recommended voltage / smoke units

[Red26]

I noted in one of the manuals packaged with a spectrum locomotive I purchased that the voltage should not exceed 16volts to the smoke unit to prevent damage. I was always under the impression that all the equipment operated on 18volts, up to 22 volts (some manufacturers seem to have even higher ratings). So what is it?

Are the smoke units wired differently from the motors to protect them from anything over 16 volts?

Does anything over 16volts render the speed of most locos prototypically too fast & therefore there is no point in exceeding 16 volts?

Does using battery power make any difference? (less voltage drop across a few inches of wire as opposed to many yards on track power)