I'm from Brazil e I have an APC BR1200N-BR (input: bivolt / output: 120V). Some days ago I've tested the output voltage using an eletronic multimeter and, for my surprise, I've noticed that, when on battery mode, the device provides a strange too low output voltage: around 90V.
When working with input power, the reading of the output voltage is around 115V. That is to say, completely normal. However, on battery mode the too low voltage has scared me, as, I think, 90~92V doesn't seem to be a reasonable margin for devices that require 120V, although, up to now, I haven't faced any problem.
Besides that, another fact has called my attention. A friend of mine has the same unity model and he, indeed, has been having troubles. His pc reboots all the time and the motherboard, when the pc loads Windows, has stability problems and fan reading issues. In his case, the output voltage measured with the multimeter, on battery mode, is: 70~75V.
Having that said, I'd like to know what would be the accurate and safe output voltage range when on battery mode, so that the devices connected to the UPS work without risk of problems.
Are you using a non True RMS or a True RMS voltmeter? If you are using a non True RMS, that is the usual voltage that you'll get on our UPS. If you could find a true RMS voltmeters you could use that and it would give you a reading of ~115V AC.
With regards to your friends problem, it can be caused by a lot of things such as: power supply is too sensitive with the UPS transfer time, power supply doesn't read or like the step approximated sine wave that BackUPS supplies.
Below are APC Knowledgebase articles that would give you ideas with your problems:
[True RMS Voltmeter Required to Read Stepped Approximated Sine Wave|http://nam-en.apc.com/app/answers/detail/a_id/1493/kw/true%20rms%20voltmeter/session/L3RpbWUvMTMxMDk2MTk4MS9zaWQvWHFtZ09mems%3D]
[Different Types of Voltmeters|http://nam-en.apc.com/app/answers/detail/a_id/4119/kw/true%20rms%20voltmeter/session/L3RpbWUvMTMxMDk2MTk4MS9zaWQvWHFtZ09mems%3D]
[Equipment connected to a Back-UPS product turns off unexpectedly and/or reboots.|http://nam-en.apc.com/app/answers/detail/a_id/20/kw/equipment%20reboots/session/L3RpbWUvMTMxMDk2MTk4MS9zaWQvWHFtZ09mems%3D]
Like what I have said on my previous post. You will need to use a True RMS voltmeter to read the proper voltage of the UPS when it is running on battery. When our BackUPS are online, it provides whatever is being fed into it unless it is trimming or boosting the voltage. When it goes to battery mode, it switches to a step approximated/ square wave. This is where the problem happen, if you are using a non True RMS voltmeter it cannot read the voltage properly. But if it is a True RMS voltmeter, it will read all the voltage points and would tell you that the voltage is ~115V AC.