A battery dropping to 10 volts without explanation is a sign of a bad cell.
Either a cell is dried out and not producing power or shorted. Either way it
needs to come out of service ASAP.

The alternator is working extra hard trying to charge a 5 cell battery and
localized heating can cuase the battery to burst. Yes it can happen to AGMs.
It will be a dryer mess. At 6 years you have exceeded its typical life so
just let go.

I would suggest a normal strting battery but soemthing larger than the group
41. I believe the Diesels were equipped with group 48 as is typical for the
larger Audi's and Mercedes.

Dennis

>From: Poppie Jagersand <poppie.jagersand@YAHOO.CA>
>Reply-To: Poppie Jagersand <poppie.jagersand@YAHOO.CA>
>To: vanagon@GERRY.VANAGON.COM
>Subject: Re: Why AGM batts don't last as long as they should
>Date: Sun, 22 Jun 2008 19:03:41 -0400
>
>Thanks to Dennis for the (as usual) very complete and factual description
>of (AGM) battery charging pitfalls.
>
>My personal subjective experience with Optima red top in the '82 Westy:
>Its CCA capacity seems to degrade over time, while the
>wetcell batteries have kept good capacity for several years,
>then gone dead quickly. The optima is now 6years old, and
>still works ok in the summer, but cranking voltage during
>last winter was a bit marginal for the 1.9l Diesel. (It would start at
>-10-15C but not below -20C. With a new battery the 1.9TD
>starts down to about -30C. At that temperature the Redline MTL gear
>oil is so thick that it is difficult to change gear until warmed up, and
>steering
>is similarly hard.)
>
>The Optima now also seems to self discharge a lot quicker than the equally
>old
>wetcell in my '95 westy. After 1 month of sitting the optima
>was discharged to 10V, while the wetcell was 12.1V. External
>parasite current is 10-20mA, and cannot explain this. (20e-3*24*30=14Ah
>and battery is 55Ah).
>
>In conclusion I'm looking for something else to replace it, but haven't
>found something really appealing on the Canadian market yet.
>Unless I learn differently I'll probably just get another Walmart battery.
>(My last Walmart battery was made by Johnson Controls, a reputable
>manufacturer and also lasted 6 years.)
>
>Martin
>
>Dennis Haynes <d23haynes57@HOTMAIL.COM> wrote:
>http://batterytender.com/battery_basics.php
>
>This AGM battery thing has been going on for a while so I thought that I
>would chime in. I have a lot of battery experience with Computer Room back
>up systems, forklifts, guided vehicles etc. I have dealt with AGM
>batteries up to 48 volt @ 240 A/H (6 hour rate) and flooded types, high
>acid up to 48 volts and 1,250 A/H. I now spec batteries and chargers for
>my companies US and Canada sites and will be doing some of Europe next
>year.
>
>Defects aside, AGM batteries generally die from sulphation, (under
>charging or stored uncharged) or the matt drying out, (overcharging,
>charging too fast, or too often). Unfortunately the fix for sulphation is
>overcharging so you end up in a catch 22.
>
>They are also temperature sensitive. Park your van in the sun and let it
>get to 130F and then start the engine and blast it with 14.2 volts during
>bulk. Yea, that hurts.
>
>Take a look at many computer room UPS installations. The electronics will
>be most any where. The battery banks are usually in the air conditioned
>room.
>
>Look at the Battery tender web site. Somewhat generic but look at the
>charging curves and voltage recommendations. Battery manufacturers often
>have slightly different recommendations due to different grid and plate
>constructions and acid concentrations. Trojan sell 3 different flooded
>style golf cart (GC-2) batteries. The T-145 is a high acid (1.325 vs.
>1.285 sg. It runs and charges at slightly higher voltage. Most important
>is temperature compensation. Some AGMs call for a 14.6 (2.47 v/c), volt
>burst near the end of the absorption stage. Some (Trojan), say never go
>above 14.1.
>
>Batteries can not be properly charged on fixed voltage systems. Smart
>chargers can't work while power demands are constantly changing. The
>voltage is a compromise. Note the usual gassing voltage of ~2.37 v/pc,
>(14.2v). Charging a battery at this voltage through the bulk or high
>current stage means you are localized gassing the whole time. A flooded
>battery will be more tolerant but start kissing that AGM good bye. We
>won't even consider Gel cells here.
>
