[C38] Battery Voltage

[Les]

Hi, Steve, 
	The multimeter is most likely a relatively inexpensive one?
	Most multimeters use one of two methods to read voltage.  The first is
a D/A, generally 10 bits, and autoranging is just starting at the
maximum range and going down the ranges until the meter reads about half
scale typically or the lowest range is reached.  Some use a method
called dual slope, where a capacitor is charged for a dedicated time by
the measured voltage, and then discharged via a lower constant current.
The time measured is equivalent to the voltage.  The first method is
subject to more errors, and produces errors on the order of 1-2 lsb's,
about 1 part per thousand, or for a 12v measurement about .1% +/- the
percentage of tolerance of all the components used to make the meter,
typically 1% resistors. .1v is 1%.  But this is still relatively
accurate.  Panel meters generally use an offset of some kind, generally
a zener diode, which is 1-2% tolerance, and then if a monitor, an A/D to
measure the voltage.  I suspect it also uses 1% resistors, so a 0.1v
error is reasonable unless they design it with tighter components.  Also
temperature, specific gravity and time after the charge will also affect
the reading.  

	OK, to get a good static reading of battery voltage, remove all loads
(switch it off and disconnect the bilge pump) or remove one of the
terminals from the battery.
	
	Wait 1/2 hour for the battery to stabilize.  This saturates the charge
and the voltage will more or less blend through the plates and
electrolyte.
	
	Read the voltage.  This will tell you the real static voltage +/- the
meter accuracy.  Check the website for your specific batteries.  Plate
construction, electrolyte specific gravity and temperature will all
affect the battery state of charge.  Make sure you are looking at the
same kind of battery.  AGM, Gell, and lead acid are all different and
each type, a starting battery will be different from a deepcycle will be
different from an industrial (golf cart).

	There is also drop in the wiring to/from the meters, depending on the
load when the voltage is read.  Some monitors have temperature
compensation (did you install that?), and will compensate for the
temperature by adjusting the voltage readout.

	Finally if you want to really split hairs, read all the manuals on your
monitor, the voltmeter, and the batteries.

	One other caution is not to use a mix of starting and deep cycle
batteries on a common charger.  It will work moderately well, but the
starting battery will be severely abused by the charge cycles of the
deep cycle charger, especially if it has the ability to do gassing
charges (the voltage rises to 15v and is held there for one or two hours
to reduce sulfation of the plates. This is sometimes called equalzing
charging.)

Regards,
Les H

On Sun, 2009-09-20 at 22:20 -0700, Steve Smolinske wrote:
> I've been doing some research and cant find what Im looking for, so
> Ill throw it out to the group.  for my start battery I have a 12v
> lead/acid, for the house bank two 6 volts in series.  Nice new voltage
> meter installed in the panel so I can see the state of charge.
> Charged them up last night and today,  The start battery settled in
> three hours later at 12.6 and the house bank 12.9.  I have found info
> all over the net stating fully charged batteries should be 12.7, 13.1
> or 13.5 first question is which should it be?  Second is why do I get
> a .1 difference when looking at the panel read out versus a multimeter
> on the terminals......the multimeter reads lower.   As always thanks. 
>  
> Steve
> Peregrine #312
> Seattle
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