The voltmeter inside are not accurate. They are more to confirm that the batteries are being charged. To test if they are getting 16VDC - use your multi meter across this meter and see what you will read.

The answer(s) are: 1. No adjustment. 2. The voltmeter reading is very inaccurate.

I suggest you go to Fry's or to a local ham radio shop and purchase a *quality* handheld multimeter. A ham radio shop can give very informed guidance. My multimeter (http://www.amazon.com/Fluke-FLUKE-87-V-Digital-Multimeter/dp/B0002YFD1K/ref=sr_1_5?s=lamps-light&srs=2529432011&ie=UTF8&qid=1446774332&sr=1-5&keywords=fluke+voltmeter) is a $400 Fluke meter, but I am sure other brands with nearly the same quality can be purchased for less. Or find a friend with a good meter and have him check the battery voltage at the bus behind the circuit breaker panel. Once you know an accurate voltage you can write on a piece of blue tape what the correction factor is and stick that to the meter front. Eg, "Voltmeter reads .6 VDC Low".

Good advice for all boat owners, Catalina or other: In future, whenever troubleshooting DC or even AC troubles, use a high quality multimeter instead of the ship's original voltmeter. An exception to this rule is that the Link Pro and similar types of monitors typically have a very accurate voltmeter, and so can be believed.