>VW and most trucks use 13.8 volts, (2.3v/c). This is an ideal voltage as
>it will keep a battery around 80-85% state of charge, (SOC) and not cause
>any damage during extended charging such as long trips. It also stays
>below the gassing limit even during the bulk charge cycle which will
>always happen after engine start and when the battery is heated from
>outside temps or use. Yes many cars run at 14.1 or higher but they usually
>do not get 5 to 7 years of service out of a maintenance free battery.
>
>Now, let's talk about cycling or cycle life. A battery is really cycled
>anytime it is discharged under ~ 50% and when it is fully charged. Full
>charging means bringing it to the gassing or absorption part of a charge
>cycle. Now if you watch a batteries voltage after charging you will see
>that even with no load, the voltage will quickly fall. So now if we set
>the alternator voltage to 14.2 volts or more here is what happens. After
>each time the engine is turned off, the battery voltage will fall. 10%
>will be gone is few hours. So long trip, stop for gas and lunch, start
>engine an hour or so later and guess what. The battery will accept a bulk
>charge and then go into the gas or absorption mode. Yes, you have just
>taken a cycle count on your AGM. My point? You do not want to do this
>frequently, especially on an AGM. The 13.8 avoids this cycle count.
>
>Another problem with a high fixed voltage is the batteries reaction after
>charge is complete. Yu need to detect when the battery is charged by
>monitoring current draw and then reduce the voltage to 13.2-13.5.
>
>Charging batteries is really not about voltage. It is the current that is
>needed to get the job done. At the start of the charge cycle, the current
>should be somewhere between 16% (preferred), to 25% of the A/H capacity. A
>fixed voltage will no do this unless the wiring is undersized or the
>alternator can�t keep up.  Again, look at the charging curves.
>
>AGM Batteries can also be damaged by discharging too fast. You want to
>discharge at 5 to 6 hour rate. So that 1,000 watt inverter that needs 100
>amps really needs 500 A/H of capacity to back it up. Good flooded
>batteries will usually handle a 2 hour rate for a short time.
>
>AGM is still a technology that is being developed. The only real advantage
>is the closed design which reduces spills, splashes, and maintenance. They
>do offer resistance to vibration damage which makes them ideal for boats.
>The down side is that they are still very expensive for what they offer
>and the reliability is not where it should be. They are not tolerant of
>bad charging or discharging practices and they (large ones) can not be
>serviced.
>
>I hope some of this helps.
>
>Dennis
>
>
>
>
>
>-----Original Message-----
>From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
>Stuart MacMillan
>Sent: Wednesday, June 18, 2008 1:59 PM
>To: vanagon@GERRY.VANAGON.COM
>Subject: Re: Why AGM batts don't last as long as they should
>
>That sounds good, you'll know the battery is fully charged when it drops
>to
>13v.  I'd still get a decent charger made to maintain Gels though for
>winter
>storage, and between trips.  The alternator simply won't charge these
>unless
>it can put out 14.4 volts consistently.
>
>Here is the one I have:  www.ctek.com  I have the 3300.
>
>Stuart
>
>-----Original Message-----
>From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
>Michael Diehr
>Sent: Wednesday, June 18, 2008 9:18 AM
>To: vanagon@GERRY.VANAGON.COM
>Subject: Re: Why AGM batts don't last as long as they should
>
> > Anytime I hear of a Vanagon with a dead AGM or two, I'm curious to
> > know
> > what is the charging voltage at the battery terminals? If anything
> > less
> > than the "magic" number of 14.2 or so, an AGM is going to perform
> > poorly or less than optimal.
>
>
>I have an optima yellow top that never seemed to get charged properly
>from the alternator.   About a year ago I added a supplemental solar
>panel (el-cheapo brand):
>   http://www.sunforceproducts.com/english/details.asp?id=89
>
>Any idea if this charge controller is putting out enough voltage?  The
>manual
>
>http://www.sunforceproducts.com/prodinfo/manuals/SunforceSolarManEng.pdf
>suggests it will charge up to 14.2 volts, and cut back in at 13
>volts.   Problem solved ?
>
>
>
>
>
>---------------------------------
>
>
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