On Wed, Jun 16, 2010 at 7:43 AM, Doug Alcock <doug.alcock@gmail.com> wrote:

> Try putting a Voltmeter on the battery and seeing if it agrees with your
> plug in meter. It might be reading a tad low..........
>

Doug has made the point I was going to make but I'll belabor it  a bit.  The
only way to get an accurate reading of the voltage level of the battery is
to measure directly at the battery itself.  There will be 'line loss' if
taking the reading at some point like the cigar lighter.  That said, the
loss will be constant (assuming a good, consistent connection at the lighter
and that is not a good assumption) and the reading you see will be a good
indicator or battery and (when the engine is running) charging state.  I
have two of the little plug in gizmos, one about 40 years old (an actual
analog meter instead of LEDs or a LCD readout) and they are great for
getting a read on the battery and charging system especially in a vehicle
that does not have a volt meter built in.  But the volt meter I have in the
'cabin' for watching the state of either the aux battery or the starting
battery (a switch controls what goes to the meter) is hard wired and then
adjusted to read the same as my Fluke reading the batteries right at the
battery.
BTW, a 'perfect' battery is going to settle out at about 12.5-12.6 volts
after the engine is off for a half an hour or more, assuming no load on it.
Anything from about 12.2 v or above is what you will normally see.