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PhaseShift:
The following is not a complete answer.
If you use a meter with an input coupling capacitor, (Examples: Simpson 260 in the OUTPUT mode, or Fluke 27 or 87 in AC position), the AC and DC voltage sources are isolated and added together, then you will read the AC voltage.
If you use any of the above meters in the DC position, then the reading is the DC voltage.
If you have an RMS reading meter that works for DC thru an adequate upper frequency, then the reading will be the RMS value of the composite waveform.
If your meter is a peak reading type, then it's the peak that is measured. Note: + and - peaks could be different and you would need to switch polarity to get the two different values.
Undesired current will flow if you parallel a DC voltage source with an AC voltage source.
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