In article , says...

Is there an easy way to calibrate (low voltage) digital voltmeter? Is
there- say a battery that can be usd as a reference?

No battery in the consumer market is precise enough. In order to
properly calibrate any voltmeter, you need a true calibration standard
such as the Fluke 5700 series.

Regrettably, such equipment usually carries a five-figure price
tag. There are older units -- DC and AC references and such -- that you
can often find on Greed-bay or at electronic surplus places, but keep in
mind that you may need multiple units for DC, AC, and Ohms.

Happy hunting.

--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR,
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"