From Bob Nelson - "Lionelsoni":

 

Do you have a "true RMS" voltmeter?  Most AC voltmeters assume that you are measuring a sinusoidal waveform, which the CW-80 definitely does not put out.  Here is a correction chart for use with an ordinary voltmeter:

Meter          RMS 
  0          0
  0.5          1.6
  1          2.7
  1.5          3.7
  2          4.6
  2.5          5.4
  3          6.1
  3.5          6.9
  4          7.5
  4.5          8.2
  5          8.8
  5.5          9.5
  6          10
  6.5          10.6
  7          11.2
  7.5          11.7
  8          12.2
  8.5          12.7
  9          13.2
  9.5          13.7
 10          14.1
 10.5          14.6
 11          15
 11.5          15.4
 12          15.8
 12.5          16.2
 13          16.6
 13.5          16.9
 14          17.3
 14.5          17.6
 15          17.9
 15.5          18.2
 16          18.5
 16.5          18.8
 17          19
 17.5          19.3
 18          19.5
 18.5          19.7
 19          19.8
 19.5          19.9
 20          20

 

You can see that when the CW-80 is putting out its full 18 volts, an ordinary meter (which I'm assuming is what you've got) should read about 15 volts, which is not too far from what you're seeing on the accessory output.