I have the the same gauge - and it normally reads
~12.2-12.3V off, and ~13.9V running with no
accessories on. Use of accessories will drop voltage -
so that with both heater fans, headlights, stereo, etc
- it may be 12.9 or 13.0V. When the batteries have
either just sat for a while, or I've used them, I'll
see voltage of ~ 13.5-13.7V running. This will
gradually creep up to the 13.9 value once it's had
time to fully charge...

John

--- Larry Alofs <lalofs@RCN.COM> wrote:

> To me, both readings seem about half a volt low.
>
> Larry A.
>