I'm by no means a battery expert, but I've always used voltage readings to determine how much charge a battery has.  After a good deep charge I allow the van to sit overnight.  The next morning I'll check the voltage at the terminals (also factor in temperature) to determine how well a battery is holding up.  In warmer weather (70's +) on a newer battery I can generally hold 12.8v.  In really cold temperatures (20s - 30's) on the same battery I've seen 12.6-12.7.  FWIW, this is on a sealed AGM battery.

Here's a link to a good site that provides some excellent battery information.  It additionally has a great voltage to state of charge chart that I keep handy in the van.
http://www.ccis.com/home/mnemeth/12volt/12volt.htm

Erik

On Jan 26, 2010, at 12:09 PM, Al Knoll wrote:

> specific gravity of the electrolyte at a particular temperature will give
> you what you seek
> wont work for sealed batteries though
> 
> On Tue, Jan 26, 2010 at 11:49 AM, Don Hanson <dhanson928@gmail.com> wrote:
> 
>> Is there a way to tell about the auxilliary battery condition in a VW (or
>> any rig, for that matter)?  I've been working with all kinds of batteries
>> over the years and I always have been puzzled about how to find out what I
>> might expect from the battery.  I have a digital multimeter and I get
>> voltage readings but I'm told that is no real indicator of how good the
>> battery is charged or is capabale of being charged...
>> Is there an instrument that I could wire into my systems (I have a few
>> different vehicles with multiple battery systems) where I might get an
>> actual meaningful readout that would tell me..."Hey, don't expect much more
>> from this battery till you charge it more" or "this battery is shot" or
>> "all
>> your batteries are in fine shape with 100% charge"?
>> I have seen battery condition indicator gauges in yachts..I've seen
>> voltage indicators on some rigs.  My travel trailer has an LED readout and
>> for that matter, so does my semi-westie van...but those things seem pretty
>> much a joke...
>>  What should I wire into my systems to give me a better handle on how much
>> juice I have in the batteries?  Is there anything that works..or do I just
>> have to keep guesstimating as I have been doing all these years?
>> Thanks, Don Hanson
>>