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Date:         Sun, 22 Jun 2008 19:03:41 -0400
Reply-To:     Poppie Jagersand <poppie.jagersand@YAHOO.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Poppie Jagersand <poppie.jagersand@YAHOO.CA>
Subject:      Re: Why AGM batts don't last as long as they should
In-Reply-To:  <E036FBCE7F1344B7AB45C2D5417119FE@MASTERPC>
Content-Type: text/plain; charset=iso-8859-1

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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