-inch, Mid 2010)
CyberPower CP 1500C battery backup connected with USB

Attachments:
'iMac.spx', 'Screen Shot 2015-11-05 at 8.48.46 AM.png' and 'Screen Shot 2015-11-05 at 8.48.48 AM.png' were successfully uploaded.