A 12V battery at high load may only give around 11V or even less, then there are resistive losses in the low voltage wiring and components that reduce the final voltage even further.
Also, the output is likely synthesised by a PWM signal and controlled with feedback to maintain the correct output voltage, and enough "headroom" in the duty cycle so the power devices never lock on from cycle to cycle (100% duty cycle).
That totally depends on the rest of the unit circuit & how it is built? Is it an off-line or on-line design?
(Switching in with a clunk when AC fails, or permanently stabilising the power through it?)
That totally depends on the rest of the unit circuit & how it is built? Is it an off-line or on-line design?
(Switching in with a clunk when AC fails, or permanently stabilising the power through it?)
If you are using the transformer without connections to the original electronics it should be OK, at least for a short time to test the output & see what you get?
The transformer in a typical modified sinewave UPS does not directly make the output voltage. It makes a single DC rail that is the peak output voltage.
4 mosfets in a full bridge configuration then are switched on in timed sequence to create the positive. zero, negative, zero, repeat, output waveform.